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Android事件分发之ACTION_CANCEL机制及作用

發(fā)布時(shí)間：2023/12/20 Android 43 豆豆

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ACTION_CANCEL產(chǎn)生場(chǎng)景

在閱讀ViewGroup事件分發(fā)相關(guān)源碼過(guò)程中，有時(shí)候會(huì)見(jiàn)到ACTION_CANCEL這一事件。那么這一事件是如何產(chǎn)生的呢？按照網(wǎng)上的說(shuō)法，當(dāng)手指從當(dāng)前view移出后，當(dāng)前view就會(huì)收到ACTION_CANCEL這一事件，這一定是正確的嗎？下面我們來(lái)看兩個(gè)例子：

例子1:

import android.content.Context import android.support.constraint.ConstraintLayout import android.util.AttributeSet import android.util.Log import android.view.MotionEventclass CustomViewGroup @JvmOverloads constructor(context: Context, attrs: AttributeSet? = null, defStyleAttr: Int = 0 ) : ConstraintLayout(context, attrs, defStyleAttr) {var hasInterceptMoveEvent = falseoverride fun onInterceptTouchEvent(ev: MotionEvent?): Boolean {// Log.d("TAG", "${ev?.action}:CustomViewGroup onInterceptTouchEvent")if (!hasInterceptMoveEvent && ev?.action == MotionEvent.ACTION_MOVE) {hasInterceptMoveEvent = truereturn true}return false}override fun dispatchTouchEvent(ev: MotionEvent?): Boolean {Log.d("TAG", "${getAction(ev?.action)}:CustomViewGroup dispatchTouchEvent")return super.dispatchTouchEvent(ev)}}

我們自定義一個(gè)ViewGroup，覆蓋它的onInterceptTouchEvent方法和dispatchTouchEvent方法。在onInterceptTouchEvent方法中我們只攔截了一次MotionEvent.ACTION_MOVE事件。hasInterceptMoveEvent用于控制只攔截一次。而在dispatchTouchEvent方法中，我們打印出當(dāng)前ViewGroup處理的事件。看下getAction方法定義：

fun getAction(action: Int?): String {return when (action) {MotionEvent.ACTION_DOWN -> "MotionEvent.ACTION_DOWN"MotionEvent.ACTION_MOVE -> "MotionEvent.ACTION_MOVE"MotionEvent.ACTION_UP -> "MotionEvent.ACTION_UP"MotionEvent.ACTION_CANCEL -> "MotionEvent.ACTION_CANCEL"else -> "OTHER"} }

其實(shí)就是根據(jù)Action對(duì)應(yīng)的Int值轉(zhuǎn)化為字符串，讓我們的Log更加直觀。

而在ViewGroup內(nèi)部，我們放置了一個(gè)自定義Button。代碼如下：

class CusButton @JvmOverloads constructor(context: Context, attrs: AttributeSet? = null, defStyleAttr: Int = 0 ) : Button(context, attrs, defStyleAttr) {override fun dispatchTouchEvent(event: MotionEvent?): Boolean {Log.d("TAG", "${getAction(event?.action)}:CusButton dispatchTouchEvent")return true} }

