Android中ACTION_CANCEL的触发机制与滑出子view的情况
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2022-06-23 23:30:17
目录action_cancel的触发时机2，action_down初始化操作3，在子view处理事件的过程中被从父view中移除时4，子view被设置了pflag_cancel_next_up_eve...
看完本文你将了解：
- action_cancel的触发时机
- 滑出子view区域会发生什么？为什么不响应onclick()事件
首先看一下官方的解释：
/**
* constant for {@link #getactionmasked}: the current gesture has been aborted.
* you will not receive any more points in it. you should treat this as
* an up event, but not perform any action that you normally would.
*/
public static final int action_cancel = 3;
说人话就是：当前的手势被中止了，你不会再收到任何事件了，你可以把它当做一个action_up事件，但是不要执行正常情况下的逻辑。
action_cancel的触发时机
有四种情况会触发action_cancel:
- 在子view处理事件的过程中，父view对事件拦截
- action_down初始化操作
- 在子view处理事件的过程中被从父view中移除时
- 子view被设置了pflag_cancel_next_up_event标记时
1，父view拦截事件
首先要了解viewgroup什么情况下会拦截事件，look the fuck resource code：
/**
* {@inheritdoc}
*/
@override
public boolean dispatchtouchevent(motionevent ev) {
...
boolean handled = false;
if (onfiltertoucheventforsecurity(ev)) {
final int action = ev.getaction();
final int actionmasked = action & motionevent.action_mask;
...
// check for interception.
final boolean intercepted;
// 判断条件一
if (actionmasked == motionevent.action_down
|| mfirsttouchtarget != null) {
final boolean disallowintercept = (mgroupflags & flag_disallow_intercept) != 0;
// 判断条件二
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;
}
...
}
...
}
有两个条件
- motionevent.action_down事件或者mfirsttouchtarget非空也就是有子view在处理事件
- 子view没有做拦截，也就是没有调用
viewparent#requestdisallowintercepttouchevent(true)
如果满足上面的两个条件才会执行onintercepttouchevent(ev)。
如果viewgroup拦截了事件，则intercepted变量为true，接着往下看：
@override
public boolean dispatchtouchevent(motionevent ev) {
boolean handled = false;
if (onfiltertoucheventforsecurity(ev)) {
...
// check for interception.
final boolean intercepted;
if (actionmasked == motionevent.action_down
|| mfirsttouchtarget != null) {
final boolean disallowintercept = (mgroupflags & flag_disallow_intercept) != 0;
if (!disallowintercept) {
// 当mfirsttouchtarget != null，也就是子view处理了事件
// 此时如果父viewgroup拦截了事件，intercepted==true
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;
}
...
// dispatch to touch targets.
if (mfirsttouchtarget == null) {
...
} 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) {
...
} else {
// 判断一：此时cancelchild == true
final boolean cancelchild = resetcancelnextupflag(target.child)
|| intercepted;
// 判断二：给child发送cancel事件
if (dispatchtransformedtouchevent(ev, cancelchild,
target.child, target.pointeridbits)) {
handled = true;
}
...
}
...
}
}
...
}
...
return handled;
}
以上判断一处cancelchild为true，然后进入判断二中一看究竟：
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) {
// 将event设置成action_cancel
event.setaction(motionevent.action_cancel);
if (child == null) {
...
} else {
// 分发给child
handled = child.dispatchtouchevent(event);
}
event.setaction(oldaction);
return handled;
}
...
}
当参数cancel为ture时会将event设置为motionevent.action_cancel，然后分发给child。
2，action_down初始化操作
public boolean dispatchtouchevent(motionevent ev) {
boolean handled = false;
if (onfiltertoucheventforsecurity(ev)) {
final int action = ev.getaction();
final int actionmasked = action & motionevent.action_mask;
// 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.
// 取消并清除所有的touch目标
cancelandcleartouchtargets(ev);
resettouchstate();
}
...
}
...
}
系统可能会由于app切换、anr等原因丢失了up，cancel事件。
因此需要在action_down时丢弃掉所有前面的状态，具体代码如下：
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);
// 分发事件同情况一
dispatchtransformedtouchevent(event, true, target.child, target.pointeridbits);
}
...
