概述

当一个Activity接收到焦点,它会请求对其布局进行绘制。Android框架会处理绘制的流程,但Activity必须提供布局层次的一个根节点。

绘制从布局的根节点开始,它需要对布局树进行测量并进行绘制。绘制通过遍历布局树对每个与失效区域(invalid region)相交的View进行绘制。依次地,每个ViewGroup请求它的每个子View进行绘制(使用draw()方法)并且每个View负责绘制它自身。因为布局树采用前序遍历,这意味着父View会在子View之前(也就是图层的下方)绘制,同时该父View的兄弟View会以它们在树中出现的顺序进行绘制。

框架不会绘制失效区域外的View对象,并且会处理好View背景的绘制,你可以通过调用invalidate()方法来强制一个View进行绘制。

布局的绘制需要经过两个过程的处理:

  • 测量

    测量过程在measure(int, int)方法中实现,它是对View树的自上而下的遍历。在递归过程中,每个View将尺寸规范沿树从上往下推,在测量过程的结尾,每个View都存储其测量结果。

  • 布局

    布局过程在layout(int, int, int, int)中实现,它也是自上而下的。在这个过程中,每个父View负责对其所有子View的位置进行定位,它需要使用测量过程中计算出的视图尺寸。

当一个View对象的measure()方法返回时,其getMeasuredWidth()getMeasuredHeight()值必须被设定,对于该View的所有子孙View对象也是一样。一个View对象的测量的宽度和高度必须遵循父View施加的约束。这可以保证在测量过程的结尾,所有父视图都接收子视图的测量。一个父视图可能会在其子视图上多次调用measure()方法。例如,父视图可能使用未指定的尺寸规格对所有子视图进行测量,来找到子视图想要有多大,如果所有子视图不受约束的尺寸之和太大或者太小,这时候父视图会使用实际的尺寸规格再次调用measure()(也就是说,如果子视图不同意它们自己获取的空间,那么父视图会在第二次测量过程中介入并设定测量规则)。

为了初始化一个布局,可以调用requestLayout()。通常当一个View相信它当前的边界已经不适合它了,那么它会在自身上调用该方法。

测量过程采用两个类对尺寸进行沟通:

  • ViewGroup.LayoutParams

    用于View对象来告知它们的父级它们想要如何被测量和定位。基础ViewGroup.LayoutParams仅描述了View在宽度和高度上想要有多大。对于每个尺寸,它可以指定如下值:

    • 一个确切的数字
    • MATCH_PARENT,表示View想和父级一样大(不包括父级的padding)
    • WRAP_CONTENT,表示View想要刚好足够包围其内容的大小(包括padding)
  • MeasureSpec

    用于从上往下将测量需求从父级推给子级。MeasureSpec可以有如下三种模式:

    • UNSPECIFIED:一个父视图可以用他来确定一个子View所期望的尺寸。例如,一个LinearLayout可以在其子视图上调用measure(),其中将高度设置为UNSPECIFIED并且宽度设置为EXACTLY 240来找出给定一个240像素的宽度时,子视图期望有多高。
    • EXACTLY:用于父视图给子视图施加一个确定的尺寸。子视图必须使用该尺寸,并且保证它所有的后代视图都是符合该尺寸限制的。
    • AT MOST:用于父视图给子视图施加一个最大的尺寸。子视图必须保证它以及其所有的后代都符合该尺寸的限制。

源码分析

整体流程

绘制从布局的根节点开始,它需要测量和和绘制布局树。 从ViewRootImpl的首次布局请求开始:

ViewRootImpl.java

    final class TraversalRunnable implements Runnable {
        @Override
        public void run() {
            doTraversal();
        }
    }

    @Override
    public void requestLayout() {
        if (!mHandlingLayoutInLayoutRequest) {
            checkThread();
            mLayoutRequested = true;
            scheduleTraversals();
        }
    }

    void scheduleTraversals() {
        if (!mTraversalScheduled) {
            mTraversalScheduled = true;
            mTraversalBarrier = mHandler.getLooper().getQueue().postSyncBarrier();
            mChoreographer.postCallback(
                    Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
            if (!mUnbufferedInputDispatch) {
                scheduleConsumeBatchedInput();
            }
            notifyRendererOfFramePending();
            pokeDrawLockIfNeeded();
        }
    }

    void doTraversal() {
        if (mTraversalScheduled) {
            mTraversalScheduled = false;
            mHandler.getLooper().getQueue().removeSyncBarrier(mTraversalBarrier);

            if (mProfile) {
                Debug.startMethodTracing("ViewAncestor");
            }

            performTraversals();

            if (mProfile) {
                Debug.stopMethodTracing();
                mProfile = false;
            }
        }
    }

    private void performTraversals() {
        // cache mView since it is used so much below...
        final View host = mView;

        if (DBG) {
            System.out.println("======================================");
            System.out.println("performTraversals");
            host.debug();
        }

        if (host == null || !mAdded)
            return;

        mIsInTraversal = true;
        mWillDrawSoon = true;
        boolean windowSizeMayChange = false;
        boolean newSurface = false;
        boolean surfaceChanged = false;
        WindowManager.LayoutParams lp = mWindowAttributes;

        int desiredWindowWidth;
        int desiredWindowHeight;

        final int viewVisibility = getHostVisibility();
        final boolean viewVisibilityChanged = !mFirst
                && (mViewVisibility != viewVisibility || mNewSurfaceNeeded);
        final boolean viewUserVisibilityChanged = !mFirst &&
                ((mViewVisibility == View.VISIBLE) != (viewVisibility == View.VISIBLE));

        WindowManager.LayoutParams params = null;
        if (mWindowAttributesChanged) {
            mWindowAttributesChanged = false;
            surfaceChanged = true;
            params = lp;
        }
        CompatibilityInfo compatibilityInfo =
                mDisplay.getDisplayAdjustments().getCompatibilityInfo();
        if (compatibilityInfo.supportsScreen() == mLastInCompatMode) {
            params = lp;
            mFullRedrawNeeded = true;
            mLayoutRequested = true;
            if (mLastInCompatMode) {
                params.privateFlags &= ~WindowManager.LayoutParams.PRIVATE_FLAG_COMPATIBLE_WINDOW;
                mLastInCompatMode = false;
            } else {
                params.privateFlags |= WindowManager.LayoutParams.PRIVATE_FLAG_COMPATIBLE_WINDOW;
                mLastInCompatMode = true;
            }
        }

        mWindowAttributesChangesFlag = 0;

        Rect frame = mWinFrame;
        if (mFirst) {
            mFullRedrawNeeded = true;
            mLayoutRequested = true;

            final Configuration config = mContext.getResources().getConfiguration();
            if (shouldUseDisplaySize(lp)) {
                // NOTE -- system code, won't try to do compat mode.
                Point size = new Point();
                mDisplay.getRealSize(size);
                desiredWindowWidth = size.x;
                desiredWindowHeight = size.y;
            } else {
                desiredWindowWidth = dipToPx(config.screenWidthDp);
                desiredWindowHeight = dipToPx(config.screenHeightDp);
            }

            // We used to use the following condition to choose 32 bits drawing caches:
            // PixelFormat.hasAlpha(lp.format) || lp.format == PixelFormat.RGBX_8888
            // However, windows are now always 32 bits by default, so choose 32 bits
            mAttachInfo.mUse32BitDrawingCache = true;
            mAttachInfo.mHasWindowFocus = false;
            mAttachInfo.mWindowVisibility = viewVisibility;
            mAttachInfo.mRecomputeGlobalAttributes = false;
            mLastConfigurationFromResources.setTo(config);
            mLastSystemUiVisibility = mAttachInfo.mSystemUiVisibility;
            // Set the layout direction if it has not been set before (inherit is the default)
            if (mViewLayoutDirectionInitial == View.LAYOUT_DIRECTION_INHERIT) {
                host.setLayoutDirection(config.getLayoutDirection());
            }
            host.dispatchAttachedToWindow(mAttachInfo, 0);
            mAttachInfo.mTreeObserver.dispatchOnWindowAttachedChange(true);
            dispatchApplyInsets(host);
            //Log.i(mTag, "Screen on initialized: " + attachInfo.mKeepScreenOn);

        } else {
            desiredWindowWidth = frame.width();
            desiredWindowHeight = frame.height();
            if (desiredWindowWidth != mWidth || desiredWindowHeight != mHeight) {
                if (DEBUG_ORIENTATION) Log.v(mTag, "View " + host + " resized to: " + frame);
                mFullRedrawNeeded = true;
                mLayoutRequested = true;
                windowSizeMayChange = true;
            }
        }

        if (viewVisibilityChanged) {
            mAttachInfo.mWindowVisibility = viewVisibility;
            host.dispatchWindowVisibilityChanged(viewVisibility);
            if (viewUserVisibilityChanged) {
                host.dispatchVisibilityAggregated(viewVisibility == View.VISIBLE);
            }
            if (viewVisibility != View.VISIBLE || mNewSurfaceNeeded) {
                endDragResizing();
                destroyHardwareResources();
            }
            if (viewVisibility == View.GONE) {
                // After making a window gone, we will count it as being
                // shown for the first time the next time it gets focus.
                mHasHadWindowFocus = false;
            }
        }

        // Non-visible windows can't hold accessibility focus.
        if (mAttachInfo.mWindowVisibility != View.VISIBLE) {
            host.clearAccessibilityFocus();
        }

        // Execute enqueued actions on every traversal in case a detached view enqueued an action
        getRunQueue().executeActions(mAttachInfo.mHandler);

        boolean insetsChanged = false;

        boolean layoutRequested = mLayoutRequested && (!mStopped || mReportNextDraw);
        if (layoutRequested) {

            final Resources res = mView.getContext().getResources();

            if (mFirst) {
                // make sure touch mode code executes by setting cached value
                // to opposite of the added touch mode.
                mAttachInfo.mInTouchMode = !mAddedTouchMode;
                ensureTouchModeLocally(mAddedTouchMode);
            } else {
                if (!mPendingOverscanInsets.equals(mAttachInfo.mOverscanInsets)) {
                    insetsChanged = true;
                }
                if (!mPendingContentInsets.equals(mAttachInfo.mContentInsets)) {
                    insetsChanged = true;
                }
                if (!mPendingStableInsets.equals(mAttachInfo.mStableInsets)) {
                    insetsChanged = true;
                }
                if (!mPendingVisibleInsets.equals(mAttachInfo.mVisibleInsets)) {
                    mAttachInfo.mVisibleInsets.set(mPendingVisibleInsets);
                    if (DEBUG_LAYOUT) Log.v(mTag, "Visible insets changing to: "
                            + mAttachInfo.mVisibleInsets);
                }
                if (!mPendingOutsets.equals(mAttachInfo.mOutsets)) {
                    insetsChanged = true;
                }
                if (mPendingAlwaysConsumeNavBar != mAttachInfo.mAlwaysConsumeNavBar) {
                    insetsChanged = true;
                }
                if (lp.width == ViewGroup.LayoutParams.WRAP_CONTENT
                        || lp.height == ViewGroup.LayoutParams.WRAP_CONTENT) {
                    windowSizeMayChange = true;

                    if (shouldUseDisplaySize(lp)) {
                        // NOTE -- system code, won't try to do compat mode.
                        Point size = new Point();
                        mDisplay.getRealSize(size);
                        desiredWindowWidth = size.x;
                        desiredWindowHeight = size.y;
                    } else {
                        Configuration config = res.getConfiguration();
                        desiredWindowWidth = dipToPx(config.screenWidthDp);
                        desiredWindowHeight = dipToPx(config.screenHeightDp);
                    }
                }
            }

