上一篇分析了View也就是子控件的事件传递机制，现在我们来看一下父容器也就是ViewGroup的事件传递
ViewGroup事件分发
ViewGroup和普通的view相比，多了一个onInterceptTouchEvent，我们还是通过自定义一个ViewGroup来查看ViewGroup的事件传递
- MyRelativeLayout.java
/**
* @author wxblack-mac
* @DESCRIBE:
* @DATE 2019/3/28 09:58
* GOOD LUCK
*/
public class MyRelativeLayout extends RelativeLayout {
public MyRelativeLayout(Context context, AttributeSet attrs) {
super(context, attrs);
}
@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
Log.d(TAG, "dispatchTouchEvent: action" + ev.getAction());
return super.dispatchTouchEvent(ev);
}
@Override
public boolean onTouchEvent(MotionEvent event) {
Log.d(TAG, "onTouchEvent: event" + event.getAction());
return super.onTouchEvent(event);
}
@Override
public boolean onInterceptTouchEvent(MotionEvent ev) {
Log.d(TAG, "onInterceptTouchEvent: event" + ev.getAction());
return super.onInterceptTouchEvent(ev);
}
}
- activity_main.xml
<?xml version="1.0" encoding="utf-8"?>
<com.d9ing.toucheventlsn.MyRelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/rl"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".MainActivity">
<com.d9ing.toucheventlsn.MyButton
android:id="@+id/btn"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="BUTTON" />
</com.d9ing.toucheventlsn.MyRelativeLayout>
然后我们执行程序点击MyButton查看事件的执行情况
log日志输出
从日志的打印我们可以看出，点击事件先发生在ViewGroup上
我们可以看到ViewGroup的事件响应顺序是：
dispatchTouchEvent—->onInterceptTouchEvent—>onTouchListener—->onTouchEvent
我们看ViewGroup的源码来分析一下我们观察到的流程
在ViewGroup的dispatchTouchEvent方法中，我们看到调用了onInterceptTouchEvent，看onInterceptTouchEvent的返回是true还是false
如果是true表示拦截事件，不会给子View分发事件，会自己消费这个事件
如果是false表示不拦截，会遍历所有的子控件，将事件分发给子控件
if (!canceled && !intercepted) {
// If the event is targeting accessibility focus we give it to the
// view that has accessibility focus and if it does not handle it
// we clear the flag and dispatch the event to all children as usual.
// We are looking up the accessibility focused host to avoid keeping
// state since these events are very rare.
...省略一些代码
// Check for interception.
final boolean intercepted;
if (actionMasked == MotionEvent.ACTION_DOWN
|| mFirstTouchTarget != null) {
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;
}
我们可以看到，在dispatchTouchEvent方法中ViewGroup会遍历子View,然后会调用dispatchTransformedTouchEvent
for (int i = childrenCount - 1; i >= 0; i--) {
final int childIndex = getAndVerifyPreorderedIndex(
childrenCount, i, customOrder);
.....省略一些代码
resetCancelNextUpFlag(child);
if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) {
// Child wants to receive touch within its bounds.
mLastTouchDownTime = ev.getDownTime();
if (preorderedList != null) {
// childIndex points into presorted list, find original index
for (int j = 0; j < childrenCount; j++) {
if (children[childIndex] == mChildren[j]) {
mLastTouchDownIndex = j;
break;
}
}
} else {
mLastTouchDownIndex = childIndex;
}
mLastTouchDownX = ev.getX();
mLastTouchDownY = ev.getY();
newTouchTarget = addTouchTarget(child, idBitsToAssign);
alreadyDispatchedToNewTouchTarget = true;
break;
}
我们看dispatchTransformedTouchEvent中的方法
// 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);
}
结合ViewGroup的源码还有View的源码我们可以知道，如果View的Clickable、longClieckable等设置为false，则handled会返回false dispatchTransformedTouchEvent方法的作用就是判断ViewGroup是否有子控件或者是否有子控件需要消耗事件。
如果ViewGroup如果存在子控件的情况下，如果子控件的child是一个ViewGroup这个时候就会递归执行继续遍历ViewGroup。
就会将事件分发给子View，会执行子View的dispatchTouchEvent方法
如果ViewGroup不存在子View， handled = super.dispatchTouchEvent(transformedEvent);会返回false。dispatchTransformedTouchEvent也会返回false。如果返回false后表示没有要消费事件的View。
然后接下来就会分发事件到真正的触摸对象。
// 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);
}
这里我们可以看到当mFirstTouchTarget为null的时候表示没有子控件。在dispatchTransformedTouchEvent方法中会调用父类的dispatchTouchEvent。将事件传递会父类或者父容器的dispatchTouchEvent。这样事件就又回到了父类当中。
如果存在子View，ViewGroup会找到真正触摸发生的对象。
// 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;
}
if (cancelChild) {
if (predecessor == null) {
mFirstTouchTarget = next;
} else {
predecessor.next = next;
}
target.recycle();
target = next;
continue;
}
}
predecessor = target;
target = next;
}
}
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;
}
if (cancelChild) {
if (predecessor == null) {
mFirstTouchTarget = next;
} else {
predecessor.next = next;
}
target.recycle();
target = next;
continue;
}
}
predecessor = target;
target = next;
}
从这段代码中我们就能看到 会不断的循环找下一个触摸的目标。需要最终找到消费掉这个事件的对象，没有找到后，也就是代码中的mFirstTouchTarget这个为空的时候，事件就会被自己消费掉。
总结
看了这么多的源码，感觉已经懵逼了。我们来画一张图来总结这个事件传递的过程
事件分发源码流程图1.png
这个图并不很完整，只是说明了正常的情况，还有一些情况我们在这解释一下：
上图中，在最后一个子View的onTouch事件中，如果子View返回了true则子View会将事件消费掉。如果onTouch返回false代表子View不消费事件，则事件会传递回父容器的dispatchTouchEvent经过super.dispatch传递到父容器也就是上图第二个容器的onTouch事件。如果父容器的onTouch返回的是true代表事件被父容器消费。如果返回的是false代表父容器不消费这个事件，事件会继续想上传，传给父容器的dispatchTouchEvent，在dispatchTouchEvent内部会传递给父容器的onTouch,如果onTouch返回true代表本事件被消费，返回false表示不消费事件。则事件继续向上传。
所以在调用表示的就是，在子view的onTouch返回false后会传递事件到父容器的onTouch再向上传递到父容器的onTouch。但是实际的事件传递要多了一个传递到dispatchTouchEvent。再由dispatchTouchEvent执行super.dispatchTouchEvent调用onTouch。