}
}
ps：在dispatchdetachedfromwindow()中也会调用cancelandcleartouchtargets()
3，在子view处理事件的过程中被从父view中移除时
public void removeview(view view) {
if (removeviewinternal(view)) {
requestlayout();
invalidate(true);
}
}
private boolean removeviewinternal(view view) {
final int index = indexofchild(view);
if (index >= 0) {
removeviewinternal(index, view);
return true;
}
return false;
}
private void removeviewinternal(int index, view view) {
...
canceltouchtarget(view);
...
}
private void canceltouchtarget(view view) {
touchtarget predecessor = null;
touchtarget target = mfirsttouchtarget;
while (target != null) {
final touchtarget next = target.next;
if (target.child == view) {
...
// 创建action_cancel事件
motionevent event = motionevent.obtain(now, now,
motionevent.action_cancel, 0.0f, 0.0f, 0);
event.setsource(inputdevice.source_touchscreen);
分发给目标view
view.dispatchtouchevent(event);
event.recycle();
return;
}
predecessor = target;
target = next;
}
}
4，子view被设置了pflag_cancel_next_up_event标记时
在情况一种的两个判断处：
// 判断一：此时cancelchild == true
final boolean cancelchild = resetcancelnextupflag(target.child)
|| intercepted;
// 判断二：给child发送cancel事件
if (dispatchtransformedtouchevent(ev, cancelchild,
target.child, target.pointeridbits)) {
handled = true;
}
当 resetcancelnextupflag(target.child) 为true时同样也会导致cancel，查看代码：
/**
* indicates whether the view is temporarily detached.
*
* @hide
*/
static final int pflag_cancel_next_up_event = 0x04000000;
private static boolean resetcancelnextupflag(view view) {
if ((view.mprivateflags & pflag_cancel_next_up_event) != 0) {
view.mprivateflags &= ~pflag_cancel_next_up_event;
return true;
}
return false;
}
根据注释大概意思是，该view暂时detached，detached是什么意思？就是和attached相反的那个，具体什么时候打了这个标记，我觉得没必要深究。
以上四种情况最重要的就是第一种，后面的只需了解即可。
滑出子view区域会发生什么？
了解了什么情况下会触发action_cancel，那么针对问题：滑出子view区域会触发action_cancel吗？这个问题就很明确了：不会。
实践是检验真理的唯一标准，代码撸起来：
public class mybutton extends androidx.appcompat.widget.appcompatbutton {
@override
public boolean ontouchevent(motionevent event) {
switch (event.getaction()) {
case motionevent.action_down:
logutil.d("action_down");
break;
case motionevent.action_move:
logutil.d("action_move");
break;
case motionevent.action_up:
logutil.d("action_up");
break;
case motionevent.action_cancel:
logutil.d("action_cancel");
break;
}
return super.ontouchevent(event);
}
}
一波操作以后日志如下：
(mybutton.java:32) -->action_down
(mybutton.java:36) -->action_move
(mybutton.java:36) -->action_move
(mybutton.java:36) -->action_move
(mybutton.java:36) -->action_move
(mybutton.java:36) -->action_move
(mybutton.java:39) -->action_up
滑出view后依然可以收到action_move和action_up事件。
为什么有人会认为滑出view后会收到action_cancel呢？
我想是因为滑出view后，view的onclick()不会触发了，所以有人就以为是触发了action_cancel。
那么为什么滑出view后不会触发onclick呢？再来看看view的源码：
在view的ontouchevent()中：
case motionevent.action_move:
// be lenient about moving outside of buttons
// 判断是否超出view的边界
if (!pointinview(x, y, mtouchslop)) {
// outside button
if ((mprivateflags & pressed) != 0) {
// 这里改变状态为 not pressed
// need to switch from pressed to not pressed
mprivateflags &= ~pressed;
}
}
break;
case motionevent.action_up:
boolean prepressed = (mprivateflags & pflag_prepressed) != 0;
// 可以看到当move出view范围后，这里走不进去了
if ((mprivateflags & pflag_pressed) != 0 || prepressed) {
...
performclick();
...
}
mignorenextupevent = false;
break;
1，在action_move中会判断事件的位置是否超出view的边界，如果超出边界则将mprivateflags置为not pressed状态。
2，在action_up中判断只有当mprivateflags包含pressed状态时才会执行performclick()等。
因此滑出view后不会执行onclick()。
结论：
- 滑出view范围后，如果父view没有拦截事件，则会继续受到
action_move和action_up等事件。 - 一旦滑出view范围，view会被移除
pressed标记，这个是不可逆的，然后在action_up中不会执行performclick()等逻辑。
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