Android View事件处理机制与分发流程

View类还包含一系列嵌套接口以及您可以更加轻松定义的回调。这些接口称为事件侦听器，是您捕获用户与UI之间交互的票证。

尽管您通常会使用事件侦听器来侦听用户交互，但有时您确实需要扩展View类以构建自定义组件。也许，您想扩展 Button类来丰富某些内容的样式。在这种情况下，您将能够使用该类的事件处理程序为类定义默认事件行为。

API

事件侦听器（Event Listeners）

事件侦听器是View类中包含一个回调方法的接口。当用户与UI项目之间的交互触发已注册此视图的侦听器时，Android框架将调用这些方法。

各事件侦听器接口包含的回调方法如下：

onClick()
在 View.OnClickListener 中。当用户触摸项目（处于触摸模式下）时，或者使用导航键或轨迹球聚焦于项目，然后按适用的“Enter”键或按下轨迹球时，将调用此方法。
onLongClick()
在 View.OnLongClickListener 中。当用户触摸并按住项目（处于触摸模式下）时，或者使用导航键或轨迹球聚焦于项目，然后按住适用的“Enter”键或按住轨迹球（持续一秒钟）时，将调用此方法。
onFocusChange()
在 View.OnFocusChangeListener 中。当用户使用导航键或轨迹球导航到或远离项目时，将调用此方法。 onKey() 在 View.OnKeyListener 中。当用户聚焦于项目并按下或释放设备上的硬按键时，将调用此方法。
onTouch()
在 View.OnTouchListener 中。当用户执行可视为触摸事件的操作时，其中包括按下、释放或屏幕上的任何移动手势（在项目边界内），将调用此方法。
onCreateContextMenu()
在 View.OnCreateContextMenuListener 中。当（因持续“长按”而）生成上下文菜单时，将调用此方法。请参见菜单开发者指南中有关上下文菜单的阐述。这些方法是其相应接口的唯一成员。

请注意，上述示例中的 onClick() 回调没有返回值，但是其他某些事件侦听器方法必须返回布尔值。具体原因取决于事件。对于这几个事件侦听器，必须返回布尔值的原因如下：

onLongClick()：此方法返回一个布尔值，表示您是否已处理完事件，以及是否应该将它继续传下去。也就是说，返回 true 表示您已经处理事件且事件应就此停止；如果您尚未处理事件和/或事件应该继续传递给其他任何点击侦听器，则返回 false。
onKey()：此方法返回一个布尔值，表示您是否已处理完事件，以及是否应该将它继续传下去。也就是说，返回 true 表示您已经处理事件且事件应就此停止；如果您尚未处理事件和/或事件应该继续传递给其他任何按键侦听器，则返回 false。
onTouch()：此方法返回一个布尔值，表示侦听器是否处理完此事件。重要的是，此事件可以拥有多个分先后顺序的操作。因此，如果在收到关闭操作事件时返回 false，则表示您并未处理完此事件，而且对其后续操作也不感兴趣。因此，您无需执行事件内的任何其他操作，如手势或最终操作事件。

事件处理程序(Event Handlers)

如果您从视图构建自定义组件，则将能够定义几种用作默认事件处理程序的回调方法。在有关自定义组件的文档中，您将了解某些用于事件处理的常见回调，其中包括：

onKeyDown(int, KeyEvent)：在发生新的按键事件时调用
onKeyUp(int, KeyEvent)：在发生按键弹起事件时调用
onTrackballEvent(MotionEvent)：在发生轨迹球运动事件时调用
onTouchEvent(MotionEvent)：在发生触摸屏运动事件时调用
onFocusChanged(boolean, int, Rect)：在视图获得或失去焦点时调用

还有一些其他方法值得您注意，尽管它们并非 View 类的一部分，但可能会直接影响所能采取的事件处理方式。因此，在管理布局内更复杂的事件时，请考虑使用以下其他方法：

Activity.dispatchTouchEvent(MotionEvent)：此方法允许 Activity 在分派给窗口之前截获所有触摸事件。
ViewGroup.onInterceptTouchEvent(MotionEvent)：此方法允许 ViewGroup 监视分派给子视图的事件。
ViewParent.requestDisallowInterceptTouchEvent(boolean)：对父视图调用此方法表明不应使用 onInterceptTouchEvent(MotionEvent) 截获触摸事件。

事件分发流程

将一个动作(鼠标、手写笔、触摸、轨迹球)事件MotionEvent进行分发过程，目的是将产生的动作事件分发给特定的对象进行消费。分发的层级由上至下从Activity到Window再到View。

这里的事件分发指各个组件的dispatchTouchEvent(MotionEvent ev)方法:

