详细分析Android中onTouch事件传递机制

onTach介绍

ontach是Android系统中整个事件机制的基础。Android中的其他事件，如onClick、onLongClick等都是以onTach为基础的。

onTach包括从手指按下到离开手机屏幕的整个过程，在微观形式上，具体表现为action_down、action_move和action_up等过程。

onTach两种主要定义形式如下：

1.在自定义控件中，常见的有重写onTouchEvent(MotionEvent ev)方法。如在开发中经常可以看到重写的onTouchEvent方法，

并且其中有针对不同的微观表现（action_down、action_move和action_up等）做出的相应判断，执行逻辑并可能返回不同的布尔值。

2.在代码中，直接对现有控件设置setOnTouchListener监听器。并重写监听器的onTouch方法。onTouch回调函数中有view和MotionEvent

onTouch事件传递机制

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：

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。

总结

以上就是关于Android中onTouch事件传递机制的详细分析，希望对各位Android开发者们的学习或者工作能有一定的帮助，如果有疑问大家可以留言交流。