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
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：
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<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 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：
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。