在分析Android事件分发机制前，明确android的两大基础控件类型:View和ViewGroup。View即普通的控件，没有子布局的，如Button、TextView. ViewGroup继承自View，表示可以有子控件，如Linearlayout、Listview这些。今天我们先来了解View的事件分发机制。
先看下代码，非常简单，只有一个Button，分别给它注册了OnClick和OnTouch的点击事件。
btn.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
Log.i("Tag", "This is button onClick event");
}
});
btn.setOnTouchListener(new View.OnTouchListener() {
@Override
public boolean onTouch(View v, MotionEvent event) {
Log.i("Tag", "This is button onTouch action" + event.getAction());
return false;
}
});
运行一下项目，结果如下：
I/Tag: This is button onTouch action0
I/Tag: This is button onTouch action2
I/Tag: This is button onTouch action2
I/Tag: This is button onTouch action1
I/Tag: This is button onClick event
可以看到，onTouch是有先于onClick执行的，因此事件的传递顺序是先onTouch，在到OnClick。具体为什么这样，下面会通过源码来说明。这时，我们可能注意到了，onTouch的方法是有返回值，这里是返回false，我们将它改为true再运行一次，结果如下：
I/Tag: This is button onTouch action0
I/Tag: This is button onTouch action2
I/Tag: This is button onTouch action2
I/Tag: This is button onTouch action2
I/Tag: This is button onTouch action1
对比两次结果，我们发现onClick方法不再执行，为什么会这样，下面我将通过源码给大家一步步理清这个思路。
查看源码时，首先要知道所有View类型控件事件入口都是dispatchTouchEvent()，所以我们直接进入到View这个类里面的dispatchTouchEvent（）方法看一下。
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)) {
//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;
}
从源码第25行处可以看到，mOnTouchListener.onTouch()的方法首先被执行，如果li != null && li.mOnTouchListener != null&& (mViewFlags & ENABLED_MASK) == ENABLED&& li.mOnTouchListener.onTouch(this, event)都为真的话，result赋值为true，否则就执行onTouchEvent(event)方法。
从上面可以看到要符合条件有四个，
1、ListenerInfo li，它是view中的一个静态类，里面定义view的事件的监听等等，所以有涉及到view的事件，ListenerInfo都会被实例化，因此li不为null
2、mOnTouchiListener是在setOnTouchListener方法里面赋值的，只要touch事件被注册，mOnTouchiListener一定不会null
3、 (mViewFlags & ENABLED_MASK) == ENABLED，是判断当前点击的控件是否是enable的，button默认为enable，这个条件也恒定为true，
4、重点来了，li.mOnTouchListener.onTouch(this, event)就是回调控件onTouch方法，当这个条件也为true时，result=true,onTouchEvent（event）将不会被执行。如果onTouch返回false，就会再执行onTouchEvent(event)方法。
我们接着再进入到onTouchEvent方法查看源码。
public boolean onTouchEvent(MotionEvent event) {
final float x = event.getX();
final float y = event.getY();
final int viewFlags = mViewFlags;
final int action = event.getAction();
if ((viewFlags & ENABLED_MASK) == DISABLED) {
if (action == MotionEvent.ACTION_UP && (mPrivateFlags & PFLAG_PRESSED) != 0) {
setPressed(false);
}
// 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)
|| (viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE);
}
if (mTouchDelegate != null) {
if (mTouchDelegate.onTouchEvent(event)) {
return true;
}
}
if (((viewFlags & CLICKABLE) == CLICKABLE ||
(viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE) ||
(viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE) {
switch (action) {
case MotionEvent.ACTION_UP:
boolean prepressed = (mPrivateFlags & PFLAG_PREPRESSED) != 0;
if ((mPrivateFlags & PFLAG_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 (prepressed) {
// The button is being released before we actually
// showed it as pressed. Make it show the pressed
// state now (before scheduling the click) to ensure
// the user sees it.
