【Android】Android事件分发机制的学习笔记和理解

前言：

Android事件分发机制让我头疼了很久，很多概念容易混淆，在简书上看到一篇绝赞的文章，在阅读view源码的帮助下，终于可以说是大致搞懂了整个的流程，以下做一些笔记和自己的简略概括总结
这里附上原文地址，再次感谢作者一文读懂Android View事件分发机制
首先确定几个面试官可能会问到的问题

事件分发机制的分发过程
onTouchEvent,onTouch方法的区别
如何去理解dispatchTouchEvent 和 onInterceptTouchEvent，onTouchEvent
如何使用requestDisallowInterceptTouchEvent方法，如何调用这个方法可以解决子view和父view之间的冲突
各个方法的返回值都代表了什么，怎么去理解

一事件分发机制的分发过程

引用原文一张流程图，秒懂:

简而言之，就是Activity的dispatchTouchEvent（）–>Window–>DecorView（顶级的ViewGrop）的dispatchTouchEvent（）点击事件传到了 ViewGrop里，之后再由viewgrop传入他自己的子view里

ViewGrop：可以有很多子view ，所以有dispatchTouchEvent， onInterceptTouchEvent，onTouchEvent（除了分发和触摸事件，还有拦截事件）
View：只有dispatchTouchEvent ，onTouchEvent（只有分发和触摸事件）

参照源码可以更加方便的便于理解：

我们模拟一个情景，来方便理解，引用https://blog.csdn.net/qq_30379689/article/details/53967177这篇文章中的情形

<?xml version="1.0" encoding="utf-8"?>                --------------------- 作者：Hensen_ 来源：CSDN 原文：https://blog.csdn.net/qq_30379689/article/details/53967177 版权声明：本文为博主原创文章，转载请附上博文链接！

文章中只是说出了假如我们重写各个方法会出现的情况，并没有说明具体流程，下面我来根据view的源码，分析一下自己的思考过程。
直接分析viewgrop的源码，我把他大致简化一下

 public boolean dispatchTouchEvent(MotionEvent ev) {        //省略        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) {                   // 这里通过requestDisallowInterceptTouchEvent 方法可以设置 FLAG_SPLIT_MOTION_EVENTS 这个标签，来控制是否拦截事件                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 preorderedList = buildTouchDispatchChildList();                        final boolean customOrder = preorderedList == null                                && isChildrenDrawingOrderEnabled();                        final View[] children = mChildren;                        //这里是遍历所有的子view                        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;                            }     //在这个方法里会设置newTouchTarget，使得newTouchTarget 不为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);                            //事件在这个dispatchTransformedTouchEvent方法里进行分发                            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();                                //给mFirstTouchTarget赋值                                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.            //如果mFirstTouchTarget 不为空，即 这个viewgrop有子view，            if (mFirstTouchTarget == null) {                // No touch targets so treat this as an ordinary view.                // 事件在这个dispatchTransformedTouchEvent方法里进行分发,表示事件还未消耗                handled = dispatchTransformedTouchEvent(ev, canceled, null,                        TouchTarget.ALL_POINTER_IDS);            } else {              //省略            }          //省略        return handled;    }

private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel,            View child, int desiredPointerIdBits) {        final boolean handled;        // Canceling motions is a special case.  We don't need to perform any transformations        // or filtering.  The important part is the action, not the contents.        final int oldAction = event.getAction();        if (cancel || oldAction == MotionEvent.ACTION_CANCEL) {            event.setAction(MotionEvent.ACTION_CANCEL);            if (child == null) {                handled = super.dispatchTouchEvent(event);            } else {                handled = child.dispatchTouchEvent(event);            }            event.setAction(oldAction);            return handled;        }        //省略    }

 private TouchTarget addTouchTarget(@NonNull View child, int pointerIdBits) {        final TouchTarget target = TouchTarget.obtain(child, pointerIdBits);        target.next = mFirstTouchTarget;        mFirstTouchTarget = target;        return target;    }

