Android事件分发机制源码分析

Android事件分发机制源码分析
- Part1事件来源以及传递顺序
  - Activity分发事件源码
  - PhoneWindow分发事件源码
  - 小结
- Part2ViewGroup事件分发过程
  - 小结
- Part3View事件分发过程
  - 小结

Part1:事件来源以及传递顺序

Activity分发事件源码

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

当触屏、按下Home键、back键、menu键等都会触发onUserInteraction()，可以重写这个方法处理一些用户交互。可以看到Activity将事件交给Window来处理。如果Window不能消费事件Activity调用onTouchEvent()自行处理事件。PhoneWindow是Window的唯一实现类，接下来分析PhoneWindow分发事件过程。

PhoneWindow分发事件源码

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

mDecor是DecorView类的对象，DecorView继承FrameLayout，PhoneWindow只是把事件交由顶级DecorView处理。由于DecorView继承FrameLayout，FrameLayout继承ViewGroup，所以，之后的事件分发过程与ViewGroup事件分发过程一样。接下来Part2会介绍这部分。

小结

事件首先传递给Activity，Activity将事件传递给Window（PhoneWindow是其实现类），Window把事件交给顶级DecorView处理，如果Window没有消费这个事件则Activity调用oTouchEvent()自行处理事件。
事件传递顺序：

Created with Raphaël 2.1.0 Activity Window DecorView

Part2:ViewGroup事件分发过程

只针对主要流程以及相应代码进行分析，不会贴出完整代码。ViewGroup的事件分发过程主要在dispatchTouchEvent()方法中。

首先，对MotionEvent做个简单介绍。事件序列开始于ACTION_DOWN，终于ACTION_UP。对于单指操作有ACTION_DOWN、ACTION_MOVE、ACTION_UP等事件序列组成。多指操作由ACTION_DOWN、ACTION_POINTER_DOWN、ACTION_MOVE、ACTION_POINTER_UP、ACTION_UP事件序列组成。pointer可以理解为触摸点。多指对应的pointerId不变，pointerIndex在事件序列中是变化的。
更多参考：http://www.jianshu.com/p/0c863bbde8eb

事件序列的起始动作是ACTION_DOWN，在新事件序列到达时要做一些状态清除操作。

// 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.    /**由于一些特殊原因丢失ACTION_UP或者ACTION_CANCEL，    *导致事件序列结束时mFirstTouchTarget(TouchTarget链表)未被清空，    * 新事件序列到达时，要先清空mFirstTouchTarget    */    cancelAndClearTouchTargets(ev);    //主要设置 mGroupFlags &= ~FLAG_DISALLOW_INTERCEPT    resetTouchState();}

看看cancelAndClearTouchTargets()和resetTouchState()的具体实现。

/*** Cancels and clears all touch targets.*/private void cancelAndClearTouchTargets( MotionEvent event ){    if ( mFirstTouchTarget != null )    {        boolean syntheticEvent = false;        if ( event == null )        {            final long now = SystemClock.uptimeMillis();            event = MotionEvent.obtain( now, now,                            MotionEvent.ACTION_CANCEL, 0.0f, 0.0f, 0 );            event.setSource( InputDevice.SOURCE_TOUCHSCREEN );            syntheticEvent = true;        }        for ( TouchTarget target = mFirstTouchTarget; target != null; target = target.next )        {            resetCancelNextUpFlag( target.child );            dispatchTransformedTouchEvent( event, true, target.child, target.pointerIdBits );        }        clearTouchTargets();        if ( syntheticEvent )        {            event.recycle();        }    }}

dispatchTransformedTouchEvent()后面再分析，先来看clearTouchTargets()方法。

/** * Clears all touch targets. */private void clearTouchTargets(){    TouchTarget target = mFirstTouchTarget;    if ( target != null )    {        do        {            TouchTarget next = target.next;            target.recycle();            target = next;        }        while ( target != null );        mFirstTouchTarget = null;    }}

显而易见，clearTouchTargets()对mFirstTouchTarget指向的链表进行了清空操作。
接下来看看resetTouchState()方法的实现：

private void resetTouchState() {        clearTouchTargets();        resetCancelNextUpFlag(this);        mGroupFlags &= ~FLAG_DISALLOW_INTERCEPT;        mNestedScrollAxes = SCROLL_AXIS_NONE;    }

同样对mFirstTouchTarget指向的链表进行了清空，更重要的是设置了~FLAG_DISALLOW_INTERCEPT标志位。引出一个结论，子View在ACTION_DOWN时调用ViewGroup的requestDisallowInterceptTouchEvent()方法是无效的。

接下来ViewGroup检测是否要拦截事件：

/* Check for interception. */final boolean intercepted;/* ACTION_DOWN或者mFirstTouchTarget!=null时检测是否要拦截事件 */if ( actionMasked == MotionEvent.ACTION_DOWN     || mFirstTouchTarget != null ){    final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;    if ( !disallowIntercept )    {        intercepted = onInterceptTouchEvent( ev );        /* restore action in case it was changed */        ev.setAction( action );      } 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;}

