浅析Android事件分发

对于Android的UI来说,可以铺满整个屏幕,但是手机的屏幕小,所以机制的Android设计师们果断了选择了分层的布局,也就是今天为什么会有Android事件分发!

Touch事件分发中只有两个主角:ViewGroup和View。Activity的Touch事件事实上是调用它内部的ViewGroup的Touch事件，可以直接当成ViewGroup处理。

先了解一下这三个高大上的方法:事件分发(dispatchTouchEvent(MotionEvent ev))、事件拦截(onInterceptTouchEvent(MotionEvent ev))、事件响应(onTouchEvent(MotionEvent ev)),全篇的事件分发就围绕这三个方法(ViewGroup的相关事件有三个：onInterceptTouchEvent、dispatchTouchEvent、onTouchEvent。View的相关事件只有两个：dispatchTouchEvent、onTouchEvent)

事件分发：public boolean dispatchTouchEvent(MotionEvent ev)
Touch 事件发生时 Activity 的 dispatchTouchEvent(MotionEvent ev) 方法会以隧道方式（从根元素依次往下传递直到最内层子元素或在中间某一元素中由于某一条件停止传递）将事件传递给最外层 View 的 dispatchTouchEvent(MotionEvent ev) 方法，并由该 View 的 dispatchTouchEvent(MotionEvent ev) 方法对事件进行分发。dispatchTouchEvent 的事件分发逻辑如下：

如果 return true，事件会分发给当前 View 并由 dispatchTouchEvent 方法进行消费，同时事件会停止向下传递；
如果 return false，事件分发分为两种情况：如果当前 View 获取的事件直接来自 Activity，则会将事件返回给Activity 的 onTouchEvent 进行消费；如果当前 View 获取的事件来自外层父控件，则会将事件返回给父 View 的 onTouchEvent 进行消费。
如果返回系统默认的 super.dispatchTouchEvent(ev)，事件会自动的分发给当前 ViewGroup 的 onInterceptTouchEvent方法。

事件拦截：public boolean onInterceptTouchEvent(MotionEvent ev)
在外层 View 的 dispatchTouchEvent(MotionEvent ev) 方法返回系统默认的 super.dispatchTouchEvent(ev) 情况下，事件会自动的分发给当前 View 的 onInterceptTouchEvent 方法。onInterceptTouchEvent 的事件拦截逻辑如下：

如果 onInterceptTouchEvent 返回 true，则表示将事件进行拦截，并将拦截到的事件交由当前 View 的onTouchEvent 进行处理；
如果 onInterceptTouchEvent 返回 false，则表示将事件放行，当前 View上的事件会被传递到子 View上，再由子 View 的 dispatchTouchEvent 来开始这个事件的分发

事件响应：public boolean onTouchEvent(MotionEvent ev)
在 dispatchTouchEvent 返回 super.dispatchTouchEvent(ev) 并且 onInterceptTouchEvent 返回 true 或返回 super.onInterceptTouchEvent(ev) 的情况下 onTouchEvent 会被调用。onTouchEvent 的事件响应逻辑如下：

如果事件传递到当前 View 的 onTouchEvent 方法，而该方法返回了 false，那么这个事件会从当前 View向上传递，并且都是由上层 View 的 onTouchEvent 来接收，如果传递到上面的 onTouchEvent 也返回 false，这个事件就会“消失”，而且接收不到下一次事件。
如果返回了 true 则会接收并消费该事件。
如果返回 super.onTouchEvent(ev) 默认处理事件的逻辑和返回 false 时相同。

这三个高大上的方法基本逻辑先分析到这里，下面从源码看这个方法具体是如何处理的

public boolean dispatchTouchEvent(MotionEvent event) {        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)) {            //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;    }

首先检测事件的安全性,判断监听事件是否为空,是否有touch事件,判断当前控件是否为enable,回调onTouch()事件的返回值,那个down和up时停止滚动等先不管,看核心代码.
下面就开始看第三个if，首先是注册监听事件，那么监听事件又是从哪里赋值呢？

public void setOnClickListener(OnClickListener l) {        if (!isClickable()) {            setClickable(true);        }        getListenerInfo().mOnClickListener = l;    }

ListenerInfo getListenerInfo() {        if (mListenerInfo != null) {            return mListenerInfo;        }        mListenerInfo = new ListenerInfo();        return mListenerInfo;    }

