(本文基于android-2.3.3_r1代码研究)
前面研究了Android触屏事件的分发机制；本文继续从源码角度研究触屏事件的处理机制。
一. View.onTouchEvent
Android的触屏事件，最终分发给View的onTouchEvent方法来处理，我们来看看它是怎么处理的：
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/**
* Implement this method to handle touch screen motion events.
*
* @param event The motion event.
* @return True if the event was handled, false otherwise.
*/
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;
}
由第13行及第23行代码可知，当View的CLICKABLE状态位或LONG_CLICKABLE状态位为1时，即View有可点击或可长按时，onTouchEvent方法返回true；由前面的文章可知，onTouchEvent方法返回true就代表View消费了touch事件；而View的setOnClickListener方法会设置CLICKABLE状态位，setOnLongClickListener会设置LONG_CLICKABLE状态位。因此，当对View设置了OnClickListener或OnLongClickListener时，这个View就会消费父ViewGroup分发的touch事件。
由第10行处代码可知，当View是DISABLED状态时，仍然会消费分发过来的touch事件，只是不做任何处理。
1. down事件
首先看下对down事件的处理：
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case MotionEvent.ACTION_DOWN:
if (mPendingCheckForTap == null) {
mPendingCheckForTap = new CheckForTap();
}
mPrivateFlags |= PREPRESSED;
mHasPerformedLongPress = false;
postDelayed(mPendingCheckForTap, ViewConfiguration.getTapTimeout());
break;
首先创建了一个CheckForTap对象mPendingCheckForTap，CheckForTap是个Runnable接口实例；然后设置了PREPRESSED状态位；最后调用View.postDelayed方法将tap延时消息发送到主线程消息队列，延迟时间为ViewConfiguration.getTapTimeout() (android2.3.3_r1中是115ms)：
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private final class CheckForTap implements Runnable {
public void run() {
mPrivateFlags &= ~PREPRESSED;
mPrivateFlags |= PRESSED;
refreshDrawableState();
if ((mViewFlags & LONG_CLICKABLE) == LONG_CLICKABLE) {
postCheckForLongClick(ViewConfiguration.getTapTimeout());
}
}
}
当用户按下触屏并保持一定时间后，就会执行CheckForTap的run方法：首先取消了PREPRESSED状态位，设置了PRESSED状态位；然后刷新View的drawable状态；如果View可长按，则调用postCheckForLongClick方法：
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private void postCheckForLongClick(int delayOffset) {
mHasPerformedLongPress = false;
if (mPendingCheckForLongPress == null) {
mPendingCheckForLongPress = new CheckForLongPress();
}
mPendingCheckForLongPress.rememberWindowAttachCount();
postDelayed(mPendingCheckForLongPress,
ViewConfiguration.getLongPressTimeout() - delayOffset);
}
postCheckForLongClick方法中创建了CheckForLongPress对象mPendingCheckForLongPress，然后发送long click延时消息到消息队列，这次的延迟时间为ViewConfiguration.getLongPressTimeout() - ViewConfiguration.getTapTimeout()， 即长按时间减去tap时间(android2.3.3_r1中为 500ms-115ms)。CheckForLongPress同CheckForTap类似，也是一个Runnable接口实例，只不过CheckForTap是用来处理tap事件，CheckForLongPress用来处理长按事件。
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class CheckForLongPress implements Runnable {
private int mOriginalWindowAttachCount;
public void run() {
if (isPressed() && (mParent != null)
&& mOriginalWindowAttachCount == mWindowAttachCount) {
if (performLongClick()) {
mHasPerformedLongPress = true;
}
}
}
public void rememberWindowAttachCount() {
mOriginalWindowAttachCount = mWindowAttachCount;
}
}
当用户按下View的时间超过长按时间阈值时，就会调用CheckForLongPress.run()方法；在run方法中调用View.performLongClick()方法，处理用户长按事件。
2. move事件
下面再看下onTouchEvent方法对move事件的处理：
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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;
move事件的处理很简单：View接收到move事件时，如果触点移出View时，调用removeTapCallback和removeLongPressCallback，将发送到主线程消息队列的tap、long click消息移除，不再处理点击或长按事件，同时取消PRESSED状态。
3. up事件
再来看下onTouchEvent方法对up事件的处理：
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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;
第8行处，首先View请求焦点；20到25行处理点击事件，最终会调用mOnClickListener的onClick方法；29行到41行处取消View的PRESSED状态。
其中，第11行处会判断mHasPerformedLongPress的值，如果mHasPerformedLongPress为true，则不会处理用户点击事件；mHasPerformedLongPress为false才会处理。那么mHasPerformedLongPress变量是在哪里设置的呢？
mHasPerformedLongPress初始为false，在View接收到down事件时以及发送long click延时消息时都会设置为false；只有在处理长按事件时，调用View.performLongClick()方法返回为true时，才会设置为true。我们看下View.performLongClick()方法：
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/**
* Call this view's OnLongClickListener, if it is defined. Invokes the context menu if the
* OnLongClickListener did not consume the event.
*
* @return True if one of the above receivers consumed the event, false otherwise.
*/
public boolean performLongClick() {
sendAccessibilityEvent(AccessibilityEvent.TYPE_VIEW_LONG_CLICKED);
boolean handled = false;
if (mOnLongClickListener != null) {
handled = mOnLongClickListener.onLongClick(View.this);
}
if (!handled) {
handled = showContextMenu();
}
if (handled) {
performHapticFeedback(HapticFeedbackConstants.LONG_PRESS);
}
return handled;
}
可以看到，只有在回调mOnLongClickListener.onLongClick方法返回true时，performLongClick才会返回true。这就意味着当给View设置了OnLongClickListener且OnLongClickListener.onLongClick方法返回true时，长按View后松开，点击事件不再触发；当OnLongClickListener.onLongClick方法返回false时，长按View时触发长按事件，然后松开触屏后，点击事件依旧会被触发。这在前面的文章中已经用demo验证过的。
相关文章
Android Touch事件传递机制(一) – onInterceptTouchEvent & onTouchEvent
Android Touch事件传递机制(二) – OnTouchListener & OnClickListener & OnLongClickListener
Android Touch事件传递机制(三) – Touch事件分发(dispatchTouchEvent)