這個(gè)自定義Button只是將當(dāng)前dispatchTouchEvent方法收到的事件打印出來(lái)。

現(xiàn)在我們進(jìn)行如下操作：

手指按住button，然后移動(dòng)，移出button外，然后松開(kāi)。我們看下打印出的Log：

04-23 15:13:48.553 22928-22928/com.lee.myapplication D/TAG: MotionEvent.ACTION_DOWN:CustomViewGroup dispatchTouchEvent
04-23 15:13:48.553 22928-22928/com.lee.myapplication D/TAG: MotionEvent.ACTION_DOWN:CusButton dispatchTouchEvent
04-23 15:13:48.574 22928-22928/com.lee.myapplication D/TAG: MotionEvent.ACTION_MOVE:CustomViewGroup dispatchTouchEvent
04-23 15:13:48.574 22928-22928/com.lee.myapplication D/TAG: MotionEvent.ACTION_CANCEL:CusButton dispatchTouchEvent
04-23 15:13:48.591 22928-22928/com.lee.myapplication D/TAG: MotionEvent.ACTION_MOVE:CustomViewGroup dispatchTouchEvent
04-23 15:13:48.607 22928-22928/com.lee.myapplication D/TAG: MotionEvent.ACTION_MOVE:CustomViewGroup dispatchTouchEvent
04-23 15:13:48.624 22928-22928/com.lee.myapplication D/TAG: MotionEvent.ACTION_MOVE:CustomViewGroup dispatchTouchEvent
04-23 15:13:48.642 22928-22928/com.lee.myapplication D/TAG: MotionEvent.ACTION_MOVE:CustomViewGroup dispatchTouchEvent
04-23 15:13:48.658 22928-22928/com.lee.myapplication D/TAG: MotionEvent.ACTION_MOVE:CustomViewGroup dispatchTouchEvent
04-23 15:13:48.674 22928-22928/com.lee.myapplication D/TAG: MotionEvent.ACTION_MOVE:CustomViewGroup dispatchTouchEvent
04-23 15:13:48.691 22928-22928/com.lee.myapplication D/TAG: MotionEvent.ACTION_MOVE:CustomViewGroup dispatchTouchEvent
04-23 15:13:48.708 22928-22928/com.lee.myapplication D/TAG: MotionEvent.ACTION_MOVE:CustomViewGroup dispatchTouchEvent
04-23 15:13:48.724 22928-22928/com.lee.myapplication D/TAG: MotionEvent.ACTION_MOVE:CustomViewGroup dispatchTouchEvent
04-23 15:13:48.741 22928-22928/com.lee.myapplication D/TAG: MotionEvent.ACTION_MOVE:CustomViewGroup dispatchTouchEvent
04-23 15:13:48.758 22928-22928/com.lee.myapplication D/TAG: MotionEvent.ACTION_MOVE:CustomViewGroup dispatchTouchEvent
04-23 15:13:48.775 22928-22928/com.lee.myapplication D/TAG: MotionEvent.ACTION_MOVE:CustomViewGroup dispatchTouchEvent
04-23 15:13:48.791 22928-22928/com.lee.myapplication D/TAG: MotionEvent.ACTION_MOVE:CustomViewGroup dispatchTouchEvent
04-23 15:13:48.808 22928-22928/com.lee.myapplication D/TAG: MotionEvent.ACTION_MOVE:CustomViewGroup dispatchTouchEvent
04-23 15:13:48.825 22928-22928/com.lee.myapplication D/TAG: MotionEvent.ACTION_MOVE:CustomViewGroup dispatchTouchEvent
04-23 15:13:48.841 22928-22928/com.lee.myapplication D/TAG: MotionEvent.ACTION_MOVE:CustomViewGroup dispatchTouchEvent
04-23 15:13:48.859 22928-22928/com.lee.myapplication D/TAG: MotionEvent.ACTION_MOVE:CustomViewGroup dispatchTouchEvent
04-23 15:13:48.866 22928-22928/com.lee.myapplication D/TAG: MotionEvent.ACTION_MOVE:CustomViewGroup dispatchTouchEvent
04-23 15:13:48.867 22928-22928/com.lee.myapplication D/TAG: MotionEvent.ACTION_UP:CustomViewGroup dispatchTouchEvent

我們看到CusButton確實(shí)收到了一個(gè)ACTION_CANCEL事件，并且在這個(gè)事件之后，CusButton并沒(méi)有收到任何事件。所以我們大概能夠猜到：如果ViewGroup攔截了Move事件，那么這個(gè)Move事件將會(huì)轉(zhuǎn)化為Cancel事件傳遞給子view。

例子2:

