View事件分发流程
备注：该文章参考了两位大神 Kelin 、 guolin 的共三篇文章（文末附上链接，感谢两位大神的分享），个人认为结合三篇文章及源码来理解会更加印象深刻并容易理解，因此融合成了这篇文章，主要是供自己学习，需要认真学习View事件分发流程的童鞋，推荐直接阅读文末附上的三篇文章。PS:文中的图片都是拷贝这两位的成果！！（再次感谢两位大神！！！）
1、View分发
View中存在两个方法：dispatchTouchEvent()和onTouchEvent()。并且，对于一个View，可以通过setOnTouchListener(l)方法设置其onTouch方法回调。见源码（以android sdk-10为基础）：
public boolean dispatchTouchEvent(MotionEvent event) {
if (!onFilterTouchEventForSecurity(event)) {
return false;
}
if (mOnTouchListener != null && (mViewFlags & ENABLED_MASK) == ENABLED &&
mOnTouchListener.onTouch(this, event)) {
return true;
}
return onTouchEvent(event);
}
如果设置了onTouchListener，则会首先执行onTouch方法，并且根据其返回值，决定是否消费当次事件（true则消费，false则不消费）。然后执行onTouchEvent()方法，如果该方法返回的是true，则表明当次事件被View消费了；否则返回false（原因后面阐述）。接着进入onTouchEvent()方法中：
public boolean onTouchEvent(MotionEvent event) {
final int viewFlags = mViewFlags;
if ((viewFlags & ENABLED_MASK) == DISABLED) {
// A disabled view that is clickable still consumes the touch
// events, it just doesn't respond to them.
return (((viewFlags & CLICKABLE) == CLICKABLE ||
(viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE));
}
if (mTouchDelegate != null) {
if (mTouchDelegate.onTouchEvent(event)) {
return true;
}
}
if (((viewFlags & CLICKABLE) == CLICKABLE ||
(viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE)) {
switch (event.getAction()) {
case MotionEvent.ACTION_UP:
boolean prepressed = (mPrivateFlags & PREPRESSED) != 0;
if ((mPrivateFlags & PRESSED) != 0 || prepressed) {
// take focus if we don't have it already and we should in
// touch mode.
boolean focusTaken = false;
if (isFocusable() && isFocusableInTouchMode() && !isFocused()) {
focusTaken = requestFocus();
}
if (!mHasPerformedLongPress) {
// This is a tap, so remove the longpress check
removeLongPressCallback();
// Only perform take click actions if we were in the pressed state
if (!focusTaken) {
// Use a Runnable and post this rather than calling
// performClick directly. This lets other visual state
// of the view update before click actions start.
if (mPerformClick == null) {
mPerformClick = new PerformClick();
}
if (!post(mPerformClick)) {
performClick();
}
}
}
if (mUnsetPressedState == null) {
mUnsetPressedState = new UnsetPressedState();
}
if (prepressed) {
mPrivateFlags |= PRESSED;
refreshDrawableState();
postDelayed(mUnsetPressedState,
ViewConfiguration.getPressedStateDuration());
} else if (!post(mUnsetPressedState)) {
// If the post failed, unpress right now
mUnsetPressedState.run();
}
removeTapCallback();
}
break;
case MotionEvent.ACTION_DOWN:
if (mPendingCheckForTap == null) {
mPendingCheckForTap = new CheckForTap();
}
mPrivateFlags |= PREPRESSED;
mHasPerformedLongPress = false;
postDelayed(mPendingCheckForTap, ViewConfiguration.getTapTimeout());
break;
case MotionEvent.ACTION_CANCEL:
mPrivateFlags &= ~PRESSED;
refreshDrawableState();
removeTapCallback();
break;
case MotionEvent.ACTION_MOVE:
final int x = (int) event.getX();
final int y = (int) event.getY();
// Be lenient about moving outside of buttons
int slop = mTouchSlop;
if ((x < 0 - slop) || (x >= getWidth() + slop) ||
(y < 0 - slop) || (y >= getHeight() + slop)) {
// Outside button
removeTapCallback();
if ((mPrivateFlags & PRESSED) != 0) {
// Remove any future long press/tap checks
removeLongPressCallback();
// Need to switch from pressed to not pressed
mPrivateFlags &= ~PRESSED;
refreshDrawableState();
}
}
break;
}
return true;
}
return false;
}
重点看当switch条件进入ACTION_UP时，经过一系列判断，执行了performClick()，看看这个方法：
public boolean performClick() {
sendAccessibilityEvent(AccessibilityEvent.TYPE_VIEW_CLICKED);
if (mOnClickListener != null) {
playSoundEffect(SoundEffectConstants.CLICK);
mOnClickListener.onClick(this);
return true;
}
return false;
}
这里可以看出，调用了传入的OnClickListener的onClick方法。
到此View的事件分发基本流程已经分析完毕，接下来分析ViewGroup事件分发流程。
2、ViewGroup事件分发流程
ViewGroup是View的子类，其除了dispatchTouchEvent()、onTouchEvent()方法外，还存在第三个重要方法：onInterceptTouchEvent()。从源码下手，先看ViewGroup的onInterceptTouchEvent()：
public boolean onInterceptTouchEvent(MotionEvent ev) {
return false;
}
这个方法默认返回的false，表示ViewGroup默认不拦截该事件。再看ViewGroup的dispatchTouchEvent()：
public boolean dispatchTouchEvent(MotionEvent ev) {
if (!onFilterTouchEventForSecurity(ev)) {
return false;
}
final int action = ev.getAction();
final float xf = ev.getX();
final float yf = ev.getY();
final float scrolledXFloat = xf + mScrollX;
final float scrolledYFloat = yf + mScrollY;
final Rect frame = mTempRect;
boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
if (action == MotionEvent.ACTION_DOWN) {
if (mMotionTarget != null) {
// this is weird, we got a pen down, but we thought it was
// already down!