首先事件从Activity开始分发，首先它让Window进行事件分发，如果事件未被消费，则直接由Activity的onTouchEvent()消费。
Window事件分发，Window的实现类为PhoneWindow，PhoneWindow将事件直接传递给顶级DecorView进行分发。
View事件分发
- ViewGroup事件分发
  - 不拦截事件，事件将沿着View层次嵌套结构继续向下分发，直到事件被消费。如果向下分发过程中事件未被消费，则事件将沿着原来的传递路径向上传递，直到事件被消费。
  - 事件被拦截，将由拦截事件的ViewGroup来消费事件，如果未消费将事件向上传递，直到被消费。
  这里需要注意的是，对于每次接收到ACTION_DOWN事件，mFirstTouchTarget会置为空，FLAG_DISALLOW_INTERCEPT标志位被重置，ViewGroup总是调用onInterceptTouchEvent()来判断是否进行拦截。因此如果ACTION_DOWN被拦截，那么后续其它事件都由它自身来处理。如果ACTION_DOWN未被拦截，那么mFirstTouchTarget不为空，对于后续其它事件，子View通过调用ViewParent的requestDisallowInterceptTouchEvent()方法来控制对onInterceptTouchEvent(）的调用。如果后续某个事件被拦截，子View会接收到ACTION_CANCEL事件，该事件后续事件将由拦截该事件的ViewGroup来消费。如果未拦截，则按照正常分发流程处理。
- 常规View事件分发与消费
  如果OnTouchListener不为空并且enabled为true，事件由OnTouchListener消费，否则由onTouchEvent消费。如果OnTouchListener的onTouch方法返回false，事件将也由onTouchEvent消费。

源码分析

ViewGroup.java

    @Override
    public boolean dispatchTouchEvent(MotionEvent ev) {
        if (mInputEventConsistencyVerifier != null) {
            mInputEventConsistencyVerifier.onTouchEvent(ev, 1);
        }

        // If the event targets the accessibility focused view and this is it, start
        // normal event dispatch. Maybe a descendant is what will handle the click.
        if (ev.isTargetAccessibilityFocus() && isAccessibilityFocusedViewOrHost()) {
            ev.setTargetAccessibilityFocus(false);
        }

        boolean handled = false;
        if (onFilterTouchEventForSecurity(ev)) {
            final int action = ev.getAction();
            final int actionMasked = action & MotionEvent.ACTION_MASK;

            // Handle an initial down.
            if (actionMasked == MotionEvent.ACTION_DOWN) {
                // Throw away all previous state when starting a new touch gesture.
                // The framework may have dropped the up or cancel event for the previous gesture
                // due to an app switch, ANR, or some other state change.
                cancelAndClearTouchTargets(ev);
                resetTouchState();
            }

            // Check for interception.
            final boolean intercepted;
            if (actionMasked == MotionEvent.ACTION_DOWN
                    || mFirstTouchTarget != null) {
                final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
                if (!disallowIntercept) {
                    intercepted = onInterceptTouchEvent(ev);
                    ev.setAction(action); // restore action in case it was changed
                } else {
                    intercepted = false;
                }
            } else {
                // There are no touch targets and this action is not an initial down
                // so this view group continues to intercept touches.
                intercepted = true;
            }

            // If intercepted, start normal event dispatch. Also if there is already
            // a view that is handling the gesture, do normal event dispatch.
            if (intercepted || mFirstTouchTarget != null) {
                ev.setTargetAccessibilityFocus(false);
            }

            // Check for cancelation.
            final boolean canceled = resetCancelNextUpFlag(this)
                    || actionMasked == MotionEvent.ACTION_CANCEL;

            // Update list of touch targets for pointer down, if needed.
            final boolean split = (mGroupFlags & FLAG_SPLIT_MOTION_EVENTS) != 0;
            TouchTarget newTouchTarget = null;
            boolean alreadyDispatchedToNewTouchTarget = false;
            if (!canceled && !intercepted) {

                // If the event is targeting accessiiblity focus we give it to the
                // view that has accessibility focus and if it does not handle it
                // we clear the flag and dispatch the event to all children as usual.
                // We are looking up the accessibility focused host to avoid keeping
                // state since these events are very rare.
                View childWithAccessibilityFocus = ev.isTargetAccessibilityFocus()
                        ? findChildWithAccessibilityFocus() : null;

                if (actionMasked == MotionEvent.ACTION_DOWN
                        || (split && actionMasked == MotionEvent.ACTION_POINTER_DOWN)
                        || actionMasked == MotionEvent.ACTION_HOVER_MOVE) {
                    final int actionIndex = ev.getActionIndex(); // always 0 for down
                    final int idBitsToAssign = split ? 1 << ev.getPointerId(actionIndex)
                            : TouchTarget.ALL_POINTER_IDS;

                    // Clean up earlier touch targets for this pointer id in case they
                    // have become out of sync.
                    removePointersFromTouchTargets(idBitsToAssign);

                    final int childrenCount = mChildrenCount;
                    if (newTouchTarget == null && childrenCount != 0) {
                        final float x = ev.getX(actionIndex);
                        final float y = ev.getY(actionIndex);
                        // Find a child that can receive the event.
                        // Scan children from front to back.
                        final ArrayList<View> preorderedList = buildTouchDispatchChildList();
                        final boolean customOrder = preorderedList == null
                                && isChildrenDrawingOrderEnabled();
                        final View[] children = mChildren;
                        for (int i = childrenCount - 1; i >= 0; i--) {
                            final int childIndex = getAndVerifyPreorderedIndex(
                                    childrenCount, i, customOrder);
                            final View child = getAndVerifyPreorderedView(
                                    preorderedList, children, childIndex);

