Android中ACTION_CANCEL的触发机制与滑出子view的情况
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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