如果我們的ViewGroup不攔截Move事件，那么是否也會(huì)產(chǎn)生Cancel事件呢？
我們修改一下CustomViewGroup源碼：

class CustomViewGroup @JvmOverloads constructor(context: Context, attrs: AttributeSet? = null, defStyleAttr: Int = 0 ) : ConstraintLayout(context, attrs, defStyleAttr) {// var hasInterceptMoveEvent = falseoverride fun onInterceptTouchEvent(ev: MotionEvent?): Boolean {// Log.d("TAG", "${ev?.action}:CustomViewGroup onInterceptTouchEvent") // if (!hasInterceptMoveEvent && ev?.action == MotionEvent.ACTION_MOVE) { // hasInterceptMoveEvent = true // return true // }return false}override fun dispatchTouchEvent(ev: MotionEvent?): Boolean {Log.d("TAG", "${getAction(ev?.action)}:CustomViewGroup dispatchTouchEvent")return super.dispatchTouchEvent(ev)}}

即onInterceptTouchEvent方法不攔截事件。同樣的操作我們?cè)賮?lái)看下Log：

04-23 15:29:24.089 24484-24484/com.lee.myapplication D/TAG: MotionEvent.ACTION_DOWN:CustomViewGroup dispatchTouchEvent
04-23 15:29:24.089 24484-24484/com.lee.myapplication D/TAG: MotionEvent.ACTION_DOWN:CusButton dispatchTouchEvent
04-23 15:29:24.104 24484-24484/com.lee.myapplication D/TAG: MotionEvent.ACTION_MOVE:CustomViewGroup dispatchTouchEvent
04-23 15:29:24.105 24484-24484/com.lee.myapplication D/TAG: MotionEvent.ACTION_MOVE:CusButton dispatchTouchEvent
04-23 15:29:24.121 24484-24484/com.lee.myapplication D/TAG: MotionEvent.ACTION_MOVE:CustomViewGroup dispatchTouchEvent
04-23 15:29:24.121 24484-24484/com.lee.myapplication D/TAG: MotionEvent.ACTION_MOVE:CusButton dispatchTouchEvent
04-23 15:29:24.139 24484-24484/com.lee.myapplication D/TAG: MotionEvent.ACTION_MOVE:CustomViewGroup dispatchTouchEvent
04-23 15:29:24.139 24484-24484/com.lee.myapplication D/TAG: MotionEvent.ACTION_MOVE:CusButton dispatchTouchEvent
04-23 15:29:24.154 24484-24484/com.lee.myapplication D/TAG: MotionEvent.ACTION_MOVE:CustomViewGroup dispatchTouchEvent
04-23 15:29:24.155 24484-24484/com.lee.myapplication D/TAG: MotionEvent.ACTION_MOVE:CusButton dispatchTouchEvent
04-23 15:29:24.171 24484-24484/com.lee.myapplication D/TAG: MotionEvent.ACTION_MOVE:CustomViewGroup dispatchTouchEvent
04-23 15:29:24.171 24484-24484/com.lee.myapplication D/TAG: MotionEvent.ACTION_MOVE:CusButton dispatchTouchEvent
04-23 15:29:24.188 24484-24484/com.lee.myapplication D/TAG: MotionEvent.ACTION_MOVE:CustomViewGroup dispatchTouchEvent
04-23 15:29:24.188 24484-24484/com.lee.myapplication D/TAG: MotionEvent.ACTION_MOVE:CusButton dispatchTouchEvent
04-23 15:29:24.205 24484-24484/com.lee.myapplication D/TAG: MotionEvent.ACTION_MOVE:CustomViewGroup dispatchTouchEvent
04-23 15:29:24.205 24484-24484/com.lee.myapplication D/TAG: MotionEvent.ACTION_MOVE:CusButton dispatchTouchEvent
04-23 15:29:24.221 24484-24484/com.lee.myapplication D/TAG: MotionEvent.ACTION_MOVE:CustomViewGroup dispatchTouchEvent
04-23 15:29:24.221 24484-24484/com.lee.myapplication D/TAG: MotionEvent.ACTION_MOVE:CusButton dispatchTouchEvent
04-23 15:29:24.233 24484-24484/com.lee.myapplication D/TAG: MotionEvent.ACTION_MOVE:CustomViewGroup dispatchTouchEvent
04-23 15:29:24.233 24484-24484/com.lee.myapplication D/TAG: MotionEvent.ACTION_MOVE:CusButton dispatchTouchEvent
04-23 15:29:24.250 24484-24484/com.lee.myapplication D/TAG: MotionEvent.ACTION_MOVE:CustomViewGroup dispatchTouchEvent
04-23 15:29:24.250 24484-24484/com.lee.myapplication D/TAG: MotionEvent.ACTION_MOVE:CusButton dispatchTouchEvent
04-23 15:29:24.268 24484-24484/com.lee.myapplication D/TAG: MotionEvent.ACTION_MOVE:CustomViewGroup dispatchTouchEvent
04-23 15:29:24.268 24484-24484/com.lee.myapplication D/TAG: MotionEvent.ACTION_MOVE:CusButton dispatchTouchEvent
04-23 15:29:24.275 24484-24484/com.lee.myapplication D/TAG: MotionEvent.ACTION_MOVE:CustomViewGroup dispatchTouchEvent
04-23 15:29:24.276 24484-24484/com.lee.myapplication D/TAG: MotionEvent.ACTION_MOVE:CusButton dispatchTouchEvent
04-23 15:29:24.276 24484-24484/com.lee.myapplication D/TAG: MotionEvent.ACTION_UP:CustomViewGroup dispatchTouchEvent
04-23 15:29:24.276 24484-24484/com.lee.myapplication D/TAG: MotionEvent.ACTION_UP:CusButton dispatchTouchEvent