// XXX: We should probably send an ACTION_UP to the current
// target.
mMotionTarget = null;
}
// If we're disallowing intercept or if we're allowing and we didn't
// intercept
if (disallowIntercept || !onInterceptTouchEvent(ev)) {
// reset this event's action (just to protect ourselves)
ev.setAction(MotionEvent.ACTION_DOWN);
// We know we want to dispatch the event down, find a child
// who can handle it, start with the front-most child.
final int scrolledXInt = (int) scrolledXFloat;
final int scrolledYInt = (int) scrolledYFloat;
final View[] children = mChildren;
final int count = mChildrenCount;
for (int i = count - 1; i >= 0; i--) {
final View child = children[i];
if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE
|| child.getAnimation() != null) {
child.getHitRect(frame);
if (frame.contains(scrolledXInt, scrolledYInt)) {
// offset the event to the view's coordinate system
final float xc = scrolledXFloat - child.mLeft;
final float yc = scrolledYFloat - child.mTop;
ev.setLocation(xc, yc);
child.mPrivateFlags &= ~CANCEL_NEXT_UP_EVENT;
if (child.dispatchTouchEvent(ev)) {
// Event handled, we have a target now.
mMotionTarget = child;
return true;
}
// The event didn't get handled, try the next view.
// Don't reset the event's location, it's not
// necessary here.
}
}
}
}
}
boolean isUpOrCancel = (action == MotionEvent.ACTION_UP) ||
(action == MotionEvent.ACTION_CANCEL);
if (isUpOrCancel) {
// Note, we've already copied the previous state to our local
// variable, so this takes effect on the next event
mGroupFlags &= ~FLAG_DISALLOW_INTERCEPT;
}
// The event wasn't an ACTION_DOWN, dispatch it to our target if
// we have one.
final View target = mMotionTarget;
if (target == null) {
// We don't have a target, this means we're handling the
// event as a regular view.
ev.setLocation(xf, yf);
if ((mPrivateFlags & CANCEL_NEXT_UP_EVENT) != 0) {
ev.setAction(MotionEvent.ACTION_CANCEL);
mPrivateFlags &= ~CANCEL_NEXT_UP_EVENT;
}
return super.dispatchTouchEvent(ev);
}
// if have a target, see if we're allowed to and want to intercept its
// events
if (!disallowIntercept && onInterceptTouchEvent(ev)) {
final float xc = scrolledXFloat - (float) target.mLeft;
final float yc = scrolledYFloat - (float) target.mTop;
mPrivateFlags &= ~CANCEL_NEXT_UP_EVENT;
ev.setAction(MotionEvent.ACTION_CANCEL);
ev.setLocation(xc, yc);
if (!target.dispatchTouchEvent(ev)) {
// target didn't handle ACTION_CANCEL. not much we can do
// but they should have.
}
// clear the target
mMotionTarget = null;
// Don't dispatch this event to our own view, because we already
// saw it when intercepting; we just want to give the following
// event to the normal onTouchEvent().
return true;
}
if (isUpOrCancel) {
mMotionTarget = null;
}
// finally offset the event to the target's coordinate system and
// dispatch the event.
final float xc = scrolledXFloat - (float) target.mLeft;
final float yc = scrolledYFloat - (float) target.mTop;
ev.setLocation(xc, yc);
if ((target.mPrivateFlags & CANCEL_NEXT_UP_EVENT) != 0) {
ev.setAction(MotionEvent.ACTION_CANCEL);
target.mPrivateFlags &= ~CANCEL_NEXT_UP_EVENT;
mMotionTarget = null;
}
return target.dispatchTouchEvent(ev);
}
在事件为ACTION_DOWN时，经过一系列判断，会执行onInterceptTouchEvent()方法，决定是否拦截事件，如果不拦截（返回false），则从该ViewGroup中的所有子View中，找到对应的点击控件，执行其dispatchTouchEvent()，并根据其返回结果，决定是否消费掉该事件（返回true表示消费，false则继续执行后面的代码）。这里回溯一下第1节中说到的问题，这里执行了子View的dispatchTouchEvent方法，最终进入了子View的onTouchEvent方法中，如果该方法返回的是true，代表该子View消费了上述事件，则在该ViewGroup的dispatchTouchEvent方法中，直接return true，不再执行后续代码，事件不再传递。
如果这里子View的onTouchEvent方法返回的是false，则会执行后面的代码
return super.dispatchTouchEvent(ev);
这里执行的就是当前ViewGroup自己的dispatchTouchEvent方法，该方法在其父类View中实现，后续流程就跟第1节中View的事件分发流程是一样的了，就不再分析了。
3、ACTION_DOWN、ACTION_MOVE、ACTION_UP事件的分发
当前事件是ACTION_DOWN，则该事件遵循上述事件分发流程，当第一个事件ACTION_DOWN被某个View或者ViewGroup或者Activity的方法消费了，则后续所有的ACTION_MOVE、ACTION_UP事件，都不再经过上述冗长的分发流程，具体流程可见以下几张图（红色的箭头代表ACTION_DOWN 事件的流向，蓝色的箭头代表ACTION_MOVE 和 ACTION_UP 事件的流向）：
最后再附一张图，帮助记忆事件分发流程中各个方法返回值的作用。
