博文出处：Android onTouch事件传递机制解析，欢迎大家关注我的博客，谢谢！
记得刚开始学习Android的时候，对于onTouch相关的事件一头雾水。分不清onTouch()，onTouchEvent()和OnClick()之间的关系和先后顺序，觉得有必要搞清onTouch事件传递的原理。经过一段时间的琢磨以及网上相关博客的介绍，总算是了解了触摸事件传递的机制了，顺便写一篇博客来记录一下。下面就让我们来看看吧。
大家都知道一般我们使用的UI控件都是继承自共同的父类——View。所以View这个类应该掌管着onTouch事件的相关处理。那就让我们去看看：在View中寻找Touch相关的方法，其中一个很容易地引起了我们的注意：dispatchTouchEvent(MotionEvent event)。根据方法名的意思应该是负责分发触摸事件的，下面给出了源码：
/**
* Pass the touch screen motion event down to the target view, or this
* view if it is the target.
*
* @param event The motion event to be dispatched.
* @return True if the event was handled by the view, false otherwise.
*/
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;
}
源码有点长，但我们不必每一行都看。首先注意到dispatchTouchEvent的返回值是boolean类型的，注释上的解释：@return True if the event was handled by the view, false otherwise.也就是说如果该触摸事件被这个View消费了就返回true，否则返回false。在方法中首先判断了该event是否是否得到了焦点，如果没有得到焦点直接返回false。然后让我们把目光转向if (li != null && li.mOnTouchListener != null&& (mViewFlags & ENABLED_MASK) == ENABLED&& li.mOnTouchListener.onTouch(this, event))这个片段，看到这里有一个名为li的局部变量，属于 ListenerInfo 类，经 mListenerInfo 赋值得到。ListenerInfo只是一个包装类，里面封装了大量的监听器。再在 View 类中去寻找 mListenerInfo ，可以看到下面的代码：
ListenerInfo getListenerInfo() {
if (mListenerInfo != null) {
return mListenerInfo;
}
mListenerInfo = new ListenerInfo();
return mListenerInfo;
}
因此我们可以知道mListenerInfo是不为空的，所以li也不是空，第一个判断为true，然后看到li.mOnTouchListener，前面说过ListenerInfo是一个监听器的封装类，所以我们同样去追踪mOnTouchListener：
/**
* Register a callback to be invoked when a touch event is sent to this view.
* @param l the touch listener to attach to this view
*/
public void setOnTouchListener(OnTouchListener l) {
getListenerInfo().mOnTouchListener = l;
}
正是通过上面的方法来设置 mOnTouchListener 的，我想上面的方法大家肯定都很熟悉吧，正是我们平时经常用的 xxx.setOnTouchListener ，好了我们从中得知如果设置了OnTouchListener则第二个判断也为true，第三个判断为如果该View是否为enable，默认都是enable的，所以同样为true。还剩最后一个：li.mOnTouchListener.onTouch(this, event)，显然是回调了第二个判断中监听器的onTouch()方法，如果onTouch()方法返回true,则上面四个判断全部为true,dispatchTouchEvent()方法会返回true，并且不会执行if (!result && onTouchEvent(event))这个判断；而在这个判断中我们又看到了一个熟悉的方法：onTouchEvent()。所以想要执行onTouchEvent，则在上面的四个判断中必须至少有一个false。
那就假定我们在onTouch()方法中返回的是false，这样就顺利地执行了onTouchEvent，那就看看onTouchEvent的源码吧：
/**
* Implement this method to handle touch screen motion events.
* <p>
* If this method is used to detect click actions, it is recommended that
* the actions be performed by implementing and calling
* {@link #performClick()}. This will ensure consistent system behavior,
* including:
* <ul>
* <li>obeying click sound preferences
* <li>dispatching OnClickListener calls
* <li>handling {@link AccessibilityNodeInfo#ACTION_CLICK ACTION_CLICK} when
* accessibility features are enabled
* </ul>
*
* @param event The motion event.
* @return True if the event was handled, false otherwise.
*/
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;
}
这段源码比 dispatchTouchEvent 的还要长，不过同样我们挑重点的看：
if (((viewFlags & CLICKABLE) == CLICKABLE || (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE) || (viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE)
看到这句话就大概知道了主要是判断该view是否是可点击的，如果可以点击则接着执行，否则直接返回false。可以看到if里面用switch来判断是哪种触摸事件，但在最后都是返回true的。还有一点要注意：在 ACTION_UP 中会执行 performClick() 方法：
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;
}
可以看到上面的li.mOnClickListener.onClick(this);，没错，我们好像又有了新的发现。根据上面的经验，这句代码会去回调我们设置好的点击事件监听器。也就是我们平常用的xxx.setOnClickListener(listener);
/**
* Register a callback to be invoked when this view is clicked. If this view is not
* clickable, it becomes clickable.
*
* @param l The callback that will run
*
* @see #setClickable(boolean)
*/
public void setOnClickListener(@Nullable OnClickListener l) {
if (!isClickable()) {
setClickable(true);
}
getListenerInfo().mOnClickListener = l;
}
我们可以看到上面方法设置正是mListenerInfo的点击监听器，验证了上面的猜想。到了这里onTouch事件的传递机制基本已经分析完成了,也算是告一段落了。
好了，这下我们可以解决开头的问题了，顺便我们再来小结一下：在dispatchTouchEvent中，如果设置了OnTouchListener并且View是enable的，那么首先被执行的是OnTouchListener中的onTouch(View v, MotionEvent event)。若onTouch返回true,则dispatchTouchEvent不再往下执行并且返回true；不然会执行onTouchEvent，在onTouchEvent中若View是可点击的，则返回true，不然为false。还有在onTouchEvent中若View是可点击以及当前触摸事件为ACTION_UP，会执行performClick()，回调OnClickListener的onClick方法。下面是我画的一张草图：
触摸事件传递机制原理图
还有一点值得注意的地方是：假如当前事件是ACTION_DOWN，只有dispatchTouchEvent返回true了之后该View才会接收到接下来的ACTION_MOVE,ACTION_UP事件，也就是说只有事件被消费了才能接收接下来的事件。
好了，今天就到这里了，如果有什么问题可以在下面留言。
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