            // Ask host how big it wants to be
            windowSizeMayChange |= measureHierarchy(host, lp, res,
                    desiredWindowWidth, desiredWindowHeight);
        }

        if (collectViewAttributes()) {
            params = lp;
        }
        if (mAttachInfo.mForceReportNewAttributes) {
            mAttachInfo.mForceReportNewAttributes = false;
            params = lp;
        }

        if (mFirst || mAttachInfo.mViewVisibilityChanged) {
            mAttachInfo.mViewVisibilityChanged = false;
            int resizeMode = mSoftInputMode &
                    WindowManager.LayoutParams.SOFT_INPUT_MASK_ADJUST;
            // If we are in auto resize mode, then we need to determine
            // what mode to use now.
            if (resizeMode == WindowManager.LayoutParams.SOFT_INPUT_ADJUST_UNSPECIFIED) {
                final int N = mAttachInfo.mScrollContainers.size();
                for (int i=0; i<N; i++) {
                    if (mAttachInfo.mScrollContainers.get(i).isShown()) {
                        resizeMode = WindowManager.LayoutParams.SOFT_INPUT_ADJUST_RESIZE;
                    }
                }
                if (resizeMode == 0) {
                    resizeMode = WindowManager.LayoutParams.SOFT_INPUT_ADJUST_PAN;
                }
                if ((lp.softInputMode &
                        WindowManager.LayoutParams.SOFT_INPUT_MASK_ADJUST) != resizeMode) {
                    lp.softInputMode = (lp.softInputMode &
                            ~WindowManager.LayoutParams.SOFT_INPUT_MASK_ADJUST) |
                            resizeMode;
                    params = lp;
                }
            }
        }

        if (params != null) {
            if ((host.mPrivateFlags & View.PFLAG_REQUEST_TRANSPARENT_REGIONS) != 0) {
                if (!PixelFormat.formatHasAlpha(params.format)) {
                    params.format = PixelFormat.TRANSLUCENT;
                }
            }
            mAttachInfo.mOverscanRequested = (params.flags
                    & WindowManager.LayoutParams.FLAG_LAYOUT_IN_OVERSCAN) != 0;
        }

        if (mApplyInsetsRequested) {
            mApplyInsetsRequested = false;
            mLastOverscanRequested = mAttachInfo.mOverscanRequested;
            dispatchApplyInsets(host);
            if (mLayoutRequested) {
                // Short-circuit catching a new layout request here, so
                // we don't need to go through two layout passes when things
                // change due to fitting system windows, which can happen a lot.
                windowSizeMayChange |= measureHierarchy(host, lp,
                        mView.getContext().getResources(),
                        desiredWindowWidth, desiredWindowHeight);
            }
        }

        if (layoutRequested) {
            // Clear this now, so that if anything requests a layout in the
            // rest of this function we will catch it and re-run a full
            // layout pass.
            mLayoutRequested = false;
        }

        boolean windowShouldResize = layoutRequested && windowSizeMayChange
            && ((mWidth != host.getMeasuredWidth() || mHeight != host.getMeasuredHeight())
                || (lp.width == ViewGroup.LayoutParams.WRAP_CONTENT &&
                        frame.width() < desiredWindowWidth && frame.width() != mWidth)
                || (lp.height == ViewGroup.LayoutParams.WRAP_CONTENT &&
                        frame.height() < desiredWindowHeight && frame.height() != mHeight));
        windowShouldResize |= mDragResizing && mResizeMode == RESIZE_MODE_FREEFORM;

        // If the activity was just relaunched, it might have unfrozen the task bounds (while
        // relaunching), so we need to force a call into window manager to pick up the latest
        // bounds.
        windowShouldResize |= mActivityRelaunched;

        // Determine whether to compute insets.
        // If there are no inset listeners remaining then we may still need to compute
        // insets in case the old insets were non-empty and must be reset.
        final boolean computesInternalInsets =
                mAttachInfo.mTreeObserver.hasComputeInternalInsetsListeners()
                || mAttachInfo.mHasNonEmptyGivenInternalInsets;

        boolean insetsPending = false;
        int relayoutResult = 0;
        boolean updatedConfiguration = false;

        final int surfaceGenerationId = mSurface.getGenerationId();

        final boolean isViewVisible = viewVisibility == View.VISIBLE;
        final boolean windowRelayoutWasForced = mForceNextWindowRelayout;
        if (mFirst || windowShouldResize || insetsChanged ||
                viewVisibilityChanged || params != null || mForceNextWindowRelayout) {
            mForceNextWindowRelayout = false;

            if (isViewVisible) {
                // If this window is giving internal insets to the window
                // manager, and it is being added or changing its visibility,
                // then we want to first give the window manager "fake"
                // insets to cause it to effectively ignore the content of
                // the window during layout.  This avoids it briefly causing
                // other windows to resize/move based on the raw frame of the
                // window, waiting until we can finish laying out this window
                // and get back to the window manager with the ultimately
                // computed insets.
                insetsPending = computesInternalInsets && (mFirst || viewVisibilityChanged);
            }

            if (mSurfaceHolder != null) {
                mSurfaceHolder.mSurfaceLock.lock();
                mDrawingAllowed = true;
            }

            boolean hwInitialized = false;
            boolean contentInsetsChanged = false;
            boolean hadSurface = mSurface.isValid();

            try {
                if (DEBUG_LAYOUT) {
                    Log.i(mTag, "host=w:" + host.getMeasuredWidth() + ", h:" +
                            host.getMeasuredHeight() + ", params=" + params);
                }

                if (mAttachInfo.mThreadedRenderer != null) {
                    // relayoutWindow may decide to destroy mSurface. As that decision
                    // happens in WindowManager service, we need to be defensive here
                    // and stop using the surface in case it gets destroyed.
                    if (mAttachInfo.mThreadedRenderer.pauseSurface(mSurface)) {
                        // Animations were running so we need to push a frame
                        // to resume them
                        mDirty.set(0, 0, mWidth, mHeight);
                    }
                    mChoreographer.mFrameInfo.addFlags(FrameInfo.FLAG_WINDOW_LAYOUT_CHANGED);
                }
                relayoutResult = relayoutWindow(params, viewVisibility, insetsPending);

                if (DEBUG_LAYOUT) Log.v(mTag, "relayout: frame=" + frame.toShortString()
                        + " overscan=" + mPendingOverscanInsets.toShortString()
                        + " content=" + mPendingContentInsets.toShortString()
                        + " visible=" + mPendingVisibleInsets.toShortString()
                        + " visible=" + mPendingStableInsets.toShortString()
                        + " outsets=" + mPendingOutsets.toShortString()
                        + " surface=" + mSurface);

                final Configuration pendingMergedConfig =
                        mPendingMergedConfiguration.getMergedConfiguration();
                if (pendingMergedConfig.seq != 0) {
                    if (DEBUG_CONFIGURATION) Log.v(mTag, "Visible with new config: "
                            + pendingMergedConfig);
                    performConfigurationChange(mPendingMergedConfiguration, !mFirst,
                            INVALID_DISPLAY /* same display */);
                    pendingMergedConfig.seq = 0;
                    updatedConfiguration = true;
                }

                final boolean overscanInsetsChanged = !mPendingOverscanInsets.equals(
                        mAttachInfo.mOverscanInsets);
                contentInsetsChanged = !mPendingContentInsets.equals(
                        mAttachInfo.mContentInsets);
                final boolean visibleInsetsChanged = !mPendingVisibleInsets.equals(
                        mAttachInfo.mVisibleInsets);
                final boolean stableInsetsChanged = !mPendingStableInsets.equals(
                        mAttachInfo.mStableInsets);
                final boolean outsetsChanged = !mPendingOutsets.equals(mAttachInfo.mOutsets);
                final boolean surfaceSizeChanged = (relayoutResult
                        & WindowManagerGlobal.RELAYOUT_RES_SURFACE_RESIZED) != 0;
                final boolean alwaysConsumeNavBarChanged =
                        mPendingAlwaysConsumeNavBar != mAttachInfo.mAlwaysConsumeNavBar;
                if (contentInsetsChanged) {
                    mAttachInfo.mContentInsets.set(mPendingContentInsets);
                    if (DEBUG_LAYOUT) Log.v(mTag, "Content insets changing to: "
                            + mAttachInfo.mContentInsets);
                }
                if (overscanInsetsChanged) {
                    mAttachInfo.mOverscanInsets.set(mPendingOverscanInsets);
                    if (DEBUG_LAYOUT) Log.v(mTag, "Overscan insets changing to: "
                            + mAttachInfo.mOverscanInsets);
                    // Need to relayout with content insets.
                    contentInsetsChanged = true;
                }
                if (stableInsetsChanged) {
                    mAttachInfo.mStableInsets.set(mPendingStableInsets);
                    if (DEBUG_LAYOUT) Log.v(mTag, "Decor insets changing to: "
                            + mAttachInfo.mStableInsets);
                    // Need to relayout with content insets.
                    contentInsetsChanged = true;
                }
                if (alwaysConsumeNavBarChanged) {
                    mAttachInfo.mAlwaysConsumeNavBar = mPendingAlwaysConsumeNavBar;
                    contentInsetsChanged = true;
                }
                if (contentInsetsChanged || mLastSystemUiVisibility !=
                        mAttachInfo.mSystemUiVisibility || mApplyInsetsRequested
                        || mLastOverscanRequested != mAttachInfo.mOverscanRequested
                        || outsetsChanged) {
                    mLastSystemUiVisibility = mAttachInfo.mSystemUiVisibility;
                    mLastOverscanRequested = mAttachInfo.mOverscanRequested;
                    mAttachInfo.mOutsets.set(mPendingOutsets);
                    mApplyInsetsRequested = false;
                    dispatchApplyInsets(host);
                }
                if (visibleInsetsChanged) {
                    mAttachInfo.mVisibleInsets.set(mPendingVisibleInsets);
                    if (DEBUG_LAYOUT) Log.v(mTag, "Visible insets changing to: "
                            + mAttachInfo.mVisibleInsets);
                }

                if (!hadSurface) {
                    if (mSurface.isValid()) {
                        // If we are creating a new surface, then we need to
                        // completely redraw it.  Also, when we get to the
                        // point of drawing it we will hold off and schedule
                        // a new traversal instead.  This is so we can tell the
                        // window manager about all of the windows being displayed
                        // before actually drawing them, so it can display then
                        // all at once.
                        newSurface = true;
                        mFullRedrawNeeded = true;
                        mPreviousTransparentRegion.setEmpty();