我們可以看到Button不會(huì)收到Cancel事件。即時(shí)手指滑出了button，仍然可以收到Move和Up事件。

ACTION_CANCEL作用

我們知道如果某一個(gè)子View處理了Down事件，那么隨之而來(lái)的Move和Up事件也會(huì)交給它處理。但是交給它處理之前，父View還是可以攔截事件的，如果攔截了事件，那么子View就會(huì)收到一個(gè)Cancel事件，并且不會(huì)收到后續(xù)的Move和Up事件。
下面我們通過(guò)源碼驗(yàn)證上面這段話：

@Override public boolean dispatchTouchEvent(MotionEvent ev) {boolean handled = false;final int action = ev.getAction();final int actionMasked = action & MotionEvent.ACTION_MASK;// Check for interception.final boolean intercepted;if (actionMasked == MotionEvent.ACTION_DOWN|| mFirstTouchTarget != null) {//----------源碼1-----------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.TouchTarget newTouchTarget = null;boolean alreadyDispatchedToNewTouchTarget = false;if (!canceled && !intercepted) {// ...}// Dispatch to touch targets.if (mFirstTouchTarget == null) {// ...} else {// ---------------源碼2----------------// 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;//-------------- 源碼3---------if (dispatchTransformedTouchEvent(ev, cancelChild,target.child, target.pointerIdBits)) {handled = true;}//-------------- 源碼4---------if (cancelChild) {if (predecessor == null) {mFirstTouchTarget = next;} else {predecessor.next = next;}target.recycle();target = next;continue;}}predecessor = target;target = next;}}return handled; }