从以上这段代码会看出mListenerInfo一定会被赋值,当然限定于ListenerInfo有的监听事件,监听事件不为空时,监听事件就存在onTouchListener,第三个条件基本都满足,Android控件默认的情况下都是enable,除非你手动设置,第四个条件,如果回调的onTouch()事件返回的为true,dispatchTouchEvent(event)直接返回true,否则继续走onTouchEvent(event)
从上面可以看出onTouchEvent(event)就是dispatchTouchEvent(event)的小棋子,小棋子被执行的话,返回值就是dispatchTouchEvent(event)的返回值,也就验证了之前说的如果dispatchTouchEvent(event)返回true就交给onTouchEvent(event)去执行

public boolean onTouchEvent(MotionEvent event) {        final float x = event.getX();        final float y = event.getY();        final int viewFlags = mViewFlags;        if ((viewFlags & ENABLED_MASK) == DISABLED) {            if (event.getAction() == MotionEvent.ACTION_UP && (mPrivateFlags & PFLAG_PRESSED) != 0) {                setPressed(false);            }            // A disabled view that is clickable still consumes the touch            // events, it just doesn't respond to them.            return (((viewFlags & CLICKABLE) == CLICKABLE ||                    (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE));        }        if (mTouchDelegate != null) {            if (mTouchDelegate.onTouchEvent(event)) {                return true;            }        }        if (((viewFlags & CLICKABLE) == CLICKABLE ||                (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE)) {            switch (event.getAction()) {                case MotionEvent.ACTION_UP:                    boolean prepressed = (mPrivateFlags & PFLAG_PREPRESSED) != 0;                    if ((mPrivateFlags & PFLAG_PRESSED) != 0 || prepressed) {                        // take focus if we don't have it already and we should in                        // touch mode.                        boolean focusTaken = false;                        if (isFocusable() && isFocusableInTouchMode() && !isFocused()) {                            focusTaken = requestFocus();                        }                        if (prepressed) {                            // The button is being released before we actually                            // showed it as pressed. Make it show the pressed                            // state now (before scheduling the click) to ensure                            // the user sees it.                            setPressed(true, x, y);                       }                        if (!mHasPerformedLongPress) {                            // This is a tap, so remove the longpress check                            removeLongPressCallback();                            // Only perform take click actions if we were in the pressed state                            if (!focusTaken) {                                // Use a Runnable and post this rather than calling                                // performClick directly. This lets other visual state                                // of the view update before click actions start.                                if (mPerformClick == null) {                                    mPerformClick = new PerformClick();                                }                                if (!post(mPerformClick)) {                                    performClick();                                }                            }                        }                        if (mUnsetPressedState == null) {                            mUnsetPressedState = new UnsetPressedState();                        }                        if (prepressed) {                            postDelayed(mUnsetPressedState,                                    ViewConfiguration.getPressedStateDuration());                        } else if (!post(mUnsetPressedState)) {                            // If the post failed, unpress right now                            mUnsetPressedState.run();                        }                        removeTapCallback();                    }                    break;                case MotionEvent.ACTION_DOWN:                    mHasPerformedLongPress = false;                    if (performButtonActionOnTouchDown(event)) {                        break;                    }                    // Walk up the hierarchy to determine if we're inside a scrolling container.                    boolean isInScrollingContainer = isInScrollingContainer();                    // For views inside a scrolling container, delay the pressed feedback for                    // a short period in case this is a scroll.                    if (isInScrollingContainer) {                        mPrivateFlags |= PFLAG_PREPRESSED;                        if (mPendingCheckForTap == null) {                            mPendingCheckForTap = new CheckForTap();                        }                        mPendingCheckForTap.x = event.getX();                        mPendingCheckForTap.y = event.getY();                        postDelayed(mPendingCheckForTap, ViewConfiguration.getTapTimeout());                    } else {                        // Not inside a scrolling container, so show the feedback right away                        setPressed(true, x, y);                        checkForLongClick(0);                    }                    break;                case MotionEvent.ACTION_CANCEL:                    setPressed(false);                    removeTapCallback();                    removeLongPressCallback();                    break;                case MotionEvent.ACTION_MOVE:                    drawableHotspotChanged(x, y);                    // Be lenient about moving outside of buttons                    if (!pointInView(x, y, mTouchSlop)) {                        // Outside button                        removeTapCallback();                        if ((mPrivateFlags & PFLAG_PRESSED) != 0) {                            // Remove any future long press/tap checks                            removeLongPressCallback();                            setPressed(false);                        }                    }                    break;            }            return true;        }        return false;    }

好长的代码,还是看核心代码,前面介绍了Android的控件默认都是enable的,所以第一个if跳过,直接看第三个if,控件是否可点击(或者是是否有长按事件)进入switch中,下面就开始Down,Move,Up
先来看ACTION_DOWN,首先判断当前点击down事件的操作,如果点击到menu时,显示菜单,返回true,程序直接跳出

protected boolean performButtonActionOnTouchDown(MotionEvent event) {        if ((event.getButtonState() & MotionEvent.BUTTON_SECONDARY) != 0) {            if (showContextMenu(event.getX(), event.getY(), event.getMetaState())) {                return true;            }        }        return false;    }