                        // Only initialize up-front if transparent regions are not
                        // requested, otherwise defer to see if the entire window
                        // will be transparent
                        if (mAttachInfo.mThreadedRenderer != null) {
                            try {
                                hwInitialized = mAttachInfo.mThreadedRenderer.initialize(
                                        mSurface);
                                if (hwInitialized && (host.mPrivateFlags
                                        & View.PFLAG_REQUEST_TRANSPARENT_REGIONS) == 0) {
                                    // Don't pre-allocate if transparent regions
                                    // are requested as they may not be needed
                                    mSurface.allocateBuffers();
                                }
                            } catch (OutOfResourcesException e) {
                                handleOutOfResourcesException(e);
                                return;
                            }
                        }
                    }
                } else if (!mSurface.isValid()) {
                    // If the surface has been removed, then reset the scroll
                    // positions.
                    if (mLastScrolledFocus != null) {
                        mLastScrolledFocus.clear();
                    }
                    mScrollY = mCurScrollY = 0;
                    if (mView instanceof RootViewSurfaceTaker) {
                        ((RootViewSurfaceTaker) mView).onRootViewScrollYChanged(mCurScrollY);
                    }
                    if (mScroller != null) {
                        mScroller.abortAnimation();
                    }
                    // Our surface is gone
                    if (mAttachInfo.mThreadedRenderer != null &&
                            mAttachInfo.mThreadedRenderer.isEnabled()) {
                        mAttachInfo.mThreadedRenderer.destroy();
                    }
                } else if ((surfaceGenerationId != mSurface.getGenerationId()
                        || surfaceSizeChanged || windowRelayoutWasForced)
                        && mSurfaceHolder == null
                        && mAttachInfo.mThreadedRenderer != null) {
                    mFullRedrawNeeded = true;
                    try {
                        // Need to do updateSurface (which leads to CanvasContext::setSurface and
                        // re-create the EGLSurface) if either the Surface changed (as indicated by
                        // generation id), or WindowManager changed the surface size. The latter is
                        // because on some chips, changing the consumer side's BufferQueue size may
                        // not take effect immediately unless we create a new EGLSurface.
                        // Note that frame size change doesn't always imply surface size change (eg.
                        // drag resizing uses fullscreen surface), need to check surfaceSizeChanged
                        // flag from WindowManager.
                        mAttachInfo.mThreadedRenderer.updateSurface(mSurface);
                    } catch (OutOfResourcesException e) {
                        handleOutOfResourcesException(e);
                        return;
                    }
                }

                final boolean freeformResizing = (relayoutResult
                        & WindowManagerGlobal.RELAYOUT_RES_DRAG_RESIZING_FREEFORM) != 0;
                final boolean dockedResizing = (relayoutResult
                        & WindowManagerGlobal.RELAYOUT_RES_DRAG_RESIZING_DOCKED) != 0;
                final boolean dragResizing = freeformResizing || dockedResizing;
                if (mDragResizing != dragResizing) {
                    if (dragResizing) {
                        mResizeMode = freeformResizing
                                ? RESIZE_MODE_FREEFORM
                                : RESIZE_MODE_DOCKED_DIVIDER;
                        startDragResizing(mPendingBackDropFrame,
                                mWinFrame.equals(mPendingBackDropFrame), mPendingVisibleInsets,
                                mPendingStableInsets, mResizeMode);
                    } else {
                        // We shouldn't come here, but if we come we should end the resize.
                        endDragResizing();
                    }
                }
                if (!USE_MT_RENDERER) {
                    if (dragResizing) {
                        mCanvasOffsetX = mWinFrame.left;
                        mCanvasOffsetY = mWinFrame.top;
                    } else {
                        mCanvasOffsetX = mCanvasOffsetY = 0;
                    }
                }
            } catch (RemoteException e) {
            }

            if (DEBUG_ORIENTATION) Log.v(
                    TAG, "Relayout returned: frame=" + frame + ", surface=" + mSurface);

            mAttachInfo.mWindowLeft = frame.left;
            mAttachInfo.mWindowTop = frame.top;

            // !!FIXME!! This next section handles the case where we did not get the
            // window size we asked for. We should avoid this by getting a maximum size from
            // the window session beforehand.
            if (mWidth != frame.width() || mHeight != frame.height()) {
                mWidth = frame.width();
                mHeight = frame.height();
            }

            if (mSurfaceHolder != null) {
                // The app owns the surface; tell it about what is going on.
                if (mSurface.isValid()) {
                    // XXX .copyFrom() doesn't work!
                    //mSurfaceHolder.mSurface.copyFrom(mSurface);
                    mSurfaceHolder.mSurface = mSurface;
                }
                mSurfaceHolder.setSurfaceFrameSize(mWidth, mHeight);
                mSurfaceHolder.mSurfaceLock.unlock();
                if (mSurface.isValid()) {
                    if (!hadSurface) {
                        mSurfaceHolder.ungetCallbacks();

                        mIsCreating = true;
                        SurfaceHolder.Callback callbacks[] = mSurfaceHolder.getCallbacks();
                        if (callbacks != null) {
                            for (SurfaceHolder.Callback c : callbacks) {
                                c.surfaceCreated(mSurfaceHolder);
                            }
                        }
                        surfaceChanged = true;
                    }
                    if (surfaceChanged || surfaceGenerationId != mSurface.getGenerationId()) {
                        SurfaceHolder.Callback callbacks[] = mSurfaceHolder.getCallbacks();
                        if (callbacks != null) {
                            for (SurfaceHolder.Callback c : callbacks) {
                                c.surfaceChanged(mSurfaceHolder, lp.format,
                                        mWidth, mHeight);
                            }
                        }
                    }
                    mIsCreating = false;
                } else if (hadSurface) {
                    mSurfaceHolder.ungetCallbacks();
                    SurfaceHolder.Callback callbacks[] = mSurfaceHolder.getCallbacks();
                    if (callbacks != null) {
                        for (SurfaceHolder.Callback c : callbacks) {
                            c.surfaceDestroyed(mSurfaceHolder);
                        }
                    }
                    mSurfaceHolder.mSurfaceLock.lock();
                    try {
                        mSurfaceHolder.mSurface = new Surface();
                    } finally {
                        mSurfaceHolder.mSurfaceLock.unlock();
                    }
                }
            }

            final ThreadedRenderer threadedRenderer = mAttachInfo.mThreadedRenderer;
            if (threadedRenderer != null && threadedRenderer.isEnabled()) {
                if (hwInitialized
                        || mWidth != threadedRenderer.getWidth()
                        || mHeight != threadedRenderer.getHeight()
                        || mNeedsRendererSetup) {
                    threadedRenderer.setup(mWidth, mHeight, mAttachInfo,
                            mWindowAttributes.surfaceInsets);
                    mNeedsRendererSetup = false;
                }
            }

            if (!mStopped || mReportNextDraw) {
                boolean focusChangedDueToTouchMode = ensureTouchModeLocally(
                        (relayoutResult&WindowManagerGlobal.RELAYOUT_RES_IN_TOUCH_MODE) != 0);
                if (focusChangedDueToTouchMode || mWidth != host.getMeasuredWidth()
                        || mHeight != host.getMeasuredHeight() || contentInsetsChanged ||
                        updatedConfiguration) {
                    int childWidthMeasureSpec = getRootMeasureSpec(mWidth, lp.width);
                    int childHeightMeasureSpec = getRootMeasureSpec(mHeight, lp.height);

                    if (DEBUG_LAYOUT) Log.v(mTag, "Ooops, something changed!  mWidth="
                            + mWidth + " measuredWidth=" + host.getMeasuredWidth()
                            + " mHeight=" + mHeight
                            + " measuredHeight=" + host.getMeasuredHeight()
                            + " coveredInsetsChanged=" + contentInsetsChanged);

                     // Ask host how big it wants to be
                    performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);

                    // Implementation of weights from WindowManager.LayoutParams
                    // We just grow the dimensions as needed and re-measure if
                    // needs be
                    int width = host.getMeasuredWidth();
                    int height = host.getMeasuredHeight();
                    boolean measureAgain = false;

                    if (lp.horizontalWeight > 0.0f) {
                        width += (int) ((mWidth - width) * lp.horizontalWeight);
                        childWidthMeasureSpec = MeasureSpec.makeMeasureSpec(width,
                                MeasureSpec.EXACTLY);
                        measureAgain = true;
                    }
                    if (lp.verticalWeight > 0.0f) {
                        height += (int) ((mHeight - height) * lp.verticalWeight);
                        childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(height,
                                MeasureSpec.EXACTLY);
                        measureAgain = true;
                    }

                    if (measureAgain) {
                        if (DEBUG_LAYOUT) Log.v(mTag,
                                "And hey let's measure once more: width=" + width
                                + " height=" + height);
                        performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
                    }

                    layoutRequested = true;
                }
            }
        } else {
            // Not the first pass and no window/insets/visibility change but the window
            // may have moved and we need check that and if so to update the left and right
            // in the attach info. We translate only the window frame since on window move
            // the window manager tells us only for the new frame but the insets are the
            // same and we do not want to translate them more than once.
            maybeHandleWindowMove(frame);
        }

        final boolean didLayout = layoutRequested && (!mStopped || mReportNextDraw);
        boolean triggerGlobalLayoutListener = didLayout
                || mAttachInfo.mRecomputeGlobalAttributes;
        if (didLayout) {
            performLayout(lp, mWidth, mHeight);

            // By this point all views have been sized and positioned
            // We can compute the transparent area

            if ((host.mPrivateFlags & View.PFLAG_REQUEST_TRANSPARENT_REGIONS) != 0) {
                // start out transparent
                // TODO: AVOID THAT CALL BY CACHING THE RESULT?
                host.getLocationInWindow(mTmpLocation);
                mTransparentRegion.set(mTmpLocation[0], mTmpLocation[1],
                        mTmpLocation[0] + host.mRight - host.mLeft,
                        mTmpLocation[1] + host.mBottom - host.mTop);

                host.gatherTransparentRegion(mTransparentRegion);
                if (mTranslator != null) {
                    mTranslator.translateRegionInWindowToScreen(mTransparentRegion);
                }

                if (!mTransparentRegion.equals(mPreviousTransparentRegion)) {
                    mPreviousTransparentRegion.set(mTransparentRegion);
                    mFullRedrawNeeded = true;
                    // reconfigure window manager
                    try {
                        mWindowSession.setTransparentRegion(mWindow, mTransparentRegion);
                    } catch (RemoteException e) {
                    }
                }
            }

            if (DBG) {
                System.out.println("======================================");
                System.out.println("performTraversals -- after setFrame");
                host.debug();
            }
        }

        if (triggerGlobalLayoutListener) {
            mAttachInfo.mRecomputeGlobalAttributes = false;
            mAttachInfo.mTreeObserver.dispatchOnGlobalLayout();
        }

        if (computesInternalInsets) {
            // Clear the original insets.
            final ViewTreeObserver.InternalInsetsInfo insets = mAttachInfo.mGivenInternalInsets;
            insets.reset();

            // Compute new insets in place.
            mAttachInfo.mTreeObserver.dispatchOnComputeInternalInsets(insets);
            mAttachInfo.mHasNonEmptyGivenInternalInsets = !insets.isEmpty();

