Android事件分发机制(二)
前言
接上节，在这一小节中，我将着重从View和ViewGroup的源码中去探索事件分发的流程是否如上小节分析的那样，带着上一节所留下的疑问开始愉快的阅读源码吧。
源码分析
先放上小节中总结的流程图：
在Activity中最终也是走的ViewGroup.dispatchTouchEvent，所以直接看ViewGroup就可以了：
首先我们要明确一次完整的事件分发包括ACTION_DOWN，若干ACTION_MOVE，ACTION_UP事件
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.
cancelAndClearTouchTargets(ev);
resetTouchState();
}
此处做重置工作，如果当前的MotionEvent是ACTION_DOWN，则cancel所有未执行完的事件，并清除所有的TouchTarget，这个TouchTarget是什么?它记录了所有被点击的View和MotionEvent的id，如果有设置
FLAG_DISALLOW_INTERCEPT也会被一并清除掉。
// 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;
}
// If intercepted, start normal event dispatch. Also if there is already
// a view that is handling the gesture, do normal event dispatch.
if (intercepted || mFirstTouchTarget != null) {
ev.setTargetAccessibilityFocus(false);
}
此处判断事件是否被拦截，在这里我们看到了熟悉的onInterceptTouchEvent，当前事件为ACTION_DOWN时，首先会判断有没有设置FLAG_DISALLOW_INTERCEPT标识，这个标识是通过requestDisallowInterceptTouchEvent设置的，一般是子View控制父类不去拦截事件，前面分析到了这个flag会在执行ACTION_DOWN事件时被重置，如果不允许被拦截，那么事件当然交予子View去处理啦，反之，则会执行onInterceptTouchEvent方法。如果是其他的事件，则需要考虑mFirstTouchTarget是否为null，在下面的代码中可以看到如果事件交予子控件处理，那么mFirstTouchTarget将被赋值，因此如果事件没有交予子View处理，mFirstTouchTarget就是null值，那么接下来的所有事件都不会交予子View处理了，而且也不会执行onInterceptTouchEvent。
final View[] children = mChildren;
for (int i = childrenCount - 1; i >= 0; i--) {
final int childIndex = getAndVerifyPreorderedIndex(childrenCount, i, customOrder);
final View child = getAndVerifyPreorderedView(preorderedList, children, childIndex);
if (childWithAccessibilityFocus != null) {
if (childWithAccessibilityFocus != child) {
continue;
}
childWithAccessibilityFocus = null;
i = childrenCount - 1;
}
// 判断子View是否能收到点击事件和点击事件是否在View内发生
if (!canViewReceivePointerEvents(child) || !isTransformedTouchPointInView(x, y, child, null)) {
ev.setTargetAccessibilityFocus(false);
// 如果子View收不到事件，进行下一从循环，一直到找到目标子View
continue;
}
newTouchTarget = getTouchTarget(child);
if (newTouchTarget != null) {
// Child is already receiving touch within its bounds.
// Give it the new pointer in addition to the ones it is handling.
newTouchTarget.pointerIdBits |= idBitsToAssign;
break;
}
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;
}
// The accessibility focus didn't handle the event, so clear
// the flag and do a normal dispatch to all children.
ev.setTargetAccessibilityFocus(false);
}
// 判断子View能收到点击事件的条件
private static boolean canViewReceivePointerEvents(@NonNull View child) {
return (child.mViewFlags & VISIBILITY_MASK) == VISIBLE
|| child.getAnimation() != null;
}
接下来分发事件到子View，如果子View能收到点击事件，并且点击事件在子View的范围之内，这里判断子View能否收到点击事件的条件在于它是否可见，或子View的mCurrentAnimation不为null。事件交由子View去处理，如果子View处理了该次事件，则会通过addTouchTarget记录起来。确定了事件被消费后，就会结束循环。
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;
}
}
如果没有子View处理这次事件，则会执行super.dispatchTouchEvent，交给View的dispatchTouchEvent处理，代码如下：
public boolean dispatchTouchEvent(MotionEvent event) {
// If the event should be handled by accessibility focus first.
if (event.isTargetAccessibilityFocus()) {
// We don't have focus or no virtual descendant has it, do not handle the event.
if (!isAccessibilityFocusedViewOrHost()) {
return false;
}
// We have focus and got the event, then use normal event dispatch.
event.setTargetAccessibilityFocus(false);
}
boolean result = false;
if (mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onTouchEvent(event, 0);
}
final int actionMasked = event.getActionMasked();
if (actionMasked == MotionEvent.ACTION_DOWN) {
// Defensive cleanup for new gesture
stopNestedScroll();
}
if (onFilterTouchEventForSecurity(event)) {
if ((mViewFlags & ENABLED_MASK) == ENABLED && handleScrollBarDragging(event)) {
result = true;
}
//noinspection SimplifiableIfStatement
ListenerInfo li = mListenerInfo;
if (li != null && li.mOnTouchListener != null
&& (mViewFlags & ENABLED_MASK) == ENABLED
&& li.mOnTouchListener.onTouch(this, event)) {
result = true;
}
if (!result && onTouchEvent(event)) {
result = true;
}
}
if (!result && mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onUnhandledEvent(event, 0);
}
// Clean up after nested scrolls if this is the end of a gesture;
// also cancel it if we tried an ACTION_DOWN but we didn't want the rest
// of the gesture.
if (actionMasked == MotionEvent.ACTION_UP ||
actionMasked == MotionEvent.ACTION_CANCEL ||
(actionMasked == MotionEvent.ACTION_DOWN && !result)) {
stopNestedScroll();
}
return result;
}
在View中的事件分发逻辑就比ViewGroup少多了，咱们挑重点看
ListenerInfo li = mListenerInfo;
if (li != null && li.mOnTouchListener != null && (mViewFlags & ENABLED_MASK) == ENABLED && li.mOnTouchListener.onTouch(this, event)) {
result = true;
}
if (!result && onTouchEvent(event)) {
result = true;
}
在这里，可以看到View会先判断OnTouch事件，如果有OnTouchListener成功处理了这次事件，那么就不会执行onTouchEvent方法了。
至此，Android的事件分发机制就基本分析完了，总的来说，ViewGroup将事件分发给子View，并询问子View是否能处理这次事件，如果事件被拦截了，或者没有子View处理，则执行自己的onTouchEvent，并将dispatchTouchEvent的结果反馈给父类。
下面有几个疑问，想问下读者，也顺便提醒下自己
- 如果ACTION_DOWN事件没有被处理过，那么mFirstTouchTarget一定为null吗？
- 可以看到在将事件分发给子View主要是通过dispatchTransformedTouchEvent方法的，在ViewGroup的dispatchTouchEvent中会遍历一次所有的子View，然后通过dispatchTransformedTouchEvent去询问子View是否有处理过事件，但是在dispatchTouchEvent的最后面可以看到，对于mFirstTouchTarget != null时，会再对它做一次事件分发，为什么要这么做呢？