源码解析View事件的分发机制
首先，Android中View的事件分发机制有3个重要的方法
dispatchTouchEvent()onInterceptTouchEvent()onTouchEvent()
先来看dispatchTouchEvent（）这个方法
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);
}
第一个判断指的是当前View被其它窗口覆盖，直接返回false，第二个判断首先判断mOnTouchListener不为null，并且view是enable的状态，然后 mOnTouchListener.onTouch(this, event)返回true，这三个条件如果都满足，直接return true ; 也就是下面的onTouchEvent(event）不会被执行了；
这个mOnTouchListener其实就是我们设置的View事件
public void setOnTouchListener(OnTouchListener l) {
mOnTouchListener = l;
}
如果设置了View的事件setOnTouchListener，并且return true，那么View自己的onTouchEvent就不会被执行了，那么如果我们return false呢？看下面
看一下onTouchEvent（）的源码
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;
}
if ((viewFlags & ENABLED_MASK) == DISABLED)如果当前View是Disabled状态且是可点击则会消费掉事件（return true);
这里有个最重要的方法是 if (((viewFlags & CLICKABLE) == CLICKABLE ||
(viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE))，如果短按或长按，就判断事件类型，有3个event状态，分别是
MotionEvent.ACTION_DOWN
MotionEvent.ACTION_MOVE
MotionEvent.ACTION_UP
onInterceptTouchEvent（）方法是在dispatchTouchEvent里面调用：
public boolean dispatchTouchEvent(Motion e){ boolean result=false; if(onInterceptTouchEvent(e)){ //如果当前View截获事件，那么事件就会由当前View处理，即调用onTouchEvent() result=onTouchEvent(e); }else{ //如果不截获那么交给其子View来分发 result=child.dispatchTouchEvent(e); } return result;}
从以上可以看出，这三个方法的联系。onInterceptTouchEvent()方法在dispatchTouchEvent中调用，来判断自己是否需要截取事件，如果该方法返回为true，那么View将消费该事件，即会调用onTouchEvent()方法。如果返回false,那么通过调用子View的dispatchTouchEvent()将事件交由子View来处理。
onTouchEvent()和onInterceptTouchEvent()一样也是在dispatchTouchEvent中调用的。用来处理点击事件，包括ACTION_DOWN,ACTION_MOVE,ACTION_UP。如果返回结果为false表示不消费该事件，并且也不会截获接下来的事件序列。如果返回为true表示当前View消费该事件。