            // Tell the window manager.
            if (insetsPending || !mLastGivenInsets.equals(insets)) {
                mLastGivenInsets.set(insets);

                // Translate insets to screen coordinates if needed.
                final Rect contentInsets;
                final Rect visibleInsets;
                final Region touchableRegion;
                if (mTranslator != null) {
                    contentInsets = mTranslator.getTranslatedContentInsets(insets.contentInsets);
                    visibleInsets = mTranslator.getTranslatedVisibleInsets(insets.visibleInsets);
                    touchableRegion = mTranslator.getTranslatedTouchableArea(insets.touchableRegion);
                } else {
                    contentInsets = insets.contentInsets;
                    visibleInsets = insets.visibleInsets;
                    touchableRegion = insets.touchableRegion;
                }

                try {
                    mWindowSession.setInsets(mWindow, insets.mTouchableInsets,
                            contentInsets, visibleInsets, touchableRegion);
                } catch (RemoteException e) {
                }
            }
        }

        if (mFirst && sAlwaysAssignFocus) {
            // handle first focus request
            if (DEBUG_INPUT_RESIZE) Log.v(mTag, "First: mView.hasFocus()="
                    + mView.hasFocus());
            if (mView != null) {
                if (!mView.hasFocus()) {
                    mView.restoreDefaultFocus();
                    if (DEBUG_INPUT_RESIZE) Log.v(mTag, "First: requested focused view="
                            + mView.findFocus());
                } else {
                    if (DEBUG_INPUT_RESIZE) Log.v(mTag, "First: existing focused view="
                            + mView.findFocus());
                }
            }
        }

        final boolean changedVisibility = (viewVisibilityChanged || mFirst) && isViewVisible;
        final boolean hasWindowFocus = mAttachInfo.mHasWindowFocus && isViewVisible;
        final boolean regainedFocus = hasWindowFocus && mLostWindowFocus;
        if (regainedFocus) {
            mLostWindowFocus = false;
        } else if (!hasWindowFocus && mHadWindowFocus) {
            mLostWindowFocus = true;
        }

        if (changedVisibility || regainedFocus) {
            // Toasts are presented as notifications - don't present them as windows as well
            boolean isToast = (mWindowAttributes == null) ? false
                    : (mWindowAttributes.type == WindowManager.LayoutParams.TYPE_TOAST);
            if (!isToast) {
                host.sendAccessibilityEvent(AccessibilityEvent.TYPE_WINDOW_STATE_CHANGED);
            }
        }

        mFirst = false;
        mWillDrawSoon = false;
        mNewSurfaceNeeded = false;
        mActivityRelaunched = false;
        mViewVisibility = viewVisibility;
        mHadWindowFocus = hasWindowFocus;

        if (hasWindowFocus && !isInLocalFocusMode()) {
            final boolean imTarget = WindowManager.LayoutParams
                    .mayUseInputMethod(mWindowAttributes.flags);
            if (imTarget != mLastWasImTarget) {
                mLastWasImTarget = imTarget;
                InputMethodManager imm = InputMethodManager.peekInstance();
                if (imm != null && imTarget) {
                    imm.onPreWindowFocus(mView, hasWindowFocus);
                    imm.onPostWindowFocus(mView, mView.findFocus(),
                            mWindowAttributes.softInputMode,
                            !mHasHadWindowFocus, mWindowAttributes.flags);
                }
            }
        }

        // Remember if we must report the next draw.
        if ((relayoutResult & WindowManagerGlobal.RELAYOUT_RES_FIRST_TIME) != 0) {
            reportNextDraw();
        }

        boolean cancelDraw = mAttachInfo.mTreeObserver.dispatchOnPreDraw() || !isViewVisible;

        if (!cancelDraw && !newSurface) {
            if (mPendingTransitions != null && mPendingTransitions.size() > 0) {
                for (int i = 0; i < mPendingTransitions.size(); ++i) {
                    mPendingTransitions.get(i).startChangingAnimations();
                }
                mPendingTransitions.clear();
            }

            performDraw();
        } else {
            if (isViewVisible) {
                // Try again
                scheduleTraversals();
            } else if (mPendingTransitions != null && mPendingTransitions.size() > 0) {
                for (int i = 0; i < mPendingTransitions.size(); ++i) {
                    mPendingTransitions.get(i).endChangingAnimations();
                }
                mPendingTransitions.clear();
            }
        }

        mIsInTraversal = false;
    }

     /**
     * Figures out the measure spec for the root view in a window based on it's
     * layout params.
     *
     * @param windowSize
     *            The available width or height of the window
     *
     * @param rootDimension
     *            The layout params for one dimension (width or height) of the
     *            window.
     *
     * @return The measure spec to use to measure the root view.
     */
    private static int getRootMeasureSpec(int windowSize, int rootDimension) {
        int measureSpec;
        switch (rootDimension) {

        case ViewGroup.LayoutParams.MATCH_PARENT:
            // Window can't resize. Force root view to be windowSize.
            measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.EXACTLY);
            break;
        case ViewGroup.LayoutParams.WRAP_CONTENT:
            // Window can resize. Set max size for root view.
            measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.AT_MOST);
            break;
        default:
            // Window wants to be an exact size. Force root view to be that size.
            measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY);
            break;
        }
        return measureSpec;
    }

    private void performMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) {
        if (mView == null) {
            return;
        }
        Trace.traceBegin(Trace.TRACE_TAG_VIEW, "measure");
        try {
            mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);
        } finally {
            Trace.traceEnd(Trace.TRACE_TAG_VIEW);
        }
    }

    private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth,
            int desiredWindowHeight) {
        mLayoutRequested = false;
        mScrollMayChange = true;
        mInLayout = true;

        final View host = mView;
        if (host == null) {
            return;
        }
        if (DEBUG_ORIENTATION || DEBUG_LAYOUT) {
            Log.v(mTag, "Laying out " + host + " to (" +
                    host.getMeasuredWidth() + ", " + host.getMeasuredHeight() + ")");
        }

        Trace.traceBegin(Trace.TRACE_TAG_VIEW, "layout");
        try {
            host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());

            mInLayout = false;
            int numViewsRequestingLayout = mLayoutRequesters.size();
            if (numViewsRequestingLayout > 0) {
                // requestLayout() was called during layout.
                // If no layout-request flags are set on the requesting views, there is no problem.
                // If some requests are still pending, then we need to clear those flags and do
                // a full request/measure/layout pass to handle this situation.
                ArrayList<View> validLayoutRequesters = getValidLayoutRequesters(mLayoutRequesters,
                        false);
                if (validLayoutRequesters != null) {
                    // Set this flag to indicate that any further requests are happening during
                    // the second pass, which may result in posting those requests to the next
                    // frame instead
                    mHandlingLayoutInLayoutRequest = true;

                    // Process fresh layout requests, then measure and layout
                    int numValidRequests = validLayoutRequesters.size();
                    for (int i = 0; i < numValidRequests; ++i) {
                        final View view = validLayoutRequesters.get(i);
                        Log.w("View", "requestLayout() improperly called by " + view +
                                " during layout: running second layout pass");
                        view.requestLayout();
                    }
                    measureHierarchy(host, lp, mView.getContext().getResources(),
                            desiredWindowWidth, desiredWindowHeight);
                    mInLayout = true;
                    host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());

                    mHandlingLayoutInLayoutRequest = false;

                    // Check the valid requests again, this time without checking/clearing the
                    // layout flags, since requests happening during the second pass get noop'd
                    validLayoutRequesters = getValidLayoutRequesters(mLayoutRequesters, true);
                    if (validLayoutRequesters != null) {
                        final ArrayList<View> finalRequesters = validLayoutRequesters;
                        // Post second-pass requests to the next frame
                        getRunQueue().post(new Runnable() {
                            @Override
                            public void run() {
                                int numValidRequests = finalRequesters.size();
                                for (int i = 0; i < numValidRequests; ++i) {
                                    final View view = finalRequesters.get(i);
                                    Log.w("View", "requestLayout() improperly called by " + view +
                                            " during second layout pass: posting in next frame");
                                    view.requestLayout();
                                }
                            }
                        });
                    }
                }

            }
        } finally {
            Trace.traceEnd(Trace.TRACE_TAG_VIEW);
        }
        mInLayout = false;
    }

    private void performDraw() {
        if (mAttachInfo.mDisplayState == Display.STATE_OFF && !mReportNextDraw) {
            return;
        } else if (mView == null) {
            return;
        }

        final boolean fullRedrawNeeded = mFullRedrawNeeded;
        mFullRedrawNeeded = false;

        mIsDrawing = true;
        Trace.traceBegin(Trace.TRACE_TAG_VIEW, "draw");
        try {
            draw(fullRedrawNeeded);
        } finally {
            mIsDrawing = false;
            Trace.traceEnd(Trace.TRACE_TAG_VIEW);
        }

        // For whatever reason we didn't create a HardwareRenderer, end any
        // hardware animations that are now dangling
        if (mAttachInfo.mPendingAnimatingRenderNodes != null) {
            final int count = mAttachInfo.mPendingAnimatingRenderNodes.size();
            for (int i = 0; i < count; i++) {
                mAttachInfo.mPendingAnimatingRenderNodes.get(i).endAllAnimators();
            }
            mAttachInfo.mPendingAnimatingRenderNodes.clear();
        }

        if (mReportNextDraw) {
            mReportNextDraw = false;

            // if we're using multi-thread renderer, wait for the window frame draws
            if (mWindowDrawCountDown != null) {
                try {
                    mWindowDrawCountDown.await();
                } catch (InterruptedException e) {
                    Log.e(mTag, "Window redraw count down interruped!");
                }
                mWindowDrawCountDown = null;
            }

            if (mAttachInfo.mThreadedRenderer != null) {
                mAttachInfo.mThreadedRenderer.fence();
                mAttachInfo.mThreadedRenderer.setStopped(mStopped);
            }

            if (LOCAL_LOGV) {
                Log.v(mTag, "FINISHED DRAWING: " + mWindowAttributes.getTitle());
            }

            if (mSurfaceHolder != null && mSurface.isValid()) {
                SurfaceCallbackHelper sch = new SurfaceCallbackHelper(this::postDrawFinished);
                SurfaceHolder.Callback callbacks[] = mSurfaceHolder.getCallbacks();

                sch.dispatchSurfaceRedrawNeededAsync(mSurfaceHolder, callbacks);
            } else {
                pendingDrawFinished();
            }
        }
    }    

测量

ViewRootImpl的mView是所在Window的顶级布局DecorView,它继承了FrameLayout布局,执行的measure()继承于View: View.java

