彻底搞懂Android事件分发机制

pakolagij 发布于2019-08-19 10:31 / 1740人阅读

摘要：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事件分发机制触摸事件有哪些以及怎么从传递到大家可以上网查下，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到的

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.

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)

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 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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