详细分析Android中onTouch事件传递机制

2019-12-12 05:06:15

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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：