這段代碼進(jìn)行了一定的精簡(jiǎn)，但是大致流程還是很清楚的。當(dāng)子view處理完Down事件之后，mFirstTouchTarget不為null，那么是可以走到源碼1處的，如果子view沒(méi)有對(duì)ViewGroup進(jìn)行不攔截的設(shè)置，那么disallowIntercept為false，此時(shí)會(huì)走到onInterceptTouchEvent方法，如果我們攔截了Move事件，那么onInterceptTouchEvent返回true。也就是說(shuō)intercepted為true；
此時(shí)會(huì)走到源碼2處，遍歷每一個(gè)將要處理事件的view，alreadyDispatchedToNewTouchTarget表示事件是否已經(jīng)交給view處理，此時(shí)當(dāng)然是false。到源碼3處時(shí)，由于intercepted為true，所以cancelChild為true，我們看下dispatchTransformedTouchEvent方法：

/*** Transforms a motion event into the coordinate space of a particular child view,* filters out irrelevant pointer ids, and overrides its action if necessary.* If child is null, assumes the MotionEvent will be sent to this ViewGroup instead.*/private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel,View child, int desiredPointerIdBits) {final boolean handled;// Canceling motions is a special case. We don't need to perform any transformations// or filtering. The important part is the action, not the contents.final int oldAction = event.getAction();if (cancel || oldAction == MotionEvent.ACTION_CANCEL) {event.setAction(MotionEvent.ACTION_CANCEL);if (child == null) {handled = super.dispatchTouchEvent(event);} else {handled = child.dispatchTouchEvent(event);}event.setAction(oldAction);return handled;}// 。。。}

注意，走到dispatchTransformedTouchEvent時(shí)，cancel這個(gè)參數(shù)為true。通過(guò)event.setAction(MotionEvent.ACTION_CANCEL);這句話將MotionEvent的Action設(shè)置為了ACTION_CANCEL，并且交給子view處理。

通過(guò)這里我源碼我們知道了ACTION_CANCEL的產(chǎn)生過(guò)程。那么為什么產(chǎn)生ACTION_CANCEL后，子view無(wú)法收到后續(xù)事件了呢？

在源碼4處，有這樣一段代碼：

if (cancelChild) {if (predecessor == null) {mFirstTouchTarget = next;} else {predecessor.next = next;}target.recycle();target = next;continue;}

cancelChild為true，會(huì)將當(dāng)前target節(jié)點(diǎn)從鏈表中刪除。那么后續(xù)事件到來(lái)時(shí)，view在mFirstTouchTarget鏈表中不存在，自然就不會(huì)交給它處理了。

FLAG_DISALLOW_INTERCEPT的作用

我們通過(guò)上面的代碼可以看出，即時(shí)是MOVE和UP事件，在傳遞給子View之前也是可以通過(guò)ViewGroup的onInterceptTouchEvent方法攔截的，如果攔截了，那么該事件就會(huì)變成Cancel事件傳遞給子view。
那么是否有辦法，子view不讓ViewGroup攔截時(shí)間呢？

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;}

可以看到disallowIntercept變量為true的時(shí)候，會(huì)跳過(guò)onInterceptTouchEvent方法。換句話如果設(shè)置了FLAG_DISALLOW_INTERCEPT這個(gè)flag，那么ViewGoup則不會(huì)攔截Move和Up事件。

我們?cè)僬业皆O(shè)置FLAG_DISALLOW_INTERCEPT這個(gè)flag的地方。

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

這段代碼，充分展示了位運(yùn)算的強(qiáng)大即高效。子View可以通過(guò)設(shè)置requestDisallowInterceptTouchEvent(true)來(lái)達(dá)到禁止父ViewGroup攔截事件的目的。

但是需要注意的是，FLAG_DISALLOW_INTERCEPT這個(gè)flag無(wú)法對(duì)Down事件生效。因?yàn)樵贒own時(shí)，會(huì)清空FLAG_DISALLOW_INTERCEPT。

// 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();}/*** Resets all touch state in preparation for a new cycle.*/private void resetTouchState() {clearTouchTargets();resetCancelNextUpFlag(this);mGroupFlags &= ~FLAG_DISALLOW_INTERCEPT;mNestedScrollAxes = SCROLL_AXIS_NONE;}

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