Android事件分发机制源码解析

在 更简单的学习Android事件分发 中，使用日志、比喻、流程图相结合的方式，以更简单的方法去分析了Android的事件分发机制。本篇文章将采用分析源码的方式，更深入的解析Android的事件分发机制。

一、一切从Activity开始

Android的触摸事件，是由windowManagerService进行采集，之后传递到Activiy进行处理。我们这里从Activity#dispatchTouchEvent方法开始解析

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

上述代码中， onUserInteraction() 是一个空的实现，我们直接来看下

getWindow().superDispatchTouchEvent(ev) 方法。window是一个抽象的方法，不过系统给它提供了一个实现类PhoneWindow，我们这里看下它的 superDispatchTouchEvent(ev) 方法。

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

上述代码调用了DecorView类的 superDispatchTouchEvent 方法，继续跟进

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

上述代码调用了父类的 dispatchTouchEvent 方法，DecorView的父类为FrameLayout，其直接继承了 ViewGroup#dispatchTouchEvent 方法。

二、ViewGroup中的事件分发

ViewGroup#dispatchTouchEvent方法比较长，这里只截取部分进行分析

public boolean dispatchTouchEvent(MotionEvent ev) {

    ...
        // 在ACTION_DOWN事件时，初始化Touch标记
        // 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) {
            // 是否拦截的标志位，假如设置requestDisallowInterceptTouchEvent(true)，
            // 则为true，不拦截事件
            final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
            if (!disallowIntercept) {
                // 默认返回false
                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;
        }

        ...

        // 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;
        // 不是ACTION_CANCEL事件，并且不拦截事件
        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) {
                    // 获取触摸坐标
                    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 = buildOrderedChildList();
                    final boolean customOrder = preorderedList == null
                            && isChildrenDrawingOrderEnabled();
                    final View[] children = mChildren;
                    // 遍历所有子View
                    for (int i = childrenCount - 1; i >= 0; i--) {
                        final int childIndex = customOrder
                                ? getChildDrawingOrder(childrenCount, i) : i;
                        final View child = (preorderedList == null)
                                ? children[childIndex] : preorderedList.get(childIndex);

                        ...

                        resetCancelNextUpFlag(child);
                        // 把事件(ACTION_DOWN、ACTION_POINTER_DOWN、ACTION_HOVER_MOVE)传递给子View处理
                        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;
                        }
                        ...
                    }
                    ...
                }
                ...
                }
            }
        }

        // 分发事件到目标View
        // Dispatch to touch targets.
        if (mFirstTouchTarget == null) {
            // 没有找到事件分发目标的情况，将会调用自己的onTouchEvent方法
            // 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;
            // 这里找到了事件分发的目标
            while (target != null) {
                final TouchTarget next = target.next;
                // ACTION_DOWN已经完成事件分发，并消费了事件，直接返回true
                if (alreadyDispatchedToNewTouchTarget && target == newTouchTarget) {
                    handled = true;
                } else {
                    final boolean cancelChild = resetCancelNextUpFlag(target.child)
                            || intercepted;
                    // 其余事件则需要传递给目标View进行处理
                    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;
            }
        }

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

    ...
    return handled;
}

// 默认返回false
public boolean onInterceptTouchEvent(MotionEvent ev) {
    return false;
}

我们现在来看看传递事件的 dispatchTransformedTouchEvent 方法，同样我也只是截取了其中比较关键的部分

private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel,
        View child, int desiredPointerIdBits) {
    final boolean handled;
    ...
    final MotionEvent transformedEvent;
    // 对transformedEvent的一系列计算
    ...
    if (child == null) {
        // 如果没有子View，则执行super.dispatchTouchEvent方法，
        // 调用自己的onTouchEvent方法
        handled = super.dispatchTouchEvent(transformedEvent);
    } else {
        final float offsetX = mScrollX - child.mLeft;
        final float offsetY = mScrollY - child.mTop;
        transformedEvent.offsetLocation(offsetX, offsetY);
        if (! child.hasIdentityMatrix()) {
            transformedEvent.transform(child.getInverseMatrix());
        }
        // 如果有子View，则调用子View#dispatchTouchEvent方法
        handled = child.dispatchTouchEvent(transformedEvent);
    }

    // Done.
    transformedEvent.recycle();
    return handled;
}

三、View中的事件处理

ViewGroup中不拦截事件，调用子View#dispatchTouchEvent方法进行处理

public boolean dispatchTouchEvent(MotionEvent event) {

    ...
    if (onFilterTouchEventForSecurity(event)) {
        // 如果设置了OnTouchListener，使用onTouch对事件进行处理，
        // 并返回true，则不需要再执行onTouchEvent方法
        //noinspection SimplifiableIfStatement
        ListenerInfo li = mListenerInfo;
        if (li != null && li.mOnTouchListener != null
                && (mViewFlags & ENABLED_MASK) == ENABLED
                && li.mOnTouchListener.onTouch(this, event)) {
            result = true;
        }

        if (!result && onTouchEvent(event)) {
            result = true;
        }
    }
    ...

    return result;
}

这里继续看看View#onTouchEvent方法

public boolean onTouchEvent(MotionEvent event) {
    final float x = event.getX();
    final float y = event.getY();
    final int viewFlags = mViewFlags;
    final int action = event.getAction();

    ...

    if (((viewFlags & CLICKABLE) == CLICKABLE ||
            (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE) ||
            (viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE) {
        switch (action) {
            case MotionEvent.ACTION_UP:
                ...
                // 移除长按
                removeLongPressCallback();
                ...
                    // 检查单击
                    performClick();
                ...
                break;

            case MotionEvent.ACTION_DOWN:
                ...
                // 检测是否为长按
                checkForLongClick(0);
                ...
                break;

            ....
        }

        return true;
    }

    return false;
}

上述代码，主要是检查View是否可以点击，如果可点击，则会返回true，同时也会执行可点击的事件。

四、小结

通过本文的源码解析，我们可以更深入的理解Android的事件分发。可以简单的推出一个流程 : Activity→PhoneWindow→DecorView→ViewGroup→View。如果在阅读过程中，有任何疑问与问题，欢迎与我联系。

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