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Android事件分发之ACTION_CANCEL机制及作用

發(fā)布時間：2023/12/20 Android 34 豆豆

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ACTION_CANCEL產(chǎn)生場景

在閱讀ViewGroup事件分發(fā)相關(guān)源碼過程中，有時候會見到ACTION_CANCEL這一事件。那么這一事件是如何產(chǎn)生的呢？按照網(wǎng)上的說法，當(dāng)手指從當(dāng)前view移出后，當(dāng)前view就會收到ACTION_CANCEL這一事件，這一定是正確的嗎？下面我們來看兩個例子：

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?？聪耮etAction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"} }

其實就是根據(jù)Action對應(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} }

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

現(xiàn)在我們進行如下操作：

手指按住button，然后移動，移出button外，然后松開。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確實收到了一個ACTION_CANCEL事件，并且在這個事件之后，CusButton并沒有收到任何事件。所以我們大概能夠猜到：如果ViewGroup攔截了Move事件，那么這個Move事件將會轉(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。同樣的操作我們再來看下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不會收到Cancel事件。即時手指滑出了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子View處理了Down事件，那么隨之而來的Move和Up事件也會交給它處理。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就會收到一個Cancel事件，并且不會收到后續(xù)的Move和Up事件。
下面我們通過源碼驗證上面這段話：

@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; }

這段代碼進行了一定的精簡，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沒有對ViewGroup進行不攔截的設(shè)置，那么disallowIntercept為false，此時會走到onInterceptTouchEvent方法，如果我們攔截了Move事件，那么onInterceptTouchEvent返回true。也就是說intercepted為true；
此時會走到源碼2處，遍歷每一個將要處理事件的view，alreadyDispatchedToNewTouchTarget表示事件是否已經(jīng)交給view處理，此時當(dāng)然是false。到源碼3處時，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，cancel這個參數(shù)為true。通過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。

通過這里我源碼我們知道了ACTION_CANCEL的產(chǎn)生過程。那么為什么產(chǎn)生ACTION_CANCEL后，子view無法收到后續(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，會將當(dāng)前target節(jié)點從鏈表中刪除。那么后續(xù)事件到來時，view在mFirstTouchTarget鏈表中不存在，自然就不會交給它處理了。

FLAG_DISALLOW_INTERCEPT的作用

我們通過上面的代碼可以看出，即時是MOVE和UP事件，在傳遞給子View之前也是可以通過ViewGroup的onInterceptTouchEvent方法攔截的，如果攔截了，那么該事件就會變成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的時候，會跳過onInterceptTouchEvent方法。換句話如果設(shè)置了FLAG_DISALLOW_INTERCEPT這個flag，那么ViewGoup則不會攔截Move和Up事件。

我們再找到設(shè)置FLAG_DISALLOW_INTERCEPT這個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，充分展示了位運算的強大即高效。子View可以通過設(shè)置requestDisallowInterceptTouchEvent(true)來達到禁止父ViewGroup攔截事件的目的。

但是需要注意的是，FLAG_DISALLOW_INTERCEPT這個flag無法對Down事件生效。因為在Down時，會清空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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