前言

Android的事件分發機制也是老生常談了，這篇文章并不是籠統的介紹這個機制，而是針對ACTION_DOWN這個事件探討相關的細節。

dispatchTouchEvent

說到Android事件分發，一定繞不開dispatchTouchEvent函數，View和ViewGroup的該函數有很大的不同。

我們來看看ViewGroup的dispatchTouchEvent函數，它的部分源碼如下：

@Override public boolean dispatchTouchEvent(MotionEvent ev) {...if (onFilterTouchEventForSecurity(ev)) {...boolean alreadyDispatchedToNewTouchTarget = false;if (!canceled && !intercepted) {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 downfinal int idBitsToAssign = split ? 1 << ev.getPointerId(actionIndex): TouchTarget.ALL_POINTER_IDS;removePointersFromTouchTargets(idBitsToAssign);final int childrenCount = mChildrenCount;if (newTouchTarget == null && childrenCount != 0) {...for (int i = childrenCount - 1; i >= 0; i--) {...if (!child.canReceivePointerEvents()|| !isTransformedTouchPointInView(x, y, child, null)) {ev.setTargetAccessibilityFocus(false);continue;}...if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) {...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();}...}}// 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;}...}predecessor = target;target = next;}}...}if (!handled && mInputEventConsistencyVerifier != null) {mInputEventConsistencyVerifier.onUnhandledEvent(ev, 1);}return handled; }

可以看到整個分發有幾個關鍵因素：intercepted、canceled、mFirstTouchTarget、alreadyDispatchedToNewTouchTarget。

intercepted、canceled比較好理解，重點來說說后面兩個因素的是如何影響整個分發的。

ACTION_DOWN

一個完整的事件應該包含ACTION_DOWN、ACTION_MOVE、ACTION_UP。其中ACTION_DOWN是開始也是關鍵。

從上面dispatchTouchEvent源碼中可以看到首先單獨對ACTION_DOWN事件進行了處理，對所有child進行遍歷，是從后向前遍歷的，所以在處理上面的也就是最后添加的view會先得到事件

for (int i = childrenCount - 1; i >= 0; i--) {

對于每個child，會先判斷事件是不是發生在它的區域內，不是則不處理：

if (!child.canReceivePointerEvents()|| !isTransformedTouchPointInView(x, y, child, null)) {ev.setTargetAccessibilityFocus(false);continue; }

如果在區域內，則繼續執行，下面dispatchTransformedTouchEvent這個函數就是下發事件的，我們來看下部分源碼：

private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel,View child, int desiredPointerIdBits) {...if (newPointerIdBits == oldPointerIdBits) {if (child == null || child.hasIdentityMatrix()) {if (child == null) {handled = super.dispatchTouchEvent(event);} else {final float offsetX = mScrollX - child.mLeft;final float offsetY = mScrollY - child.mTop;event.offsetLocation(offsetX, offsetY);handled = child.dispatchTouchEvent(event);event.offsetLocation(-offsetX, -offsetY);}return handled;}transformedEvent = MotionEvent.obtain(event);} else {transformedEvent = event.split(newPointerIdBits);}// Perform any necessary transformations and dispatch.if (child == null) {handled = super.dispatchTouchEvent(transformedEvent);} else {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);}// Done.transformedEvent.recycle();return handled; }

有不少邏輯在里面，但是仔細觀察可以發現，不論那個條件，執行的代碼都比較類似，如下：

if (child == null) {handled = super.dispatchTouchEvent(event); } else {...handled = child.dispatchTouchEvent(event);... }

當child不為null的時候，執行child的dispatchTouchEvent；為null時執行父類的dispatchTouchEvent，即View的dispatchTouchEvent函數，這個函數里會執行onTouchEvent等。所以在ViewGroup是沒有onTouchEvent等函數的代碼。

由于這時child不為null，所以執行了child的dispatchTouchEvent函數.

回到之前的ACTION_DOWN流程中，根據dispatchTransformedTouchEvent返回值進行不同的處理：

返回ture

如果返回true，即有一個child消費了ACTION_DOWN事件，可以看到后續執行了addTouchTarget函數，同時將alreadyDispatchedToNewTouchTarget置為true。

if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) {...newTouchTarget = addTouchTarget(child, idBitsToAssign);alreadyDispatchedToNewTouchTarget = true;break; }

addTouchTarget函數源碼如下：

private TouchTarget addTouchTarget(@NonNull View child, int pointerIdBits) {final TouchTarget target = TouchTarget.obtain(child, pointerIdBits);target.next = mFirstTouchTarget; //初始mFirstTouchTarget為null，所以這里next是nullmFirstTouchTarget = target;return target; }

關鍵的一點是對mFirstTouchTarget進行了賦值。所以說true的處理是為mFirstTouchTarget賦值，將alreadyDispatchedToNewTouchTarget置為true
最后的break則跳出循環，不再遍歷其他child。

返回false

如果返回false，即沒有任何一個child消費ACTION_DOWN事件，直接跳過if代碼，這樣mFirstTouchTarget為null。

mFirstTouchTarget

那么mFirstTouchTarget、alreadyDispatchedToNewTouchTarget這兩個屬性在分發過程中的作用是什么？我們分別來說：

1、mFirstTouchTarget為null

當mFirstTouchTarget為null，進入if語句執行dispatchTransformedTouchEvent(ev, canceled, null,TouchTarget.ALL_POINTER_IDS)

if (mFirstTouchTarget == null) {// No touch targets so treat this as an ordinary view.handled = dispatchTransformedTouchEvent(ev, canceled, null,TouchTarget.ALL_POINTER_IDS); }

由于child是null，在dispatchTransformedTouchEvent代碼中可以看到不再給任何child分發，而是調用了super.dispatchTouchEvent，即ViewGroup自己處理

這樣ACTION_DOWN事件分發完了。其他事件分發時由于不再走ACTION_DOWN的處理過程，所以mFirstTouchTarget會一直為null，所以其他事件也不再向下分發了，直接ViewGroup自己處理

2、mFirstTouchTarget不為null

當mFirstTouchTarget不為null，進入else語句中，會執行一個while循環

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;}...}predecessor = target;target = next;} }

這時由于alreadyDispatchedToNewTouchTarget為true，所以直接給handled賦值true并不做任何處理。因為之前代碼中child對ACTION_DOWN事件已經響應，所以這里的alreadyDispatchedToNewTouchTarget是為了防止重復分發ACTION_DOWN事件。

這樣ACTION_DOWN事件分發完成后，分發其他事件時，alreadyDispatchedToNewTouchTarget被重新賦值false，由于不再走ACTION_DOWN的處理過程，所以alreadyDispatchedToNewTouchTarget就一直是false了，而mFirstTouchTarget會一直保持不變。在這個while循環中則會執行else語句，通過執行dispatchTransformedTouchEvent將事件直接分發給mFirstTouchTarget對應的child，即之前消費ACTION_DOWN事件的child。

總結

這樣我們得到幾個結論：

總結

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