編輯：關於Android編程
public boolean dispatchTouchEvent(MotionEvent ev) {
if (ev.getAction() == MotionEvent.ACTION_DOWN) {
onUserInteraction();
}
if (getWindow().superDispatchTouchEvent(ev)) {
return true;
}
return onTouchEvent(ev);
}
當觸屏、按下Home鍵、back鍵、menu鍵等都會觸發onUserInteraction()，可以重寫這個方法處理一些用戶交互。可以看到Activity將事件交給Window來處理。如果Window不能消費事件Activity調用onTouchEvent()自行處理事件。PhoneWindow是Window的唯一實現類，接下來分析PhoneWindow分發事件過程。
public boolean superDispatchTouchEvent(MotionEvent event) {
return mDecor.superDispatchTouchEvent(event);
}
mDecor是DecorView類的對象，DecorView繼承FrameLayout，PhoneWindow只是把事件交由頂級DecorView處理。由於DecorView繼承FrameLayout，FrameLayout繼承ViewGroup，所以，之後的事件分發過程與ViewGroup事件分發過程一樣。接下來Part2會介紹這部分。
事件首先傳遞給Activity，Activity將事件傳遞給Window（PhoneWindow是其實現類），Window把事件交給頂級DecorView處理，如果Window沒有消費這個事件則Activity調用oTouchEvent()自行處理事件。
事件傳遞順序：
只針對主要流程以及相應代碼進行分析，不會貼出完整代碼。ViewGroup的事件分發過程主要在dispatchTouchEvent()方法中。
首先，對MotionEvent做個簡單介紹。事件序列開始於ACTION_DOWN，終於ACTION_UP。對於單指操作有ACTION_DOWN、ACTION_MOVE、ACTION_UP等事件序列組成。多指操作由ACTION_DOWN、ACTION_POINTER_DOWN、ACTION_MOVE、ACTION_POINTER_UP、ACTION_UP事件序列組成。pointer可以理解為觸摸點。多指對應的pointerId不變，pointerIndex在事件序列中是變化的。
更多參考：http://www.jianshu.com/p/0c863bbde8eb
事件序列的起始動作是ACTION_DOWN，在新事件序列到達時要做一些狀態清除操作。
// 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.
/**由於一些特殊原因丟失ACTION_UP或者ACTION_CANCEL，
*導致事件序列結束時mFirstTouchTarget(TouchTarget鏈表)未被清空，
* 新事件序列到達時，要先清空mFirstTouchTarget
*/
cancelAndClearTouchTargets(ev);
//主要設置 mGroupFlags &= ~FLAG_DISALLOW_INTERCEPT
resetTouchState();
}
看看cancelAndClearTouchTargets()和resetTouchState()的具體實現。
/**
* 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()後面再分析，先來看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;
}
}
顯而易見，clearTouchTargets()對mFirstTouchTarget指向的鏈表進行了清空操作。
接下來看看resetTouchState()方法的實現：
private void resetTouchState() {
clearTouchTargets();
resetCancelNextUpFlag(this);
mGroupFlags &= ~FLAG_DISALLOW_INTERCEPT;
mNestedScrollAxes = SCROLL_AXIS_NONE;
}
同樣對mFirstTouchTarget指向的鏈表進行了清空，更重要的是設置了~FLAG_DISALLOW_INTERCEPT標志位。引出一個結論，子View在ACTION_DOWN時調用ViewGroup的requestDisallowInterceptTouchEvent()方法是無效的。
接下來ViewGroup檢測是否要攔截事件：
/* Check for interception. */
final boolean intercepted;
/* ACTION_DOWN或者mFirstTouchTarget!=null時檢測是否要攔截事件 */
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;
}
通過上面代碼，可以看出，ViewGroup在ACTON_DOWN或mFirstTouchTarget!=null條件時都會檢測是否需要攔截事件；在mFirstTouchTarget!=null的情況下，可以通過設置FLAG_DISALLOW_INTERCEPT或~FLAG_DISALLOW_INTERCEPT標記位來決定ViewGroup是否允許攔截ACTION_DOWN之後的事件，在允許攔截的情況下是否攔截還取決於onInterceptTouchEvent()的返回值。對於滑動沖突，方案一：使用onInterceptTouchEvent()攔截事件；方案二：使用onInterceptTouchEvent()和requestDisallowInterceptTouchEvent()一起攔截事件。個人覺得方案一更為簡單實用。
接下來ViewGroup會遍歷Children，尋找能消費事件的Child。實現代碼如下：
/*
* Check for cancelation.
* PFLAG_CANCEL_NEXT_UP_EVENT標記位文檔解釋是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類型為ACTION_CANCEL或者未被攔截 */
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 */
/* 這裡處理多觸控情況，一個View如果有多指觸摸，用32位的int記錄不同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不可見並且無動畫直接跳過，或者Point不在child范圍內 */
if ( !canViewReceivePointerEvents( child )
|| !isTransformedTouchPointInView( x, y, child, null ) )
{
ev.setTargetAccessibilityFocus( false );
continue;
}
/* mFirstTouchTarget鏈表已經存在消費該事件的Child，用於多點觸控 */
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能消費事件，Child加入到mFirstTouchTarget鏈表 */
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時ViewGroup遍歷Children，尋找能夠消費事件的Child。Child不在TouchTarget鏈表中，addTouchTarget(child, idBitsToAssign)；Child已經存在TouchTarget鏈表中，多指觸摸同一View情況，newTouchTarget.pointerIdBits |= idBitsToAssign。ViewGroup對於多指觸控不同View的解決方案是使用鏈表，View對於多指觸控的方案是使用32位int來記錄每個Pointer。
找到能夠消費事件序列的Child後，ACTION_DOWN或ACTION_POINTER_DOWN之後的事件，在ViewGroup不攔截的情況下，直接交由Child處理；一旦被攔截，在dispatchTransformedTouchEventChild()方法中eventAction會置為ACTION_CANCEL，並且Child會從TouchTarget鏈表中清除，因此接收不到後續事件序列，都將交給ViewGroup處理。實現代碼如下：
/*
* Dispatch to touch targets.
