Android事件分发机制

先放张流程图，本文的所有内容都基于这张图，如下：

讲解Android事件分发的文章很多了，这里我是在理解之后，做的一个总结。按照事件传递的流程，把每一步的事件处理和返回结果做个梳理，这里的总结只包含了ACTION_DOWN事件。

1

先看Activity的

```
    public boolean dispatchTouchEvent(MotionEvent ev) {
        if (ev.getAction() == MotionEvent.ACTION_DOWN) {
            onUserInteraction();
        }
        if (getWindow().superDispatchTouchEvent(ev)) {
            return true;
        }
        return onTouchEvent(ev);
    }

分析上述代码：首先事件交给Activity所附属的Window进行分发，我们进入getWindow().superDispatchTouchEvent(ev)这段代码:

public boolean superDispatchTouchEvent(MotionEvent event) {
    return mDecor.superDispatchTouchEvent(event);
}

分析代码可以看出，PhoneWindow将事件传递给了DecorView，也就是最顶级的View，它继承自FrameLayout，所以它是一个ViewGroup，这样事件的分发，就从Activity进入了ViewGroup。这里我们先只讨论Activity对分发事件结果的处理。

1）如果返回true，则说明该事件被消费，则终止传递。

2）如果返回false，意味着事件没人处理，所有View的onTouchEvent都返回了false，那么Activity的onTouchEvent就会调用。

我们接着往下看，当事件传递给ViewGroup的dispatchTouchEvent()，它是如何处理的。

2

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;
}

这段代码比较多了，其实我也看不下去……，所以这里我直接结合流程图，给出结论吧：

1）ViewGroup的dispatchTouchEvent()方法，如果其OnTouchListener没有被设置，则默认会去调用它的onInterceptTouchEvent()方法。如果设置了OnTouchListener，则会屏蔽掉onTouchEvent事件。（关于OnTouchListener和OnTouchEvent会在文末讲解）

2）如果返回结果为true，说明事件自行消费，例如设置了OnTouchListener，则终止传递。

3）返回false意味着事件没人处理，则回溯给Activity的onTouchEvent进行处理。

3

下面进入ViewGroup的onInterceptTouchEvent()方法：

public boolean onInterceptTouchEvent(MotionEvent ev) {
    if (ev.isFromSource(InputDevice.SOURCE_MOUSE)
            && ev.getAction() == MotionEvent.ACTION_DOWN
            && ev.isButtonPressed(MotionEvent.BUTTON_PRIMARY)
            && isOnScrollbarThumb(ev.getX(), ev.getY())) {
        return true;
    }
    return false;
}

只有ViewGroup中有此方法，View中没有。它的作用时判断“是否拦截事件”。

1）默认情况是返回false，也就是不拦截，并将事件传递给子View的dispatchTouchEvent()。

2）返回true意味着拦截此事件，并将事件分发给自己的onTouchEvent。

4

下面进入ViewGroup的onTouchEvent()方法：

1）返回true，说明消费该事件，传递终止。

2）返回false，意味着不消费该事件，将事件传递给子View的dispatchTouchEvent()进行分发。

5

View没有interceptTouchEvent()方法，因为它下面已经没有View了，不需要进行拦截。

1)与ViewGroup的此方法一样，如果自己不消费，默认情况下是传递给该View的onTouchEvent进行事件处理。

2）返回true，说明被自己消费，终止传递。

3）返回false说明该View不进行处理，回溯给父View的onTouchEvent()方法。

6

如果View能接受到事件，那么它的onTouchEvent()方法将是最后被调用的，它的事件处理规则与ViewGroup的是一样的：

1）返回true，说明自行处理，传递结束。

2）返回false，意味着不处理该事件，回溯给上一级View的onTouchEvent()方法处理。

onTouchEvent、onTouchListener、onClickListener

在区别这三者之前，先上一张View的dispatchTouchEvent()的内部执行逻辑流程图，图片来自谷哥的小弟：

1、onTouchEvent与onTouchListener都是处理Touch事件的API。不同的是，onTouchEvent是Android系统自身对于Touch事件处理的一个实现，onTouchListener是View暴露给用户的接口，方便用户处理Touch事件。

onTouchListener的优先级高于onTouchEvent，如果onTouchListener被调用，且返回true，则onTouchEvent不会被执行，如果返回false，onTouchEvent则会被执行。

2、onClickListener是在ACTIN_UP之后才会被调用，它是这三者中最后一个被调用的。