    /**
     * <p>
     * This is called to find out how big a view should be. The parent
     * supplies constraint information in the width and height parameters.
     * </p>
     *
     * <p>
     * The actual measurement work of a view is performed in
     * {@link #onMeasure(int, int)}, called by this method. Therefore, only
     * {@link #onMeasure(int, int)} can and must be overridden by subclasses.
     * </p>
     *
     *
     * @param widthMeasureSpec Horizontal space requirements as imposed by the
     *        parent
     * @param heightMeasureSpec Vertical space requirements as imposed by the
     *        parent
     *
     * @see #onMeasure(int, int)
     */
    public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
        boolean optical = isLayoutModeOptical(this);
        if (optical != isLayoutModeOptical(mParent)) {
            Insets insets = getOpticalInsets();
            int oWidth  = insets.left + insets.right;
            int oHeight = insets.top  + insets.bottom;
            widthMeasureSpec  = MeasureSpec.adjust(widthMeasureSpec,  optical ? -oWidth  : oWidth);
            heightMeasureSpec = MeasureSpec.adjust(heightMeasureSpec, optical ? -oHeight : oHeight);
        }

        // Suppress sign extension for the low bytes
        long key = (long) widthMeasureSpec << 32 | (long) heightMeasureSpec & 0xffffffffL;
        if (mMeasureCache == null) mMeasureCache = new LongSparseLongArray(2);

        final boolean forceLayout = (mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT;

        // Optimize layout by avoiding an extra EXACTLY pass when the view is
        // already measured as the correct size. In API 23 and below, this
        // extra pass is required to make LinearLayout re-distribute weight.
        final boolean specChanged = widthMeasureSpec != mOldWidthMeasureSpec
                || heightMeasureSpec != mOldHeightMeasureSpec;
        final boolean isSpecExactly = MeasureSpec.getMode(widthMeasureSpec) == MeasureSpec.EXACTLY
                && MeasureSpec.getMode(heightMeasureSpec) == MeasureSpec.EXACTLY;
        final boolean matchesSpecSize = getMeasuredWidth() == MeasureSpec.getSize(widthMeasureSpec)
                && getMeasuredHeight() == MeasureSpec.getSize(heightMeasureSpec);
        final boolean needsLayout = specChanged
                && (sAlwaysRemeasureExactly || !isSpecExactly || !matchesSpecSize);

        if (forceLayout || needsLayout) {
            // first clears the measured dimension flag
            mPrivateFlags &= ~PFLAG_MEASURED_DIMENSION_SET;

            resolveRtlPropertiesIfNeeded();

            int cacheIndex = forceLayout ? -1 : mMeasureCache.indexOfKey(key);
            if (cacheIndex < 0 || sIgnoreMeasureCache) {
                // measure ourselves, this should set the measured dimension flag back
                onMeasure(widthMeasureSpec, heightMeasureSpec);
                mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
            } else {
                long value = mMeasureCache.valueAt(cacheIndex);
                // Casting a long to int drops the high 32 bits, no mask needed
                setMeasuredDimensionRaw((int) (value >> 32), (int) value);
                mPrivateFlags3 |= PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
            }

            // flag not set, setMeasuredDimension() was not invoked, we raise
            // an exception to warn the developer
            if ((mPrivateFlags & PFLAG_MEASURED_DIMENSION_SET) != PFLAG_MEASURED_DIMENSION_SET) {
                throw new IllegalStateException("View with id " + getId() + ": "
                        + getClass().getName() + "#onMeasure() did not set the"
                        + " measured dimension by calling"
                        + " setMeasuredDimension()");
            }

            mPrivateFlags |= PFLAG_LAYOUT_REQUIRED;
        }

        mOldWidthMeasureSpec = widthMeasureSpec;
        mOldHeightMeasureSpec = heightMeasureSpec;

        mMeasureCache.put(key, ((long) mMeasuredWidth) << 32 |
                (long) mMeasuredHeight & 0xffffffffL); // suppress sign extension
    }

    /**
     * <p>
     * Measure the view and its content to determine the measured width and the
     * measured height. This method is invoked by {@link #measure(int, int)} and
     * should be overridden by subclasses to provide accurate and efficient
     * measurement of their contents.
     * </p>
     *
     * <p>
     * <strong>CONTRACT:</strong> When overriding this method, you
     * <em>must</em> call {@link #setMeasuredDimension(int, int)} to store the
     * measured width and height of this view. Failure to do so will trigger an
     * <code>IllegalStateException</code>, thrown by
     * {@link #measure(int, int)}. Calling the superclass'
     * {@link #onMeasure(int, int)} is a valid use.
     * </p>
     *
     * <p>
     * The base class implementation of measure defaults to the background size,
     * unless a larger size is allowed by the MeasureSpec. Subclasses should
     * override {@link #onMeasure(int, int)} to provide better measurements of
     * their content.
     * </p>
     *
     * <p>
     * If this method is overridden, it is the subclass's responsibility to make
     * sure the measured height and width are at least the view's minimum height
     * and width ({@link #getSuggestedMinimumHeight()} and
     * {@link #getSuggestedMinimumWidth()}).
     * </p>
     *
     * @param widthMeasureSpec horizontal space requirements as imposed by the parent.
     *                         The requirements are encoded with
     *                         {@link android.view.View.MeasureSpec}.
     * @param heightMeasureSpec vertical space requirements as imposed by the parent.
     *                         The requirements are encoded with
     *                         {@link android.view.View.MeasureSpec}.
     *
     * @see #getMeasuredWidth()
     * @see #getMeasuredHeight()
     * @see #setMeasuredDimension(int, int)
     * @see #getSuggestedMinimumHeight()
     * @see #getSuggestedMinimumWidth()
     * @see android.view.View.MeasureSpec#getMode(int)
     * @see android.view.View.MeasureSpec#getSize(int)
     */
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
                getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
    }

    /**
     * Utility to return a default size. Uses the supplied size if the
     * MeasureSpec imposed no constraints. Will get larger if allowed
     * by the MeasureSpec.
     *
     * @param size Default size for this view
     * @param measureSpec Constraints imposed by the parent
     * @return The size this view should be.
     */
    public static int getDefaultSize(int size, int measureSpec) {
        int result = size;
        int specMode = MeasureSpec.getMode(measureSpec);
        int specSize = MeasureSpec.getSize(measureSpec);

        switch (specMode) {
        case MeasureSpec.UNSPECIFIED:
            result = size;
            break;
        case MeasureSpec.AT_MOST:
        case MeasureSpec.EXACTLY:
            result = specSize;
            break;
        }
        return result;
    }

DecorView.java

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        final DisplayMetrics metrics = getContext().getResources().getDisplayMetrics();
        final boolean isPortrait =
                getResources().getConfiguration().orientation == ORIENTATION_PORTRAIT;

        final int widthMode = getMode(widthMeasureSpec);
        final int heightMode = getMode(heightMeasureSpec);

        boolean fixedWidth = false;
        mApplyFloatingHorizontalInsets = false;
        if (widthMode == AT_MOST) {
            final TypedValue tvw = isPortrait ? mWindow.mFixedWidthMinor : mWindow.mFixedWidthMajor;
            if (tvw != null && tvw.type != TypedValue.TYPE_NULL) {
                final int w;
                if (tvw.type == TypedValue.TYPE_DIMENSION) {
                    w = (int) tvw.getDimension(metrics);
                } else if (tvw.type == TypedValue.TYPE_FRACTION) {
                    w = (int) tvw.getFraction(metrics.widthPixels, metrics.widthPixels);
                } else {
                    w = 0;
                }
                if (DEBUG_MEASURE) Log.d(mLogTag, "Fixed width: " + w);
                final int widthSize = MeasureSpec.getSize(widthMeasureSpec);
                if (w > 0) {
                    widthMeasureSpec = MeasureSpec.makeMeasureSpec(
                            Math.min(w, widthSize), EXACTLY);
                    fixedWidth = true;
                } else {
                    widthMeasureSpec = MeasureSpec.makeMeasureSpec(
                            widthSize - mFloatingInsets.left - mFloatingInsets.right,
                            AT_MOST);
                    mApplyFloatingHorizontalInsets = true;
                }
            }
        }

        mApplyFloatingVerticalInsets = false;
        if (heightMode == AT_MOST) {
            final TypedValue tvh = isPortrait ? mWindow.mFixedHeightMajor
                    : mWindow.mFixedHeightMinor;
            if (tvh != null && tvh.type != TypedValue.TYPE_NULL) {
                final int h;
                if (tvh.type == TypedValue.TYPE_DIMENSION) {
                    h = (int) tvh.getDimension(metrics);
                } else if (tvh.type == TypedValue.TYPE_FRACTION) {
                    h = (int) tvh.getFraction(metrics.heightPixels, metrics.heightPixels);
                } else {
                    h = 0;
                }
                if (DEBUG_MEASURE) Log.d(mLogTag, "Fixed height: " + h);
                final int heightSize = MeasureSpec.getSize(heightMeasureSpec);
                if (h > 0) {
                    heightMeasureSpec = MeasureSpec.makeMeasureSpec(
                            Math.min(h, heightSize), EXACTLY);
                } else if ((mWindow.getAttributes().flags & FLAG_LAYOUT_IN_SCREEN) == 0) {
                    heightMeasureSpec = MeasureSpec.makeMeasureSpec(
                            heightSize - mFloatingInsets.top - mFloatingInsets.bottom, AT_MOST);
                    mApplyFloatingVerticalInsets = true;
                }
            }
        }

        getOutsets(mOutsets);
        if (mOutsets.top > 0 || mOutsets.bottom > 0) {
            int mode = MeasureSpec.getMode(heightMeasureSpec);
            if (mode != MeasureSpec.UNSPECIFIED) {
                int height = MeasureSpec.getSize(heightMeasureSpec);
                heightMeasureSpec = MeasureSpec.makeMeasureSpec(
                        height + mOutsets.top + mOutsets.bottom, mode);
            }
        }
        if (mOutsets.left > 0 || mOutsets.right > 0) {
            int mode = MeasureSpec.getMode(widthMeasureSpec);
            if (mode != MeasureSpec.UNSPECIFIED) {
                int width = MeasureSpec.getSize(widthMeasureSpec);
                widthMeasureSpec = MeasureSpec.makeMeasureSpec(
                        width + mOutsets.left + mOutsets.right, mode);
            }
        }

        super.onMeasure(widthMeasureSpec, heightMeasureSpec);

        int width = getMeasuredWidth();
        boolean measure = false;

        widthMeasureSpec = MeasureSpec.makeMeasureSpec(width, EXACTLY);

        if (!fixedWidth && widthMode == AT_MOST) {
            final TypedValue tv = isPortrait ? mWindow.mMinWidthMinor : mWindow.mMinWidthMajor;
            if (tv.type != TypedValue.TYPE_NULL) {
                final int min;
                if (tv.type == TypedValue.TYPE_DIMENSION) {
                    min = (int)tv.getDimension(metrics);
                } else if (tv.type == TypedValue.TYPE_FRACTION) {
                    min = (int)tv.getFraction(mAvailableWidth, mAvailableWidth);
                } else {
                    min = 0;
                }
                if (DEBUG_MEASURE) Log.d(mLogTag, "Adjust for min width: " + min + ", value::"
                        + tv.coerceToString() + ", mAvailableWidth=" + mAvailableWidth);

                if (width < min) {
                    widthMeasureSpec = MeasureSpec.makeMeasureSpec(min, EXACTLY);
                    measure = true;
                }
            }
        }

        // TODO: Support height?

        if (measure) {
            super.onMeasure(widthMeasureSpec, heightMeasureSpec);
        }
    }