                            // If there is a view that has accessibility focus we want it
                            // to get the event first and if not handled we will perform a
                            // normal dispatch. We may do a double iteration but this is
                            // safer given the timeframe.
                            if (childWithAccessibilityFocus != null) {
                                if (childWithAccessibilityFocus != child) {
                                    continue;
                                }
                                childWithAccessibilityFocus = null;
                                i = childrenCount - 1;
                            }

                            if (!canViewReceivePointerEvents(child)
                                    || !isTransformedTouchPointInView(x, y, child, null)) {
                                ev.setTargetAccessibilityFocus(false);
                                continue;
                            }

                            newTouchTarget = getTouchTarget(child);
                            if (newTouchTarget != null) {
                                // Child is already receiving touch within its bounds.
                                // Give it the new pointer in addition to the ones it is handling.
                                newTouchTarget.pointerIdBits |= idBitsToAssign;
                                break;
                            }

                            resetCancelNextUpFlag(child);
                            if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) {
                                // Child wants to receive touch within its bounds.
                                mLastTouchDownTime = ev.getDownTime();
                                if (preorderedList != null) {
                                    // childIndex points into presorted list, find original index
                                    for (int j = 0; j < childrenCount; j++) {
                                        if (children[childIndex] == mChildren[j]) {
                                            mLastTouchDownIndex = j;
                                            break;
                                        }
                                    }
                                } else {
                                    mLastTouchDownIndex = childIndex;
                                }
                                mLastTouchDownX = ev.getX();
                                mLastTouchDownY = ev.getY();
                                newTouchTarget = addTouchTarget(child, idBitsToAssign);
                                alreadyDispatchedToNewTouchTarget = true;
                                break;
                            }

                            // The accessibility focus didn't handle the event, so clear
                            // the flag and do a normal dispatch to all children.
                            ev.setTargetAccessibilityFocus(false);
                        }
                        if (preorderedList != null) preorderedList.clear();
                    }

                    if (newTouchTarget == null && mFirstTouchTarget != null) {
                        // Did not find a child to receive the event.
                        // Assign the pointer to the least recently added target.
                        newTouchTarget = mFirstTouchTarget;
                        while (newTouchTarget.next != null) {
                            newTouchTarget = newTouchTarget.next;
                        }
                        newTouchTarget.pointerIdBits |= idBitsToAssign;
                    }
                }
            }

            // Dispatch to touch targets.
            if (mFirstTouchTarget == null) {
                // No touch targets so treat this as an ordinary view.
                handled = dispatchTransformedTouchEvent(ev, canceled, null,
                        TouchTarget.ALL_POINTER_IDS);
            } else {
                // Dispatch to touch targets, excluding the new touch target if we already
                // dispatched to it.  Cancel touch targets if necessary.
                TouchTarget predecessor = null;
                TouchTarget target = mFirstTouchTarget;
                while (target != null) {
                    final TouchTarget next = target.next;
                    if (alreadyDispatchedToNewTouchTarget && target == newTouchTarget) {
                        handled = true;
                    } else {
                        final boolean cancelChild = resetCancelNextUpFlag(target.child)
                                || intercepted;
                        if (dispatchTransformedTouchEvent(ev, cancelChild,
                                target.child, target.pointerIdBits)) {
                            handled = true;
                        }
                        if (cancelChild) {
                            if (predecessor == null) {
                                mFirstTouchTarget = next;
                            } else {
                                predecessor.next = next;
                            }
                            target.recycle();
                            target = next;
                            continue;
                        }
                    }
                    predecessor = target;
                    target = next;
                }
            }

            // Update list of touch targets for pointer up or cancel, if needed.
            if (canceled
                    || actionMasked == MotionEvent.ACTION_UP
                    || actionMasked == MotionEvent.ACTION_HOVER_MOVE) {
                resetTouchState();
            } else if (split && actionMasked == MotionEvent.ACTION_POINTER_UP) {
                final int actionIndex = ev.getActionIndex();
                final int idBitsToRemove = 1 << ev.getPointerId(actionIndex);
                removePointersFromTouchTargets(idBitsToRemove);
            }
        }

        if (!handled && mInputEventConsistencyVerifier != null) {
            mInputEventConsistencyVerifier.onUnhandledEvent(ev, 1);
        }
        return handled;
    }

Android View事件处理机制与分发流程

API#

事件侦听器（Event Listeners）#

事件处理程序(Event Handlers)#

事件分发流程#

源码分析#

API

事件侦听器（Event Listeners）

事件处理程序(Event Handlers)

事件分发流程

源码分析