彻底搞懂Android事件分发机制

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网上看到过很多人写的事件分发机制解析,感觉表述都不是很清楚,也可能没有看到写得好的文章,所以自己重新看了一遍源码,来彻底搞清楚Android事件分发机制.

触摸事件有哪些以及怎么从Activity传递到DecorView大家可以上网查下,几个重要方法的基本调用顺序,这些很容易搜到,我们重点关注事件从ViewGroup到View的事件具体的执行过程.

图片描述

1 Android事件分发涉及到的方法主要有

dispatchTouchEvent()事件分发
onInterceptTouchEvent()拦截事件
onTouchEvent()处理事件
requestDisallowInterceptTouchEvent()请求不要拦截事件

2 测试类介绍

BigGroup类型ViewGroup
SmallGroup类型ViewGroup
TestView类型View*(clickable默认false)
图片描述

3 默认情况下的事件分发流程(View.onTouchEvent()返回super.onTouchEvent())

log如下所示:
BigGroup: dispatchTouchEvent() called with: ev = [ACTION_DOWN]
BigGroup: onInterceptTouchEvent() called with: ev = [ACTION_DOWN]
SmallGroup: dispatchTouchEvent() called with: ev = [ACTION_DOWN]
SmallGroup: onInterceptTouchEvent() called with: ev = [ACTION_DOWN]
TestView: dispatchTouchEvent() called with: ev = [ACTION_DOWN]
TestView: onTouchEvent() called with: event = [ACTION_DOWN]
SmallGroup: onTouchEvent() called with: event = [ACTION_DOWN]
BigGroup: onTouchEvent() called with: event = [ACTION_DOWN]

我们看到不管怎么点击滑动,都只会触发DOWN事件的流程,我们来看下这是为什么,首先我们来看下ViewGroup的dispatchTouchEvent()

    ///////////ViewGroup
    @Override
    public boolean dispatchTouchEvent(MotionEvent ev) {
       ...省略部分代码
        boolean handled = false;
        if (onFilterTouchEventForSecurity(ev)) {
            final int action = ev.getAction();
            final int actionMasked = action & MotionEvent.ACTION_MASK;
            //如果是DOWN事件,清除标记,恢复一些标记状态
            if (actionMasked == MotionEvent.ACTION_DOWN) {
                cancelAndClearTouchTargets(ev);
                resetTouchState();
            }

            // Check for interception.
            //判断是否拦截
            final boolean intercepted;
            //触发DOWN事件或者mFirstTouchTarget不是Null,才会走if里面的逻辑
            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.
                //不是DOWN事件 mFirstTouchTarget为空  拦截后续事件
                intercepted = true;
            }

             ...省略部分代码

            // 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;
            //事件没有被取消或者没有被拦截执行if中逻辑
            if (!canceled && !intercepted) {
                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 (childWithAccessibilityFocus != null) {
                                if (childWithAccessibilityFocus != child) {
                                    continue;
                                }
                                childWithAccessibilityFocus = null;
                                i = childrenCount - 1;
                            }
                            //判断child是否可以响应事件 是否点击在了View的范围之内
                            if (!canViewReceivePointerEvents(child)
                                    || !isTransformedTouchPointInView(x, y, child, null)) {
                                ev.setTargetAccessibilityFocus(false);
                                continue;
                            }
                            //获取上次响应事件的View,DOWN事件时候newTouchTarget为Null
                            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);
                            //很重要的一个方法,在下面我们会看这个方法做了什么
                            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();
                                //在这个方法中把child赋值给了mFirstTouchTarget
                                newTouchTarget = addTouchTarget(child, idBitsToAssign);
                                alreadyDispatchedToNewTouchTarget = true;
                                break;
                            }                           
                            ev.setTargetAccessibilityFocus(false);
                        }
                        if (preorderedList != null) preorderedList.clear();
                    }

                   ...
                }
            }

            // 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;
                }
            }

            ...
        }    

        ...
        return handled;
    }
    
    
    
    //最重要的几行代码的逻辑
    private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel,
            View child, int desiredPointerIdBits) {
        final boolean handled;
        ...省略部分代码
        // Perform any necessary transformations and dispatch.
        //child为空调用父View的dispatchTouchEvent
        if (child == null) {
            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());
            }
            //child不为空调用child的dispatchTouchEvent方法
            handled = child.dispatchTouchEvent(transformedEvent);
        }

        // Done.
        transformedEvent.recycle();
        return handled;
    }
    //默认返回false
     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;
    }
    //////////View
     public boolean dispatchTouchEvent(MotionEvent event) {
        ...

        boolean result = false;
        ...

    
        if (onFilterTouchEventForSecurity(event)) {
            if ((mViewFlags & ENABLED_MASK) == ENABLED && handleScrollBarDragging(event)) {
                result = true;
            }
            //noinspection SimplifiableIfStatement
            ListenerInfo li = mListenerInfo;
            //我们看到有mOnTouchListener,并且onTouch返回true,就没有onTouchEvent啥事了
            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;
    }
    
    public boolean onTouchEvent(MotionEvent event) {
        final float x = event.getX();
        final float y = event.getY();
        final int viewFlags = mViewFlags;
        final int action = event.getAction();

        final boolean clickable = ((viewFlags & CLICKABLE) == CLICKABLE
                || (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE)
                || (viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE;