通过上面代码，可以看出，ViewGroup在ACTON_DOWN或mFirstTouchTarget!=null条件时都会检测是否需要拦截事件；在mFirstTouchTarget!=null的情况下，可以通过设置FLAG_DISALLOW_INTERCEPT或~FLAG_DISALLOW_INTERCEPT标记位来决定ViewGroup是否允许拦截ACTION_DOWN之后的事件，在允许拦截的情况下是否拦截还取决于onInterceptTouchEvent()的返回值。对于滑动冲突，方案一：使用onInterceptTouchEvent()拦截事件；方案二：使用onInterceptTouchEvent()和requestDisallowInterceptTouchEvent()一起拦截事件。个人觉得方案一更为简单实用。

接下来ViewGroup会遍历Children，寻找能消费事件的Child。实现代码如下：

/* * Check for cancelation. * PFLAG_CANCEL_NEXT_UP_EVENT标记位文档解释是Indicates whether the view is temporarily detached */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;/* View detached或者event类型为ACTION_CANCEL或者未被拦截 */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 */        /* 这里处理多触控情况，一个View如果有多指触摸，用32位的int记录不同Pointer */        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  = 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 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;                }                /* Child不可见并且无动画直接跳过，或者Point不在child范围内 */                if ( !canViewReceivePointerEvents( child )                     || !isTransformedTouchPointInView( x, y, child, null ) )                {                    ev.setTargetAccessibilityFocus( false );                    continue;                }                /* mFirstTouchTarget链表已经存在消费该事件的Child，用于多点触控 */                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 );                /* 如果Child能消费事件，Child加入到mFirstTouchTarget链表 */                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;                }                /*                 * 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;        }    }}

在ACTION_DOW或者ACTION_POINTER_DOWN时ViewGroup遍历Children，寻找能够消费事件的Child。Child不在TouchTarget链表中，addTouchTarget(child, idBitsToAssign)；Child已经存在TouchTarget链表中，多指触摸同一View情况，newTouchTarget.pointerIdBits |= idBitsToAssign。ViewGroup对于多指触控不同View的解决方案是使用链表，View对于多指触控的方案是使用32位int来记录每个Pointer。

找到能够消费事件序列的Child后，ACTION_DOWN或ACTION_POINTER_DOWN之后的事件，在ViewGroup不拦截的情况下，直接交由Child处理；一旦被拦截，在dispatchTransformedTouchEventChild()方法中eventAction会置为ACTION_CANCEL，并且Child会从TouchTarget链表中清除，因此接收不到后续事件序列，都将交给ViewGroup处理。实现代码如下：

/* * Dispatch to touch targets. * Child不能消费事件序列，交由ViewGroup处理 */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;        /* 除去ACTION_DOWN或ACTION_POINTER_DOWN事件，因为在寻找过程中已经处理过 */        if ( alreadyDispatchedToNewTouchTarget && target == newTouchTarget )        {            handled = true;        } else {            final boolean cancelChild = resetCancelNextUpFlag( target.child )                            || intercepted;            if ( dispatchTransformedTouchEvent( ev, cancelChild,                                target.child, target.pointerIdBits ) )            {                handled = true;            }            /* PFLAG_CANCEL_NEXT_UP_EVENT重置或者ViewGroup拦截ACTION_DOWN或ACTION_POINTER_DOWN  之后的事件，清除相应TouchTarget */            if ( cancelChild )            {                if ( predecessor == null )                {                    mFirstTouchTarget = next;                } else {                    predecessor.next = next;                }                target.recycle();                target = next;                continue;            }        }        predecessor = target;        target      = next;    }}

ViewGroup通过dispatchTransformedTouchEvent()方法将事件分发给Child。

/* Perform any necessary transformations and dispatch. */if ( child == null ){    handled = super.dispatchTouchEvent( transformedEvent );} else {    /* 将event坐标转换成Child坐标系内坐标 */    final float offsetX = mScrollX - child.mLeft;    final float offsetY = mScrollY - child.mTop;    transformedEvent.offsetLocation( offsetX, offsetY );    if ( !child.hasIdentityMatrix() )    {        transformedEvent.transform( child.getInverseMatrix() );    }    handled = child.dispatchTouchEvent( transformedEvent );}

未找到可消费事件的Child，ViewGroup自行处理事件序列；否则，将event坐标转换成Child坐标系内坐标交由Child处理。

ACTION_UP触发事件序列结束时清空TouchTarget，ACTION_PONITER_UP触发时，清空相应pointer的target。

/* 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 );}

小结

ViewGroup事件分发的流程图整理如下：

Created with Raphaël 2.1.0 开始 ACTION_DOWN 清空mFirstTouchTarget链表，设置~FLAG_DISALLOW_INTERCEPT标记 ACTION_DOWN或者mFirstTouchTarget!=null disallowIntercept intercepted=false ACTION_DOWN或者ACTION_POINTER_DOWN 寻找能消费事件的Child mFirstTouchTarget==null ViewGroup处理事件 ACTION_UP或者ACTION_POINTER_UP 作相应清除操作结束事件未被分发到Child Child处理 intercepted 从TouchTarget链表清除Child对应Target onInterceptTouchEvent() intercepted=true yes no yes no yes no yes no yes no yes no yes no yes no yes no