FrameLayout.java

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        int count = getChildCount();

        final boolean measureMatchParentChildren =
                MeasureSpec.getMode(widthMeasureSpec) != MeasureSpec.EXACTLY ||
                MeasureSpec.getMode(heightMeasureSpec) != MeasureSpec.EXACTLY;
        mMatchParentChildren.clear();

        int maxHeight = 0;
        int maxWidth = 0;
        int childState = 0;

        for (int i = 0; i < count; i++) {
            final View child = getChildAt(i);
            if (mMeasureAllChildren || child.getVisibility() != GONE) {
                measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);
                final LayoutParams lp = (LayoutParams) child.getLayoutParams();
                maxWidth = Math.max(maxWidth,
                        child.getMeasuredWidth() + lp.leftMargin + lp.rightMargin);
                maxHeight = Math.max(maxHeight,
                        child.getMeasuredHeight() + lp.topMargin + lp.bottomMargin);
                childState = combineMeasuredStates(childState, child.getMeasuredState());
                if (measureMatchParentChildren) {
                    if (lp.width == LayoutParams.MATCH_PARENT ||
                            lp.height == LayoutParams.MATCH_PARENT) {
                        mMatchParentChildren.add(child);
                    }
                }
            }
        }

        // Account for padding too
        maxWidth += getPaddingLeftWithForeground() + getPaddingRightWithForeground();
        maxHeight += getPaddingTopWithForeground() + getPaddingBottomWithForeground();

        // Check against our minimum height and width
        maxHeight = Math.max(maxHeight, getSuggestedMinimumHeight());
        maxWidth = Math.max(maxWidth, getSuggestedMinimumWidth());

        // Check against our foreground's minimum height and width
        final Drawable drawable = getForeground();
        if (drawable != null) {
            maxHeight = Math.max(maxHeight, drawable.getMinimumHeight());
            maxWidth = Math.max(maxWidth, drawable.getMinimumWidth());
        }

        setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
                resolveSizeAndState(maxHeight, heightMeasureSpec,
                        childState << MEASURED_HEIGHT_STATE_SHIFT));

        count = mMatchParentChildren.size();
        if (count > 1) {
            for (int i = 0; i < count; i++) {
                final View child = mMatchParentChildren.get(i);
                final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();

                final int childWidthMeasureSpec;
                if (lp.width == LayoutParams.MATCH_PARENT) {
                    final int width = Math.max(0, getMeasuredWidth()
                            - getPaddingLeftWithForeground() - getPaddingRightWithForeground()
                            - lp.leftMargin - lp.rightMargin);
                    childWidthMeasureSpec = MeasureSpec.makeMeasureSpec(
                            width, MeasureSpec.EXACTLY);
                } else {
                    childWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec,
                            getPaddingLeftWithForeground() + getPaddingRightWithForeground() +
                            lp.leftMargin + lp.rightMargin,
                            lp.width);
                }

                final int childHeightMeasureSpec;
                if (lp.height == LayoutParams.MATCH_PARENT) {
                    final int height = Math.max(0, getMeasuredHeight()
                            - getPaddingTopWithForeground() - getPaddingBottomWithForeground()
                            - lp.topMargin - lp.bottomMargin);
                    childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(
                            height, MeasureSpec.EXACTLY);
                } else {
                    childHeightMeasureSpec = getChildMeasureSpec(heightMeasureSpec,
                            getPaddingTopWithForeground() + getPaddingBottomWithForeground() +
                            lp.topMargin + lp.bottomMargin,
                            lp.height);
                }

                child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
            }
        }
    }

ViewGroup.java

    /**
     * Ask one of the children of this view to measure itself, taking into
     * account both the MeasureSpec requirements for this view and its padding
     * and margins. The child must have MarginLayoutParams The heavy lifting is
     * done in getChildMeasureSpec.
     *
     * @param child The child to measure
     * @param parentWidthMeasureSpec The width requirements for this view
     * @param widthUsed Extra space that has been used up by the parent
     *        horizontally (possibly by other children of the parent)
     * @param parentHeightMeasureSpec The height requirements for this view
     * @param heightUsed Extra space that has been used up by the parent
     *        vertically (possibly by other children of the parent)
     */
    protected void measureChildWithMargins(View child,
            int parentWidthMeasureSpec, int widthUsed,
            int parentHeightMeasureSpec, int heightUsed) {
        final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();

        final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
                mPaddingLeft + mPaddingRight + lp.leftMargin + lp.rightMargin
                        + widthUsed, lp.width);
        final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
                mPaddingTop + mPaddingBottom + lp.topMargin + lp.bottomMargin
                        + heightUsed, lp.height);

        child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
    }

    /**
     * Does the hard part of measureChildren: figuring out the MeasureSpec to
     * pass to a particular child. This method figures out the right MeasureSpec
     * for one dimension (height or width) of one child view.
     *
     * The goal is to combine information from our MeasureSpec with the
     * LayoutParams of the child to get the best possible results. For example,
     * if the this view knows its size (because its MeasureSpec has a mode of
     * EXACTLY), and the child has indicated in its LayoutParams that it wants
     * to be the same size as the parent, the parent should ask the child to
     * layout given an exact size.
     *
     * @param spec The requirements for this view
     * @param padding The padding of this view for the current dimension and
     *        margins, if applicable
     * @param childDimension How big the child wants to be in the current
     *        dimension
     * @return a MeasureSpec integer for the child
     */
    public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
        int specMode = MeasureSpec.getMode(spec);
        int specSize = MeasureSpec.getSize(spec);

        int size = Math.max(0, specSize - padding);

        int resultSize = 0;
        int resultMode = 0;

        switch (specMode) {
        // Parent has imposed an exact size on us
        case MeasureSpec.EXACTLY:
            if (childDimension >= 0) {
                resultSize = childDimension;
                resultMode = MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.MATCH_PARENT) {
                // Child wants to be our size. So be it.
                resultSize = size;
                resultMode = MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.WRAP_CONTENT) {
                // Child wants to determine its own size. It can't be
                // bigger than us.
                resultSize = size;
                resultMode = MeasureSpec.AT_MOST;
            }
            break;

        // Parent has imposed a maximum size on us
        case MeasureSpec.AT_MOST:
            if (childDimension >= 0) {
                // Child wants a specific size... so be it
                resultSize = childDimension;
                resultMode = MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.MATCH_PARENT) {
                // Child wants to be our size, but our size is not fixed.
                // Constrain child to not be bigger than us.
                resultSize = size;
                resultMode = MeasureSpec.AT_MOST;
            } else if (childDimension == LayoutParams.WRAP_CONTENT) {
                // Child wants to determine its own size. It can't be
                // bigger than us.
                resultSize = size;
                resultMode = MeasureSpec.AT_MOST;
            }
            break;

        // Parent asked to see how big we want to be
        case MeasureSpec.UNSPECIFIED:
            if (childDimension >= 0) {
                // Child wants a specific size... let him have it
                resultSize = childDimension;
                resultMode = MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.MATCH_PARENT) {
                // Child wants to be our size... find out how big it should
                // be
                resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
                resultMode = MeasureSpec.UNSPECIFIED;
            } else if (childDimension == LayoutParams.WRAP_CONTENT) {
                // Child wants to determine its own size.... find out how
                // big it should be
                resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
                resultMode = MeasureSpec.UNSPECIFIED;
            }
            break;
        }
        //noinspection ResourceType
        return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
    }

布局

在ViewRootImpl的performLayout()方法中调用了DecorView的layout()方法,DecorView未重写FrameLayout的这一方法: FrameLayout.java

    @Override
    public final void layout(int l, int t, int r, int b) {
        if (!mSuppressLayout && (mTransition == null || !mTransition.isChangingLayout())) {
            if (mTransition != null) {
                mTransition.layoutChange(this);
            }
            super.layout(l, t, r, b);
        } else {
            // record the fact that we noop'd it; request layout when transition finishes
            mLayoutCalledWhileSuppressed = true;
        }
    }

View.java

    /**
     * Assign a size and position to a view and all of its
     * descendants
     *
     * <p>This is the second phase of the layout mechanism.
     * (The first is measuring). In this phase, each parent calls
     * layout on all of its children to position them.
     * This is typically done using the child measurements
     * that were stored in the measure pass().</p>
     *
     * <p>Derived classes should not override this method.
     * Derived classes with children should override
     * onLayout. In that method, they should
     * call layout on each of their children.</p>
     *
     * @param l Left position, relative to parent
     * @param t Top position, relative to parent
     * @param r Right position, relative to parent
     * @param b Bottom position, relative to parent
     */
    @SuppressWarnings({"unchecked"})
    public void layout(int l, int t, int r, int b) {
        if ((mPrivateFlags3 & PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT) != 0) {
            onMeasure(mOldWidthMeasureSpec, mOldHeightMeasureSpec);
            mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
        }

        int oldL = mLeft;
        int oldT = mTop;
        int oldB = mBottom;
        int oldR = mRight;

        boolean changed = isLayoutModeOptical(mParent) ?
                setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b);

        if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {
            onLayout(changed, l, t, r, b);

            if (shouldDrawRoundScrollbar()) {
                if(mRoundScrollbarRenderer == null) {
                    mRoundScrollbarRenderer = new RoundScrollbarRenderer(this);
                }
            } else {
                mRoundScrollbarRenderer = null;
            }

            mPrivateFlags &= ~PFLAG_LAYOUT_REQUIRED;

            ListenerInfo li = mListenerInfo;
            if (li != null && li.mOnLayoutChangeListeners != null) {
                ArrayList<OnLayoutChangeListener> listenersCopy =
                        (ArrayList<OnLayoutChangeListener>)li.mOnLayoutChangeListeners.clone();
                int numListeners = listenersCopy.size();
                for (int i = 0; i < numListeners; ++i) {
                    listenersCopy.get(i).onLayoutChange(this, l, t, r, b, oldL, oldT, oldR, oldB);
                }
            }
        }

        mPrivateFlags &= ~PFLAG_FORCE_LAYOUT;
        mPrivateFlags3 |= PFLAG3_IS_LAID_OUT;

        if ((mPrivateFlags3 & PFLAG3_NOTIFY_AUTOFILL_ENTER_ON_LAYOUT) != 0) {
            mPrivateFlags3 &= ~PFLAG3_NOTIFY_AUTOFILL_ENTER_ON_LAYOUT;
            notifyEnterOrExitForAutoFillIfNeeded(true);
        }
    }

DecorView.java

    @Override
    protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
        super.onLayout(changed, left, top, right, bottom);
        getOutsets(mOutsets);
        if (mOutsets.left > 0) {
            offsetLeftAndRight(-mOutsets.left);
        }
        if (mOutsets.top > 0) {
            offsetTopAndBottom(-mOutsets.top);
        }
        if (mApplyFloatingVerticalInsets) {
            offsetTopAndBottom(mFloatingInsets.top);
        }
        if (mApplyFloatingHorizontalInsets) {
            offsetLeftAndRight(mFloatingInsets.left);
        }