* Child不能消費事件序列，交由ViewGroup處理
*/
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，因為在尋找過程中已經處理過 */
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攔截ACTION_DOWN或ACTION_POINTER_DOWN 之後的事件，清除相應TouchTarget */
if ( cancelChild )
{
if ( predecessor == null )
{
mFirstTouchTarget = next;
} else {
predecessor.next = next;
}
target.recycle();
target = next;
continue;
}
}
predecessor = target;
target = next;
}
}
ViewGroup通過dispatchTransformedTouchEvent()方法將事件分發給Child。
/* Perform any necessary transformations and dispatch. */
if ( child == null )
{
handled = super.dispatchTouchEvent( transformedEvent );
} else {
/* 將event坐標轉換成Child坐標系內坐標 */
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 );
}
未找到可消費事件的Child，ViewGroup自行處理事件序列；否則，將event坐標轉換成Child坐標系內坐標交由Child處理。
ACTION_UP觸發事件序列結束時清空TouchTarget，ACTION_PONITER_UP觸發時，清空相應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 );
}
ViewGroup事件分發的流程圖整理如下：
ViewGroup的事件分發過程就分析完了，接下來分析View的事件分發過程，相對ViewGroup來說相對簡單。
注意：View只處理了單指觸控的情況，未實現多指觸控，如果有需要可以自己實現。針對View的事件分發只涉及單指情況。View的事件分發過程同樣在dispatchTouchEvent()方法中，主要對這個方法進行分析即可。
View的事件分發過程同樣在dispatchTouchEvent()方法中，主要對這個方法進行分析即可。
final int actionMasked = event.getActionMasked();
if (actionMasked == MotionEvent.ACTION_DOWN) {
// Defensive cleanup for new gesture
stopNestedScroll();
}
上面代碼表示，ACTION_DOWN事件會使View停止滾動(如果View是能夠滾動的，比如ListView)。
接下來View就要開始處理事件了，代碼如下：
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狀態是ENABLE並且調用過setOnTouchListener()方法，事件是否能被OnTouchListener消費取決於onTouch()的返回值。未調用過setOnTouchListener()方法或者OnTouchListener未消費事件，由onTouchEvent()方法來處理事件，事件是否能被消費取決於onTouchEvent()的返回值。接下來看看onTouchEvent()具體實現。
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) );
}
其實，View是CLICKABLE或者LONG_CLICKABLE時返回結果都為true(具體可查看源碼)，也就是View能夠消費事件。上面的情況是View是DISABLED狀態時，會在ACTION_UP或者(mPrivateFlags & PFLAG_PRESSED) != 0設置mPrivateFlags &= ~PFLAG_PRESSED。長按以及點擊事件執行前都會先對這個標記位進行判斷。View處於DISABLED狀態可以消費事件，但是單擊和長按事件不會執行。
if (mTouchDelegate != null) {
if (mTouchDelegate.onTouchEvent(event)) {
return true;
}
}
這段代碼表示可以給View設置一個代理對象(別的View)，使用代理對象的onTouchEvent()來處理事件。比如擴大View的接觸面積、幾個View同步處理事件都可以用到。
對於View的事件處理，主要分析對ACTION_DOWN和ACTION_UP進行分析。先來看對ACTION_DOWN事件的處理：
/*
* 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 );
}
在滾動容器中的操作只是增加了個延時操作，本質還是和不在滾動容器中一樣的。來看看checkForLongClick()方法的實現：
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的消息隊列插入一個待處理的Runable對象，並且設置延時，這也是為什麼需要長按一段時間，長按操作才會執行。長按操作的具體實現都在CheckForLongPress裡了。
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;
}
}
看到重點了，performLongClick()會執行setOnLongClickListener()方法設置的OnLongClickListener的onLongClick()方法。
最後來看看View對ACTION_UP事件的處理：
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為false (可能OnLongClickListener為空或者onLongCkcik()方法返回false)，移除隊列中的CheckForLongPress對象，然後如果OnClickListener不為空執行onClick()方法。
注意：給一個Button設置OnLongClickListener和OnClickListener，onLongClick()方法返回false。這種情況長按和點擊都會執行，驗證方法不能使用System.out.print()來進行輸出驗證，因為System.out是一個有緩存的輸出流，print()並不會立即輸出，使用println()才會立即輸出。
View的狀態是CLICKABLE或者LONG_CLICKABLE都能夠消費事件，如果是DISABLED狀態則不會觸發長按和點擊事件。單擊事件優先級最低，因為最後才會處理單擊事件。
至此，Android事件分發機制分析完畢。
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