ViewGroup的事件分发过程就分析完了，接下来分析View的事件分发过程，相对ViewGroup来说相对简单。

Part3:View事件分发过程

注意：View只处理了单指触控的情况，未实现多指触控，如果有需要可以自己实现。针对View的事件分发只涉及单指情况。View的事件分发过程同样在dispatchTouchEvent()方法中，主要对这个方法进行分析即可。

View的事件分发过程同样在dispatchTouchEvent()方法中，主要对这个方法进行分析即可。

final int actionMasked = event.getActionMasked();    if (actionMasked == MotionEvent.ACTION_DOWN) {        // Defensive cleanup for new gesture        stopNestedScroll();}

上面代码表示，ACTION_DOWN事件会使View停止滚动(如果View是能够滚动的，比如ListView)。
接下来View就要开始处理事件了，代码如下：

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;    }}

View状态是ENABLE并且调用过setOnTouchListener()方法，事件是否能被OnTouchListener消费取决于onTouch()的返回值。未调用过setOnTouchListener()方法或者OnTouchListener未消费事件，由onTouchEvent()方法来处理事件，事件是否能被消费取决于onTouchEvent()的返回值。接下来看看onTouchEvent()具体实现。

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) );}

其实，View是CLICKABLE或者LONG_CLICKABLE时返回结果都为true(具体可查看源码)，也就是View能够消费事件。上面的情况是View是DISABLED状态时，会在ACTION_UP或者(mPrivateFlags & PFLAG_PRESSED) != 0设置mPrivateFlags &= ~PFLAG_PRESSED。长按以及点击事件执行前都会先对这个标记位进行判断。View处于DISABLED状态可以消费事件，但是单击和长按事件不会执行。

if (mTouchDelegate != null) {    if (mTouchDelegate.onTouchEvent(event)) {        return true;    }}

这段代码表示可以给View设置一个代理对象(别的View)，使用代理对象的onTouchEvent()来处理事件。比如扩大View的接触面积、几个View同步处理事件都可以用到。

对于View的事件处理，主要分析对ACTION_DOWN和ACTION_UP进行分析。先来看对ACTION_DOWN事件的处理：

/* * 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 );}

在滚动容器中的操作只是增加了个延时操作，本质还是和不在滚动容器中一样的。来看看checkForLongClick()方法的实现：

private void checkForLongClick( int delayOffset ){    if ( (mViewFlags & LONG_CLICKABLE) == LONG_CLICKABLE )    {        mHasPerformedLongPress = false;        if ( mPendingCheckForLongPress == null )        {            mPendingCheckForLongPress = new CheckForLongPress();        }        mPendingCheckForLongPress.rememberWindowAttachCount();        postDelayed( mPendingCheckForLongPress,                 ViewConfiguration.getLongPressTimeout() - delayOffset );    }}

postDelayed()向Handler的消息队列插入一个待处理的Runable对象，并且设置延时，这也是为什么需要长按一段时间，长按操作才会执行。长按操作的具体实现都在CheckForLongPress里了。

private final class CheckForLongPress implements Runnable {    private int mOriginalWindowAttachCount;    @Override    public void run()    {        if ( isPressed() && (mParent != null)             && mOriginalWindowAttachCount == mWindowAttachCount )        {            if ( performLongClick() )            {                mHasPerformedLongPress = true;            }        }    }    public void rememberWindowAttachCount()    {        mOriginalWindowAttachCount = mWindowAttachCount;    }}

看到重点了，performLongClick()会执行setOnLongClickListener()方法设置的OnLongClickListener的onLongClick()方法。

最后来看看View对ACTION_UP事件的处理：

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();        }    }}

如果mHasPerformedLongPress为false (可能OnLongClickListener为空或者onLongCkcik()方法返回false)，移除队列中的CheckForLongPress对象，然后如果OnClickListener不为空执行onClick()方法。

注意：给一个Button设置OnLongClickListener和OnClickListener，onLongClick()方法返回false。这种情况长按和点击都会执行，验证方法不能使用System.out.print()来进行输出验证，因为System.out是一个有缓存的输出流，print()并不会立即输出，使用println()才会立即输出。

小结

View的状态是CLICKABLE或者LONG_CLICKABLE都能够消费事件，如果是DISABLED状态则不会触发长按和点击事件。单击事件优先级最低，因为最后才会处理单击事件。

至此，Android事件分发机制分析完毕。

Android事件分发机制源码分析

Android事件分发机制源码分析

Part1:事件来源以及传递顺序

Activity分发事件源码

PhoneWindow分发事件源码

小结

Part2:ViewGroup事件分发过程

小结

Part3:View事件分发过程

小结

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