setPressed(true, x, y);
}
if (!mHasPerformedLongPress && !mIgnoreNextUpEvent) {
// 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) {
postDelayed(mUnsetPressedState,
ViewConfiguration.getPressedStateDuration());
} else if (!post(mUnsetPressedState)) {
// If the post failed, unpress right now
mUnsetPressedState.run();
}
removeTapCallback();
}
mIgnoreNextUpEvent = false;
break;
case MotionEvent.ACTION_DOWN:
mHasPerformedLongPress = false;
if (performButtonActionOnTouchDown(event)) {
break;
}
// Walk up the hierarchy to determine if we're inside a scrolling container.
boolean isInScrollingContainer = isInScrollingContainer();
// For views inside a scrolling container, delay the pressed feedback for
// a short period in case this is a scroll.
if (isInScrollingContainer) {
mPrivateFlags |= PFLAG_PREPRESSED;
if (mPendingCheckForTap == null) {
mPendingCheckForTap = new CheckForTap();
}
mPendingCheckForTap.x = event.getX();
mPendingCheckForTap.y = event.getY();
postDelayed(mPendingCheckForTap, ViewConfiguration.getTapTimeout());
} else {
// Not inside a scrolling container, so show the feedback right away
setPressed(true, x, y);
checkForLongClick(0);
}
break;
case MotionEvent.ACTION_CANCEL:
setPressed(false);
removeTapCallback();
removeLongPressCallback();
mInContextButtonPress = false;
mHasPerformedLongPress = false;
mIgnoreNextUpEvent = false;
break;
case MotionEvent.ACTION_MOVE:
drawableHotspotChanged(x, y);
// Be lenient about moving outside of buttons
if (!pointInView(x, y, mTouchSlop)) {
// Outside button
removeTapCallback();
if ((mPrivateFlags & PFLAG_PRESSED) != 0) {
// Remove any future long press/tap checks
removeLongPressCallback();
setPressed(false);
}
}
break;
}
return true;
}
return false;
}
从源码的21行我们可以看出，该控件可点击就会进入到switch判断中，当我们触发了手指离开的实际，则会进入到MotionEvent.ACTION_UP这个case当中。我们接着往下看，在源码的50行，调用到了mPerformClick()方法，我们继续进入到这个方法的源码看看。
public boolean performClick() {
final boolean result;
final ListenerInfo li = mListenerInfo;
if (li != null && li.mOnClickListener != null) {
playSoundEffect(SoundEffectConstants.CLICK);
li.mOnClickListener.onClick(this);
result = true;
} else {
result = false;
}
sendAccessibilityEvent(AccessibilityEvent.TYPE_VIEW_CLICKED);
return result;
}
现在我们可以看到，只要ListenerInfo和mOnClickListener不为null就会调用onClick这个方法，之前说过，只要有监听事件，ListenerInfo就不为null，带mOnClickListener又是在哪里赋值呢？我们再继续看下它的源码。
public void setOnClickListener(@Nullable OnClickListener l) {
if (!isClickable()) {
setClickable(true);
}
getListenerInfo().mOnClickListener = l;
}
看到这里一切就清楚了，当我们调用setOnClickListener方法来给按钮注册一个点击事件时，就会给mOnClickListener赋值。整个分发事件的顺序是onTouch()-->onTouchEvent(event)-->performClick()-->OnClick()。
现在我们可以解决之前的问题。
1、onTouch方法是优先于OnClick，所以是执行了onTouch，再执行onClick。
2、无论是dispatchTouchEvent还是onTouchEvent，如果返回true表示这个事件已经被消费、处理了，不再往下传了。在dispathTouchEvent的源码里可以看到，如果onTouchEvent返回了true，那么它也返回true。如果dispatchTouchEvent在执行onTouch监听的时候，onTouch返回了true，那么它也返回true，这个事件提前被onTouch消费掉了。就不再执行onTouchEvent了，更别说onClick监听了。
以上就是本文的全部内容，希望对大家的学习有所帮助，也希望大家多多支持。
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