View 的dispatchTouchEvent方法，在ViewGrop类里的dispatchTransformedTouchEvent方法里进行调用，调用方式：super.dispatchTouchEvent(event);

  public boolean dispatchTouchEvent(MotionEvent event) {        // If the event should be handled by accessibility focus first.        if (event.isTargetAccessibilityFocus()) {            // We don't have focus or no virtual descendant has it, do not handle the event.            if (!isAccessibilityFocusedViewOrHost()) {                return false;            }            // We have focus and got the event, then use normal event dispatch.            event.setTargetAccessibilityFocus(false);        }        boolean result = false;        if (mInputEventConsistencyVerifier != null) {            mInputEventConsistencyVerifier.onTouchEvent(event, 0);        }        final int actionMasked = event.getActionMasked();        if (actionMasked == MotionEvent.ACTION_DOWN) {            // Defensive cleanup for new gesture            stopNestedScroll();        }        if (onFilterTouchEventForSecurity(event)) {            if ((mViewFlags & ENABLED_MASK) == ENABLED && handleScrollBarDragging(event)) {                result = true;            }            //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;            }        }        if (!result && mInputEventConsistencyVerifier != null) {            mInputEventConsistencyVerifier.onUnhandledEvent(event, 0);        }        // Clean up after nested scrolls if this is the end of a gesture;        // also cancel it if we tried an ACTION_DOWN but we didn't want the rest        // of the gesture.        if (actionMasked == MotionEvent.ACTION_UP ||                actionMasked == MotionEvent.ACTION_CANCEL ||                (actionMasked == MotionEvent.ACTION_DOWN && !result)) {            stopNestedScroll();        }        return result;    }

1、dispatchTouchEvent（分发事件）

如果在MyViewGroup01的dispatchTouchEvent方法中返回true，表示需要在MyViewGroup01消费了整个事件，即不会再分发，也不会再处理。dispatchTouchEvent方法中返回true的打印信息

//分发过程
MyViewGroup02 dispatchTouchEvent
MyViewGroup02 onInterceptTouchEvent
MyViewGroup01 dispatchTouchEvent

如果在MyViewGroup01的dispatchTouchEvent方法中返回false，表示在MyViewGroup01点击事件在本层不再继续进行分发，并交由上层控件的onTouchEvent方法进行消费。dispatchTouchEvent方法中返回false的打印信息

//分发过程
MyViewGroup02 dispatchTouchEvent
MyViewGroup02 onInterceptTouchEvent
MyViewGroup01 dispatchTouchEvent
//处理过程
MyViewGroup02 onTouchEvent

作者：Hensen_
来源：CSDN
原文：https://blog.csdn.net/qq_30379689/article/details/53967177
版权声明：本文为博主原创文章，转载请附上博文链接！

分析：当MyViewGroup01的dispatchTouchEvent为true的时候，MyViewGroup02的dispatchTouchEvent 和onInterceptTouchEvent按照顺序执行，MyViewGroup01 的dispatchTouchEvent也能执行，但是MyViewGroup01的 dispatchTouchEvent直接返回了true，所以在第一次调用dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)方法的时候（代码115行），就直接返回了true，所以接着又会执行newTouchTarget = addTouchTarget(child, idBitsToAssign)（代码132行）这个方法，这个方法则是给mFirstTouchTarget赋了值，所以当mFirstTouchTarget == null时候才会执行的handled = dispatchTransformedTouchEvent(ev, canceled, null,TouchTarget.ALL_POINTER_IDS)（代码158行）方法不会被执行，而最关键的view的onTouchEvent方法，就在chlid为null的时候会执行，所以当dispatchTransformedTouchEvent(ev, canceled, null,TouchTarget.ALL_POINTER_IDS)（代码161行）不执行的时候，就不会调用onTouchEvent方法，所以当MyViewGroup01的dispatchTouchEvent为true的时候，onTouchEvent不会执行，反之，则会执行

前言：

一事件分发机制的分发过程

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【Android】Android事件分发机制的学习笔记和理解

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一 事件分发机制的分发过程

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