        // If the application changed its SystemUI metrics, we might also have to adapt
        // our shadow elevation.
        updateElevation();
        mAllowUpdateElevation = true;

        if (changed && mResizeMode == RESIZE_MODE_DOCKED_DIVIDER) {
            getViewRootImpl().requestInvalidateRootRenderNode();
        }
    }

FrameLayout.java

    @Override
    protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
        layoutChildren(left, top, right, bottom, false /* no force left gravity */);
    }

    void layoutChildren(int left, int top, int right, int bottom, boolean forceLeftGravity) {
        final int count = getChildCount();

        final int parentLeft = getPaddingLeftWithForeground();
        final int parentRight = right - left - getPaddingRightWithForeground();

        final int parentTop = getPaddingTopWithForeground();
        final int parentBottom = bottom - top - getPaddingBottomWithForeground();

        for (int i = 0; i < count; i++) {
            final View child = getChildAt(i);
            if (child.getVisibility() != GONE) {
                final LayoutParams lp = (LayoutParams) child.getLayoutParams();

                final int width = child.getMeasuredWidth();
                final int height = child.getMeasuredHeight();

                int childLeft;
                int childTop;

                int gravity = lp.gravity;
                if (gravity == -1) {
                    gravity = DEFAULT_CHILD_GRAVITY;
                }

                final int layoutDirection = getLayoutDirection();
                final int absoluteGravity = Gravity.getAbsoluteGravity(gravity, layoutDirection);
                final int verticalGravity = gravity & Gravity.VERTICAL_GRAVITY_MASK;

                switch (absoluteGravity & Gravity.HORIZONTAL_GRAVITY_MASK) {
                    case Gravity.CENTER_HORIZONTAL:
                        childLeft = parentLeft + (parentRight - parentLeft - width) / 2 +
                        lp.leftMargin - lp.rightMargin;
                        break;
                    case Gravity.RIGHT:
                        if (!forceLeftGravity) {
                            childLeft = parentRight - width - lp.rightMargin;
                            break;
                        }
                    case Gravity.LEFT:
                    default:
                        childLeft = parentLeft + lp.leftMargin;
                }

                switch (verticalGravity) {
                    case Gravity.TOP:
                        childTop = parentTop + lp.topMargin;
                        break;
                    case Gravity.CENTER_VERTICAL:
                        childTop = parentTop + (parentBottom - parentTop - height) / 2 +
                        lp.topMargin - lp.bottomMargin;
                        break;
                    case Gravity.BOTTOM:
                        childTop = parentBottom - height - lp.bottomMargin;
                        break;
                    default:
                        childTop = parentTop + lp.topMargin;
                }

                child.layout(childLeft, childTop, childLeft + width, childTop + height);
            }
        }
    }

绘制

ViewRootImpl在performDraw()方法中调用draw()方法, ViewRootImpl.java

    private void draw(boolean fullRedrawNeeded) {
        Surface surface = mSurface;
        if (!surface.isValid()) {
            return;
        }

        if (DEBUG_FPS) {
            trackFPS();
        }

        if (!sFirstDrawComplete) {
            synchronized (sFirstDrawHandlers) {
                sFirstDrawComplete = true;
                final int count = sFirstDrawHandlers.size();
                for (int i = 0; i< count; i++) {
                    mHandler.post(sFirstDrawHandlers.get(i));
                }
            }
        }

        scrollToRectOrFocus(null, false);

        if (mAttachInfo.mViewScrollChanged) {
            mAttachInfo.mViewScrollChanged = false;
            mAttachInfo.mTreeObserver.dispatchOnScrollChanged();
        }

        boolean animating = mScroller != null && mScroller.computeScrollOffset();
        final int curScrollY;
        if (animating) {
            curScrollY = mScroller.getCurrY();
        } else {
            curScrollY = mScrollY;
        }
        if (mCurScrollY != curScrollY) {
            mCurScrollY = curScrollY;
            fullRedrawNeeded = true;
            if (mView instanceof RootViewSurfaceTaker) {
                ((RootViewSurfaceTaker) mView).onRootViewScrollYChanged(mCurScrollY);
            }
        }

        final float appScale = mAttachInfo.mApplicationScale;
        final boolean scalingRequired = mAttachInfo.mScalingRequired;

        int resizeAlpha = 0;

        final Rect dirty = mDirty;
        if (mSurfaceHolder != null) {
            // The app owns the surface, we won't draw.
            dirty.setEmpty();
            if (animating && mScroller != null) {
                mScroller.abortAnimation();
            }
            return;
        }

        if (fullRedrawNeeded) {
            mAttachInfo.mIgnoreDirtyState = true;
            dirty.set(0, 0, (int) (mWidth * appScale + 0.5f), (int) (mHeight * appScale + 0.5f));
        }

        if (DEBUG_ORIENTATION || DEBUG_DRAW) {
            Log.v(mTag, "Draw " + mView + "/"
                    + mWindowAttributes.getTitle()
                    + ": dirty={" + dirty.left + "," + dirty.top
                    + "," + dirty.right + "," + dirty.bottom + "} surface="
                    + surface + " surface.isValid()=" + surface.isValid() + ", appScale:" +
                    appScale + ", width=" + mWidth + ", height=" + mHeight);
        }

        mAttachInfo.mTreeObserver.dispatchOnDraw();

        int xOffset = -mCanvasOffsetX;
        int yOffset = -mCanvasOffsetY + curScrollY;
        final WindowManager.LayoutParams params = mWindowAttributes;
        final Rect surfaceInsets = params != null ? params.surfaceInsets : null;
        if (surfaceInsets != null) {
            xOffset -= surfaceInsets.left;
            yOffset -= surfaceInsets.top;

            // Offset dirty rect for surface insets.
            dirty.offset(surfaceInsets.left, surfaceInsets.right);
        }

        boolean accessibilityFocusDirty = false;
        final Drawable drawable = mAttachInfo.mAccessibilityFocusDrawable;
        if (drawable != null) {
            final Rect bounds = mAttachInfo.mTmpInvalRect;
            final boolean hasFocus = getAccessibilityFocusedRect(bounds);
            if (!hasFocus) {
                bounds.setEmpty();
            }
            if (!bounds.equals(drawable.getBounds())) {
                accessibilityFocusDirty = true;
            }
        }

        mAttachInfo.mDrawingTime =
                mChoreographer.getFrameTimeNanos() / TimeUtils.NANOS_PER_MS;

        if (!dirty.isEmpty() || mIsAnimating || accessibilityFocusDirty) {
            if (mAttachInfo.mThreadedRenderer != null && mAttachInfo.mThreadedRenderer.isEnabled()) {
                // If accessibility focus moved, always invalidate the root.
                boolean invalidateRoot = accessibilityFocusDirty || mInvalidateRootRequested;
                mInvalidateRootRequested = false;

                // Draw with hardware renderer.
                mIsAnimating = false;

                if (mHardwareYOffset != yOffset || mHardwareXOffset != xOffset) {
                    mHardwareYOffset = yOffset;
                    mHardwareXOffset = xOffset;
                    invalidateRoot = true;
                }

                if (invalidateRoot) {
                    mAttachInfo.mThreadedRenderer.invalidateRoot();
                }

                dirty.setEmpty();

                // Stage the content drawn size now. It will be transferred to the renderer
                // shortly before the draw commands get send to the renderer.
                final boolean updated = updateContentDrawBounds();

                if (mReportNextDraw) {
                    // report next draw overrides setStopped()
                    // This value is re-sync'd to the value of mStopped
                    // in the handling of mReportNextDraw post-draw.
                    mAttachInfo.mThreadedRenderer.setStopped(false);
                }

                if (updated) {
                    requestDrawWindow();
                }

                mAttachInfo.mThreadedRenderer.draw(mView, mAttachInfo, this);
            } else {
                // If we get here with a disabled & requested hardware renderer, something went
                // wrong (an invalidate posted right before we destroyed the hardware surface
                // for instance) so we should just bail out. Locking the surface with software
                // rendering at this point would lock it forever and prevent hardware renderer
                // from doing its job when it comes back.
                // Before we request a new frame we must however attempt to reinitiliaze the
                // hardware renderer if it's in requested state. This would happen after an
                // eglTerminate() for instance.
                if (mAttachInfo.mThreadedRenderer != null &&
                        !mAttachInfo.mThreadedRenderer.isEnabled() &&
                        mAttachInfo.mThreadedRenderer.isRequested()) {

                    try {
                        mAttachInfo.mThreadedRenderer.initializeIfNeeded(
                                mWidth, mHeight, mAttachInfo, mSurface, surfaceInsets);
                    } catch (OutOfResourcesException e) {
                        handleOutOfResourcesException(e);
                        return;
                    }

                    mFullRedrawNeeded = true;
                    scheduleTraversals();
                    return;
                }

                if (!drawSoftware(surface, mAttachInfo, xOffset, yOffset, scalingRequired, dirty)) {
                    return;
                }
            }
        }

        if (animating) {
            mFullRedrawNeeded = true;
            scheduleTraversals();
        }
    }

其中调用mAttachInfo.mThreadedRenderer.draw(mView, mAttachInfo, this)进入视图绘制。 ThreadedRenderer.java

    /**
     * Draws the specified view.
     *
     * @param view The view to draw.
     * @param attachInfo AttachInfo tied to the specified view.
     * @param callbacks Callbacks invoked when drawing happens.
     */
    void draw(View view, AttachInfo attachInfo, DrawCallbacks callbacks) {
        attachInfo.mIgnoreDirtyState = true;

        final Choreographer choreographer = attachInfo.mViewRootImpl.mChoreographer;
        choreographer.mFrameInfo.markDrawStart();

        updateRootDisplayList(view, callbacks);

        attachInfo.mIgnoreDirtyState = false;