       ...
        if (clickable || (viewFlags & TOOLTIP) == TOOLTIP) {
            switch (action) {
                case MotionEvent.ACTION_UP:
                  ...
                    break;

                case MotionEvent.ACTION_DOWN:
                   ...
                    break;

                case MotionEvent.ACTION_CANCEL:
                   ...
                    break;
            }

            return true;
        }

        return false;
    }

我们梳理下DOWN事件的执行流程:
1 触发BigGroup的dispatchTouchEvent()方法
2 给intercepted赋值,disallowIntercept默认为false,这是就会触发onInterceptTouchEvent()方法,onInterceptTouchEvent默认返回false.intercepted赋值为false
3 接下来走到if (!canceled && !intercepted)的逻辑里面去了,注意注意intercepted为true的话这个方法直接就跳过去了.在这段逻辑里遍历所有的子View,接下来进入淘汰机制,child不能获得焦点,下一个,然后看看View不可见,下一个,没有点到child范围之内,下一个.剩下来的娃会执行dispatchTransformedTouchEvent方法
4 接下来dispatchTransformedTouchEvent中调用了child的dispatchTouchEvent方法
5 执行dispatchTouchEvent的child就是我们的SmallGroup,执行逻辑同2-4
6 这次执行dispatchTouchEvent的child就是我们的TestView了
7 TestView dispatchTouchEvent中首先判断了有没有onTouchListener,判断onTouch方法的返回值.我们肯定没写的了,接下来会执行onTouchEvent
8 在TestView onTouchEvent中我们看到如果这个View可点击就返回true,不可点击就返回false.TestView不可点击,所以返回了false
9 触发了连锁反应,TestView dispatchTouchEvent返回false
10SmallGroup dispatchTransformedTouchEvent返回了false, mFirstTouchTarget为null
11SmallGroup再次执行dispatchTransformedTouchEvent child参数为null,执行super.dispatchTouchEvent(),ViewGroup的super最后就是View,所以会执行逻辑7-9,不同的事主角事SmallGroup
12BigGroup dispatchTransformedTouchEvent返回了false, mFirstTouchTarget为null,重复步骤11,主角BigGroup,DOWN事件结束

mFirstTouchTarget为null会有什么影响呢?后续事件中intercepted为true,执行dispatchTransformedTouchEvent child参数为null,执行super.dispatchTouchEvent(),事件直接传递不下来了,后续事件都接收不到了

4 onInterceptTouchEvent()直接返回true

LOG如下所示:(在SmallGroup onInterceptTouchEvent中return true)
BigGroup: dispatchTouchEvent() called with: ev = [ACTION_DOWN]
BigGroup: onInterceptTouchEvent() called with: ev = [ACTION_DOWN]
SmallGroup: dispatchTouchEvent() called with: ev = [ACTION_DOWN]
SmallGroup: onInterceptTouchEvent() called with: ev = [ACTION_DOWN]
SmallGroup: onTouchEvent() called with: event = [ACTION_DOWN]
BigGroup: onTouchEvent() called with: event = [ACTION_DOWN]
按照我们之前的步骤分析,SmallGroup的onInterceptTouchEvent中返回true,那么事件就不会向下执行,由于mFirstTouchTarget为null,所以后续事件我们都接收不到了.
如果在SmallGroup的onTouchEvent中返回true,就可以收到后续事件.但是SmallGrou的onInterceptTouchEvent只会执行一次.我们来思考下为什么?
我们在SmallGroup的onInterceptTouchEvent中返回true,那么SmallGroup的mFristTouchTarget就是null;因为我们在SmallGroup的onTouchEvent中返回true,所以SmallGroup的dispatchTouchEvent也返回true.那么GigGroup的mFristTouchTarget不是null,所以事件才能延续,由于不满足事件为DOWN,也不满足mFristTouchTarget!=null,所以不会走到拦截方法里面.

5 为什么父View不要拦截DOWN事件?

SamllGroup 的onInterceptTouchEvent()直接返回true,我们在TestView中调用getParent().requestDisallowInterceptTouchEvent(true),会有效果吗?
不会,为什么呢?onInterceptTouchEvent直接返回true拦截的话,getParent().requestDisallowInterceptTouchEvent(true)根本就执行不到,所以一般情况下不要拦截DOWN事件.
getParent().requestDisallowInterceptTouchEvent(true)成功以后会有什么现象呢?
disallowIntercept变量为true,所有的onInterceptTouchEvent方法都不会在执行.

6总结

dispatchTouchEvent()
onInterceptTouchEvent()
onTouchEvent()
requestDisallowInterceptTouchEvent()
搞明白这几个事件的顺序及调用时机,以及调用有哪些限制.我们就明白了Android的事件分发机制.mFirstTouchTarget是一个很重要的变量,大多数情况下,它影响onInterceptTouchEvent的调用情况,进而影响事件是否往下传递.onTouchEvent()的返回值很大程度上决定了dispatchTouchEvent方法的返回值.dispatchTouchEvent返回false,后面的事件就基本没它啥事了.


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