再来看一下ACTION_UP,按不按的那一堆直接跳过,直接看核心代码 performClick(),这个是干嘛的,进去看看

public boolean performClick() {        final boolean result;        final ListenerInfo li = mListenerInfo;        if (li != null && li.mOnClickListener != null) {            playSoundEffect(SoundEffectConstants.CLICK);            li.mOnClickListener.onClick(this);            result = true;        } else {            result = false;        }        sendAccessibilityEvent(AccessibilityEvent.TYPE_VIEW_CLICKED);        return result;    }

这里看到了熟悉的代码onClick(),只要li和li.mOnClickListener不为空,onClick()就会被执行,上面说过li不会为空,li.mOnClickListener赋值如下:

public void setOnClickListener(OnClickListener l) {        if (!isClickable()) {            setClickable(true);        }        getListenerInfo().mOnClickListener = l;    }

只要注册了点击事件li.mOnClickListener也不会为空,所以只要你注册点击事件onClick()就会被执行,当然前提是执行Up事件哦!
看完这段代码是不是觉得掉坑里了,Down,Move,Up,Cancel事件如何执行都会返回true,除非这个控件不能点击,为什么会返回true呢,为了能够之后后面的action,如果程序执行完Down返回false的话后面的Move.Up都不会执行
总结一下view的事件分发结果,首先当你touch到某个点或者区域的时候,先去找这个控件的dispatchTouchEvent(event),该控件没有的话继续找父类,直到找到为止,然后开始事件分发,如果该控件重写了onTouch()事件,返回true的话,dispatchTouchEvent(event)直接返回true,返回false的话,继续执行onTouchEvent(event),如果该控件没有点击事件,像ImageView,TextView等直接返回false,如果存在点击事件,像Button,ImageButton等去执行Down,Move,Up等,返回true,dispatchTouchEvent(event)对应的返回true or false

以上就是Android中view的事件分发的内容了,下面我们继续研究viewgroup的事件分发,viewgroup继承自view,所以在事件分发上也有一定的相似性,只是viewgroup更复杂一些

先来看viewgroup的dispatchTouchEvent(event)

@Override    public boolean dispatchTouchEvent(MotionEvent ev) {        if (mInputEventConsistencyVerifier != null) {            mInputEventConsistencyVerifier.onTouchEvent(ev, 1);        }        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;            }            // 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 (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 = buildOrderedChildList();                        final boolean customOrder = preorderedList == null                                && isChildrenDrawingOrderEnabled();                        final View[] children = mChildren;                        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);                            if (!canViewReceivePointerEvents(child)                                    || !isTransformedTouchPointInView(x, y, child, null)) {                                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;                            }                        }                        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;    }

喔,好长啊,还是同上,只看核心代码,首先判断当前是不是down状态或者是首次touch,然后判断disallowIntercept的值,默认情况下为false,你也可以通过调用下面这个方法改变它的值

public void requestDisallowInterceptTouchEvent(boolean disallowIntercept) {        if (disallowIntercept == ((mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0)) {            // We're already in this state, assume our ancestors are too            return;        }        if (disallowIntercept) {            mGroupFlags |= FLAG_DISALLOW_INTERCEPT;        } else {            mGroupFlags &= ~FLAG_DISALLOW_INTERCEPT;        }        // Pass it up to our parent        if (mParent != null) {            mParent.requestDisallowInterceptTouchEvent(disallowIntercept);        }    }

继续上面说的,怎么突然间出现了onInterceptTouchEvent(),这个方法就是传说中的事件拦截了

public boolean onInterceptTouchEvent(MotionEvent ev) {        return false;    }

原来这个高大上的拦截事件就两行,直接给你return false,也就说默认情况下是不拦截的,当然There are no touch targets and this action is not an initial down的情况下也是拦截的.如果当前事件被拦截,事件就不再向下分发,如果当前事件没有被拦截并且没有取消，继续走view的事件分发

最后总结一下：点击到某个点，先传递到viewgroup的
dispatchTouchEvent(event)，然后传递到onInterceptTouchEvent(event)，判断当前是否拦截，不拦截就继续走view的拦截方式，拦截就谁拦截谁处理。

最后送上一张图，总结view和viewGroup的分发流程

了解了Android的事件分发,你就可以干你想干的事了,你可以随意的改变他的分发顺序了,理论上你可以直接让它按照你设定的分发顺序分发,最好别这样干,按照系统的分发顺序达成你想做的事!

                                         杏树林研发  郭莉莉

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