        // register animating rendernodes which started animating prior to renderer
        // creation, which is typical for animators started prior to first draw
        if (attachInfo.mPendingAnimatingRenderNodes != null) {
            final int count = attachInfo.mPendingAnimatingRenderNodes.size();
            for (int i = 0; i < count; i++) {
                registerAnimatingRenderNode(
                        attachInfo.mPendingAnimatingRenderNodes.get(i));
            }
            attachInfo.mPendingAnimatingRenderNodes.clear();
            // We don't need this anymore as subsequent calls to
            // ViewRootImpl#attachRenderNodeAnimator will go directly to us.
            attachInfo.mPendingAnimatingRenderNodes = null;
        }

        final long[] frameInfo = choreographer.mFrameInfo.mFrameInfo;
        int syncResult = nSyncAndDrawFrame(mNativeProxy, frameInfo, frameInfo.length);
        if ((syncResult & SYNC_LOST_SURFACE_REWARD_IF_FOUND) != 0) {
            setEnabled(false);
            attachInfo.mViewRootImpl.mSurface.release();
            // Invalidate since we failed to draw. This should fetch a Surface
            // if it is still needed or do nothing if we are no longer drawing
            attachInfo.mViewRootImpl.invalidate();
        }
        if ((syncResult & SYNC_INVALIDATE_REQUIRED) != 0) {
            attachInfo.mViewRootImpl.invalidate();
        }
    }

    private void updateRootDisplayList(View view, DrawCallbacks callbacks) {
        Trace.traceBegin(Trace.TRACE_TAG_VIEW, "Record View#draw()");
        updateViewTreeDisplayList(view);

        if (mRootNodeNeedsUpdate || !mRootNode.isValid()) {
            DisplayListCanvas canvas = mRootNode.start(mSurfaceWidth, mSurfaceHeight);
            try {
                final int saveCount = canvas.save();
                canvas.translate(mInsetLeft, mInsetTop);
                callbacks.onPreDraw(canvas);

                canvas.insertReorderBarrier();
                canvas.drawRenderNode(view.updateDisplayListIfDirty());
                canvas.insertInorderBarrier();

                callbacks.onPostDraw(canvas);
                canvas.restoreToCount(saveCount);
                mRootNodeNeedsUpdate = false;
            } finally {
                mRootNode.end(canvas);
            }
        }
        Trace.traceEnd(Trace.TRACE_TAG_VIEW);
    }

canvas.drawRenderNode(view.updateDisplayListIfDirty()) View.java

    /**
     * Gets the RenderNode for the view, and updates its DisplayList (if needed and supported)
     * @hide
     */
    @NonNull
    public RenderNode updateDisplayListIfDirty() {
        final RenderNode renderNode = mRenderNode;
        if (!canHaveDisplayList()) {
            // can't populate RenderNode, don't try
            return renderNode;
        }

        if ((mPrivateFlags & PFLAG_DRAWING_CACHE_VALID) == 0
                || !renderNode.isValid()
                || (mRecreateDisplayList)) {
            // Don't need to recreate the display list, just need to tell our
            // children to restore/recreate theirs
            if (renderNode.isValid()
                    && !mRecreateDisplayList) {
                mPrivateFlags |= PFLAG_DRAWN | PFLAG_DRAWING_CACHE_VALID;
                mPrivateFlags &= ~PFLAG_DIRTY_MASK;
                dispatchGetDisplayList();

                return renderNode; // no work needed
            }

            // If we got here, we're recreating it. Mark it as such to ensure that
            // we copy in child display lists into ours in drawChild()
            mRecreateDisplayList = true;

            int width = mRight - mLeft;
            int height = mBottom - mTop;
            int layerType = getLayerType();

            final DisplayListCanvas canvas = renderNode.start(width, height);
            canvas.setHighContrastText(mAttachInfo.mHighContrastText);

            try {
                if (layerType == LAYER_TYPE_SOFTWARE) {
                    buildDrawingCache(true);
                    Bitmap cache = getDrawingCache(true);
                    if (cache != null) {
                        canvas.drawBitmap(cache, 0, 0, mLayerPaint);
                    }
                } else {
                    computeScroll();

                    canvas.translate(-mScrollX, -mScrollY);
                    mPrivateFlags |= PFLAG_DRAWN | PFLAG_DRAWING_CACHE_VALID;
                    mPrivateFlags &= ~PFLAG_DIRTY_MASK;

                    // Fast path for layouts with no backgrounds
                    if ((mPrivateFlags & PFLAG_SKIP_DRAW) == PFLAG_SKIP_DRAW) {
                        dispatchDraw(canvas);
                        drawAutofilledHighlight(canvas);
                        if (mOverlay != null && !mOverlay.isEmpty()) {
                            mOverlay.getOverlayView().draw(canvas);
                        }
                        if (debugDraw()) {
                            debugDrawFocus(canvas);
                        }
                    } else {
                        draw(canvas);
                    }
                }
            } finally {
                renderNode.end(canvas);
                setDisplayListProperties(renderNode);
            }
        } else {
            mPrivateFlags |= PFLAG_DRAWN | PFLAG_DRAWING_CACHE_VALID;
            mPrivateFlags &= ~PFLAG_DIRTY_MASK;
        }
        return renderNode;
    }

    /**
     * Manually render this view (and all of its children) to the given Canvas.
     * The view must have already done a full layout before this function is
     * called.  When implementing a view, implement
     * {@link #onDraw(android.graphics.Canvas)} instead of overriding this method.
     * If you do need to override this method, call the superclass version.
     *
     * @param canvas The Canvas to which the View is rendered.
     */
    @CallSuper
    public void draw(Canvas canvas) {
        final int privateFlags = mPrivateFlags;
        final boolean dirtyOpaque = (privateFlags & PFLAG_DIRTY_MASK) == PFLAG_DIRTY_OPAQUE &&
                (mAttachInfo == null || !mAttachInfo.mIgnoreDirtyState);
        mPrivateFlags = (privateFlags & ~PFLAG_DIRTY_MASK) | PFLAG_DRAWN;

        /*
         * Draw traversal performs several drawing steps which must be executed
         * in the appropriate order:
         *
         *      1. Draw the background
         *      2. If necessary, save the canvas' layers to prepare for fading
         *      3. Draw view's content
         *      4. Draw children
         *      5. If necessary, draw the fading edges and restore layers
         *      6. Draw decorations (scrollbars for instance)
         */

        // Step 1, draw the background, if needed
        int saveCount;

        if (!dirtyOpaque) {
            drawBackground(canvas);
        }

        // skip step 2 & 5 if possible (common case)
        final int viewFlags = mViewFlags;
        boolean horizontalEdges = (viewFlags & FADING_EDGE_HORIZONTAL) != 0;
        boolean verticalEdges = (viewFlags & FADING_EDGE_VERTICAL) != 0;
        if (!verticalEdges && !horizontalEdges) {
            // Step 3, draw the content
            if (!dirtyOpaque) onDraw(canvas);

            // Step 4, draw the children
            dispatchDraw(canvas);

            drawAutofilledHighlight(canvas);

            // Overlay is part of the content and draws beneath Foreground
            if (mOverlay != null && !mOverlay.isEmpty()) {
                mOverlay.getOverlayView().dispatchDraw(canvas);
            }

            // Step 6, draw decorations (foreground, scrollbars)
            onDrawForeground(canvas);

            // Step 7, draw the default focus highlight
            drawDefaultFocusHighlight(canvas);

            if (debugDraw()) {
                debugDrawFocus(canvas);
            }

            // we're done...
            return;
        }

        /*
         * Here we do the full fledged routine...
         * (this is an uncommon case where speed matters less,
         * this is why we repeat some of the tests that have been
         * done above)
         */

        boolean drawTop = false;
        boolean drawBottom = false;
        boolean drawLeft = false;
        boolean drawRight = false;

        float topFadeStrength = 0.0f;
        float bottomFadeStrength = 0.0f;
        float leftFadeStrength = 0.0f;
        float rightFadeStrength = 0.0f;

        // Step 2, save the canvas' layers
        int paddingLeft = mPaddingLeft;

        final boolean offsetRequired = isPaddingOffsetRequired();
        if (offsetRequired) {
            paddingLeft += getLeftPaddingOffset();
        }

        int left = mScrollX + paddingLeft;
        int right = left + mRight - mLeft - mPaddingRight - paddingLeft;
        int top = mScrollY + getFadeTop(offsetRequired);
        int bottom = top + getFadeHeight(offsetRequired);

        if (offsetRequired) {
            right += getRightPaddingOffset();
            bottom += getBottomPaddingOffset();
        }

        final ScrollabilityCache scrollabilityCache = mScrollCache;
        final float fadeHeight = scrollabilityCache.fadingEdgeLength;
        int length = (int) fadeHeight;

        // clip the fade length if top and bottom fades overlap
        // overlapping fades produce odd-looking artifacts
        if (verticalEdges && (top + length > bottom - length)) {
            length = (bottom - top) / 2;
        }

        // also clip horizontal fades if necessary
        if (horizontalEdges && (left + length > right - length)) {
            length = (right - left) / 2;
        }

        if (verticalEdges) {
            topFadeStrength = Math.max(0.0f, Math.min(1.0f, getTopFadingEdgeStrength()));
            drawTop = topFadeStrength * fadeHeight > 1.0f;
            bottomFadeStrength = Math.max(0.0f, Math.min(1.0f, getBottomFadingEdgeStrength()));
            drawBottom = bottomFadeStrength * fadeHeight > 1.0f;
        }

        if (horizontalEdges) {
            leftFadeStrength = Math.max(0.0f, Math.min(1.0f, getLeftFadingEdgeStrength()));
            drawLeft = leftFadeStrength * fadeHeight > 1.0f;
            rightFadeStrength = Math.max(0.0f, Math.min(1.0f, getRightFadingEdgeStrength()));
            drawRight = rightFadeStrength * fadeHeight > 1.0f;
        }

        saveCount = canvas.getSaveCount();

        int solidColor = getSolidColor();
        if (solidColor == 0) {
            final int flags = Canvas.HAS_ALPHA_LAYER_SAVE_FLAG;

            if (drawTop) {
                canvas.saveLayer(left, top, right, top + length, null, flags);
            }

            if (drawBottom) {
                canvas.saveLayer(left, bottom - length, right, bottom, null, flags);
            }

            if (drawLeft) {
                canvas.saveLayer(left, top, left + length, bottom, null, flags);
            }

            if (drawRight) {
                canvas.saveLayer(right - length, top, right, bottom, null, flags);
            }
        } else {
            scrollabilityCache.setFadeColor(solidColor);
        }

        // Step 3, draw the content
        if (!dirtyOpaque) onDraw(canvas);

        // Step 4, draw the children
        dispatchDraw(canvas);

        // Step 5, draw the fade effect and restore layers
        final Paint p = scrollabilityCache.paint;
        final Matrix matrix = scrollabilityCache.matrix;
        final Shader fade = scrollabilityCache.shader;

        if (drawTop) {
            matrix.setScale(1, fadeHeight * topFadeStrength);
            matrix.postTranslate(left, top);
            fade.setLocalMatrix(matrix);
            p.setShader(fade);
            canvas.drawRect(left, top, right, top + length, p);
        }

        if (drawBottom) {
            matrix.setScale(1, fadeHeight * bottomFadeStrength);
            matrix.postRotate(180);
            matrix.postTranslate(left, bottom);
            fade.setLocalMatrix(matrix);
            p.setShader(fade);
            canvas.drawRect(left, bottom - length, right, bottom, p);
        }

        if (drawLeft) {
            matrix.setScale(1, fadeHeight * leftFadeStrength);
            matrix.postRotate(-90);
            matrix.postTranslate(left, top);
            fade.setLocalMatrix(matrix);
            p.setShader(fade);
            canvas.drawRect(left, top, left + length, bottom, p);
        }

        if (drawRight) {
            matrix.setScale(1, fadeHeight * rightFadeStrength);
            matrix.postRotate(90);
            matrix.postTranslate(right, top);
            fade.setLocalMatrix(matrix);
            p.setShader(fade);
            canvas.drawRect(right - length, top, right, bottom, p);
        }

        canvas.restoreToCount(saveCount);

        drawAutofilledHighlight(canvas);

        // Overlay is part of the content and draws beneath Foreground
        if (mOverlay != null && !mOverlay.isEmpty()) {
            mOverlay.getOverlayView().dispatchDraw(canvas);
        }

        // Step 6, draw decorations (foreground, scrollbars)
        onDrawForeground(canvas);

        if (debugDraw()) {
            debugDrawFocus(canvas);
        }
    }

参考: