Android点击事件派发机制源码分析

概述 

一直想写篇关于Android事件派发机制的文章，却一直没写，这两天刚好是周末，有时间了，想想写一篇吧，不然总是只停留在会用的层次上但是无法了解其内部机制。我用的是4.4源码，打开看看，挺复杂的，尤其是事件是怎么从Activity派发出来的，太费解了。了解Windows消息机制的人会发现，觉得Android的事件派发机制和Windows的消息派发机制挺像的，其实这是一种典型的消息“冒泡”机制，很多平台采用这个机制，消息最先到达最底层View，然后它先进行判断是不是它所需要的，否则就将消息传递给它的子View，这样一来，消息就从水底的气泡一样向上浮了一点距离，以此类推，气泡达到顶部和空气接触，破了（消息被处理了），当然也有气泡浮出到顶层了，还没破（消息无人处理），这个消息将由系统来处理，对于Android来说，会由Activity来处理。 

Android点击事件的派发机制 

1. 从Activity传递到底层View

 点击事件用MotionEvent来表示，当一个点击操作发生时，事件最先传递给当前Activity，由Activity的dispatchTouchEvent来进行事件派发，具体的工作是由Activity内部的Window来完成的，Window会将事件传递给decor view，decor view一般就是当前界面的底层容器（即setContentView所设置的View的父容器），通过Activity.getWindow.getDecorView（）可以获得。另外，看下面代码的的时候，主要看我注释的地方，代码很多很复杂，我无法一一说明，但是我注释的地方都是关键点，是博主仔细读代码总结出来的。 

源码解读： 

事件是由哪里传递给Activity的，这个我还不清楚，但是不要紧，我们从activity开始分析，已经足够我们了解它的内部实现了。 

Code：Activity#dispatchTouchEvent

   
  public boolean dispatchTouchEvent(MotionEvent ev) {
    if (ev.getAction() == MotionEvent.ACTION_DOWN) {
      //这个函数其实是个空函数，啥也没干，如果你没重写的话，不用关心
      onUserInteraction();
    }
    //这里事件开始交给Activity所附属的Window进行派发，如果返回true，整个事件循环就结束了
    //返回false意味着事件没人处理，所有人的onTouchEvent都返回了false，那么Activity就要来做最后的收场。
    if (getWindow().superDispatchTouchEvent(ev)) {
      return true;
    }
    //这里，Activity来收场了，Activity的onTouchEvent被调用
    return onTouchEvent(ev);
  } 

Window是如何将事件传递给ViewGroup的

Code：Window#superDispatchTouchEvent 

  
  public abstract boolean superDispatchTouchEvent(MotionEvent event);

这竟然是一个抽象函数，还注明了应用开发者不要实现它或者调用它，这是什么情况？再看看如下类的说明，大意是说：这个类可以控制顶级View的外观和行为策略，而且还说这个类的唯一一个实现位于android.policy.PhoneWindow，当你要实例化这个Window类的时候，你并不知道它的细节，因为这个类会被重构，只有一个工厂方法可以使用。好吧，还是很模糊啊，不太懂，不过我们可以看一下android.policy.PhoneWindow这个类，尽管实例化的时候此类会被重构，但是重构而已，功能是类似的。 

Abstract base class for a top-level window look and behavior policy. An instance of this class should be used as the top-level view added to the window manager. It provides standard UI policies such as a background, title area, default key processing, etc.The only existing implementation of this abstract class is android.policy.PhoneWindow, which you should instantiate when needing a Window. Eventually that class will be refactored and a factory method added for creating Window instances without knowing about a particular implementation.  

Code：PhoneWindow#superDispatchTouchEvent

@Override
  public boolean superDispatchTouchEvent(MotionEvent event) {
    return mDecor.superDispatchTouchEvent(event);
  }这个逻辑很清晰了，PhoneWindow将事件传递给DecorView了，这个DecorView是啥呢，请看下面 
  private final class DecorView extends frameLayout implements RootViewSurfaceTaker

  // This is the top-level view of the window, containing the window decor.
  private DecorView mDecor;

  @Override
  public final View getDecorView() {
    if (mDecor == null) {
      installDecor();
    }
    return mDecor;
  }

顺便说一下，平时Window用的最多的就是((ViewGroup)getWindow().getDecorView().findViewById(android.R.id.content)).getChildAt(0)即通过Activity来得到内部的View。这个mDecor显然就是getWindow().getDecorView()返回的View，而我们通过setContentView设置的View是它的一个子View。目前事件传递到了DecorView 这里，由于DecorView 继承自frameLayout且是我们的父View，所以最终事件会传递给我们的View，原因先不管了，换句话来说，事件肯定会传递到我们的View，不然我们的应用如何响应点击事件呢。不过这不是我们的重点，重点是事件到了我们的View以后应该如何传递，这是对我们更有用的。从这里开始，事件已经传递到我们的顶级View了，注意：顶级View实际上是最底层View，也叫根View。 

2.底层View对事件的分发过程 

点击事件到底层View（一般是一个ViewGroup）以后，会调用ViewGroup的dispatchTouchEvent方法，然后的逻辑是这样的：如果底层ViewGroup拦截事件即onInterceptTouchEvent返回true，则事件由ViewGroup处理，这个时候，如果ViewGroup的mOnTouchListener被设置，则会onTouch会被调用，否则，onTouchEvent会被调用，也就是说，如果都提供的话，onTouch会屏蔽掉onTouchEvent。在onTouchEvent中，如果设置了mOnClickListener，则onClick会被调用。如果顶层ViewGroup不拦截事件，则事件会传递给它的在点击事件链上的子View，这个时候，子View的dispatchTouchEvent会被调用，到此为止，事件已经从最底层View传递给了上一层View，接下来的行为和其底层View一致，如此循环，完成整个事件派发。另外要说明的是，ViewGroup默认是不拦截点击事件的，其onInterceptTouchEvent返回false。 

源码解读： 

Code：ViewGroup#dispatchTouchEvent 

  @Override
  public boolean dispatchTouchEvent(MotionEvent ev) {
    if (mInputEventConsistencyVerifier != null) {
      mInputEventConsistencyVerifier.onTouchEvent(ev, 1);
    }

    boolean handled = false;
    if (onFilterTouchEventForSecurity(ev)) {
      final int action = ev.getAction();
      final int actionMasked = action & MotionEvent.ACTION_MASK;

      // Handle an initial down.
      if (actionMasked == MotionEvent.ACTION_DOWN) {
 // Throw away all previous state when starting a new touch gesture.
 // The framework may have dropped the up or cancel event for the previous gesture
 // due to an app switch, ANR, or some other state change.
 cancelAndClearTouchTargets(ev);
 resetTouchState();
      }

      // Check for interception.
      final boolean intercepted;
      if (actionMasked == MotionEvent.ACTION_DOWN
   || mFirstTouchTarget != null) {
 final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
 if (!disallowIntercept) {
//这里判断是否拦截点击事件，如果拦截，则intercepted=true
   intercepted = onInterceptTouchEvent(ev);
   ev.setAction(action); // restore action in case it was changed
 } else {
   intercepted = false;
 }
      } else {
 // There are no touch targets and this action is not an initial down
 // so this view group continues to intercept touches.
 intercepted = true;
      }

      // Check for cancelation.
      final boolean canceled = resetCancelNextUpFlag(this)
   || actionMasked == MotionEvent.ACTION_CANCEL;

      // Update list of touch targets for pointer down, if needed.
      final boolean split = (mGroupFlags & FLAG_SPLIT_MOTION_EVENTS) != 0;
      TouchTarget newTouchTarget = null;
      boolean alreadyDispatchedTonewTouchTarget = false;
//这里面一大堆是派发事件到子View，如果intercepted是true，则直接跳过
      if (!canceled && !intercepted) {
 if (actionMasked == MotionEvent.ACTION_DOWN
     || (split && actionMasked == MotionEvent.ACTION_POINTER_DOWN)
     || actionMasked == MotionEvent.ACTION_HOVER_MOVE) {
   final int actionIndex = ev.getActionIndex(); // always 0 for down
   final int idBitsToAssign = split ? 1 << ev.getPointerId(actionIndex)
: TouchTarget.ALL_POINTER_IDS;

   // Clean up earlier touch targets for this pointer id in case they
   // have become out of sync.
   removePointersFromTouchTargets(idBitsToAssign);

   final int childrenCount = mChildrenCount;
   if (newTouchTarget == null && childrenCount != 0) {
     final float x = ev.getX(actionIndex);
     final float y = ev.getY(actionIndex);
     // Find a child that can receive the event.
     // Scan children from front to back.
     final View[] children = mChildren;

     final boolean customOrder = isChildrenDrawingOrderEnabled();
     for (int i = childrenCount - 1; i >= 0; i--) {
final int childIndex = customOrder ?
    getChildDrawingOrder(childrenCount, i) : i;
final View child = children[childIndex];
if (!canViewReceivePointerEvents(child)
    || !isTransformedTouchPointInView(x, y, child, null)) {
  continue;
}

newTouchTarget = getTouchTarget(child);
if (newTouchTarget != null) {
  // Child is already receiving touch within its bounds.
  // Give it the new pointer in addition to the ones it is handling.
  newTouchTarget.pointerIdBits |= idBitsToAssign;
  break;
}

resetCancelNextUpFlag(child);
if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) {
  // Child wants to receive touch within its bounds.
  mLastTouchDownTime = ev.getDownTime();
  mLastTouchDownIndex = childIndex;
  mLastTouchDownX = ev.getX();
  mLastTouchDownY = ev.getY();
  //注意下面两句，如果有子View处理了点击事件，则newTouchTarget会被赋值，
  //同时alreadyDispatchedToNewTouchTarget也会为true，这两个变量是直接影响下面的代码逻辑的。
  newTouchTarget = addTouchTarget(child, idBitsToAssign);
  alreadyDispatchedTonewTouchTarget = true;
  break;
}
     }
   }

   if (newTouchTarget == null && mFirstTouchTarget != null) {
     // Did not find a child to receive the event.
     // Assign the pointer to the least recently added target.
     newTouchTarget = mFirstTouchTarget;
     while (newTouchTarget.next != null) {
newTouchTarget = newTouchTarget.next;
     }
     newTouchTarget.pointerIdBits |= idBitsToAssign;
   }
 }
      }

      // Dispatch to touch targets.
     //这里如果当前ViewGroup拦截了事件，或者其子View的onTouchEvent都返回了false，则事件会由ViewGroup处理
      if (mFirstTouchTarget == null) {
 // No touch targets so treat this as an ordinary view.
//这里就是ViewGroup对点击事件的处理
 handled = dispatchTransformedTouchEvent(ev, canceled, null,
     TouchTarget.ALL_POINTER_IDS);
      } else {
 // Dispatch to touch targets, excluding the new touch target if we already
 // dispatched to it. Cancel touch targets if necessary.
 TouchTarget predecessor = null;
 TouchTarget target = mFirstTouchTarget;
 while (target != null) {
   final TouchTarget next = target.next;
   if (alreadyDispatchedTonewTouchTarget && target == newTouchTarget) {
     handled = true;
   } else {
     final boolean cancelChild = resetCancelNextUpFlag(target.child)
  || intercepted;
     if (dispatchTransformedTouchEvent(ev, cancelChild,
  target.child, target.pointerIdBits)) {
handled = true;
     }
     if (cancelChild) {
if (predecessor == null) {
  mFirstTouchTarget = next;
} else {
  predecessor.next = next;
}
target.recycle();
target = next;
continue;
     }
   }
   predecessor = target;
   target = next;
 }
      }

      // Update list of touch targets for pointer up or cancel, if needed.
      if (canceled
   || actionMasked == MotionEvent.ACTION_UP
   || actionMasked == MotionEvent.ACTION_HOVER_MOVE) {
 resetTouchState();
      } else if (split && actionMasked == MotionEvent.ACTION_POINTER_UP) {
 final int actionIndex = ev.getActionIndex();
 final int idBitsToRemove = 1 << ev.getPointerId(actionIndex);
 removePointersFromTouchTargets(idBitsToRemove);
      }
    }

    if (!handled && mInputEventConsistencyVerifier != null) {
      mInputEventConsistencyVerifier.onUnhandledEvent(ev, 1);
    }
    return handled;
  }

 下面再看ViewGroup对点击事件的处理

Code：ViewGroup#dispatchTransformedTouchEvent

   
  private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel,
      View child, int desiredPointerIdBits) {
    final boolean handled;

    // Canceling motions is a special case. We don't need to perform any transformations
    // or filtering. The important part is the action, not the contents.
    final int oldAction = event.getAction();
    if (cancel || oldAction == MotionEvent.ACTION_CANCEL) {
      event.setAction(MotionEvent.ACTION_CANCEL);
      if (child == null) {
     //这里就是ViewGroup对点击事件的处理，其调用了View的dispatchTouchEvent方法
 handled = super.dispatchTouchEvent(event);
      } else {
 handled = child.dispatchTouchEvent(event);
      }
      event.setAction(oldAction);
      return handled;
    }

    // Calculate the number of pointers to deliver.
    final int oldPointerIdBits = event.getPointerIdBits();
    final int newPointerIdBits = oldPointerIdBits & desiredPointerIdBits;

    // If for some reason we ended up in an inconsistent state where it looks like we
    // might produce a motion event with no pointers in it, then drop the event.
    if (newPointerIdBits == 0) {
      return false;
    }

    // If the number of pointers is the same and we don't need to perform any fancy
    // irreversible transformations, then we can reuse the motion event for this
    // dispatch as long as we are careful to revert any changes we make.
    // Otherwise we need to make a copy.
    final MotionEvent transformedEvent;
    if (newPointerIdBits == oldPointerIdBits) {
      if (child == null || child.hasIdentityMatrix()) {
 if (child == null) {
   handled = super.dispatchTouchEvent(event);
 } else {
   final float offsetX = mScrollX - child.mLeft;
   final float offsetY = mScrollY - child.mTop;
   event.offsetLocation(offsetX, offsetY);

   handled = child.dispatchTouchEvent(event);

   event.offsetLocation(-offsetX, -offsetY);
 }
 return handled;
      }
      transformedEvent = MotionEvent.obtain(event);
    } else {
      transformedEvent = event.split(newPointerIdBits);
    }

    // Perform any necessary transformations and dispatch.
    if (child == null) {
      handled = super.dispatchTouchEvent(transformedEvent);
    } else {
      final float offsetX = mScrollX - child.mLeft;
      final float offsetY = mScrollY - child.mTop;
      transformedEvent.offsetLocation(offsetX, offsetY);
      if (! child.hasIdentityMatrix()) {
 transformedEvent.transform(child.getInverseMatrix());
      }

      handled = child.dispatchTouchEvent(transformedEvent);
    }

    // Done.
    transformedEvent.recycle();
    return handled;
  }

再看

Code：View#dispatchTouchEvent

  
  public boolean dispatchTouchEvent(MotionEvent event) {
    if (mInputEventConsistencyVerifier != null) {
      mInputEventConsistencyVerifier.onTouchEvent(event, 0);
    }

    if (onFilterTouchEventForSecurity(event)) {
      //noinspection SimplifiableIfStatement
      ListenerInfo li = mListenerInfo;
      if (li != null && li.monTouchListener != null && (mViewFlags & ENABLED_MASK) == ENABLED
   && li.mOnTouchListener.onTouch(this, event)) {
 return true;
      }

      if (onTouchEvent(event)) {
 return true;
      }
    }

    if (mInputEventConsistencyVerifier != null) {
      mInputEventConsistencyVerifier.onUnhandledEvent(event, 0);
    }
    return false;
  }

这段代码比较简单，View对事件的处理是这样的：如果设置了OnTouchListener就调用onTouch，否则就直接调用onTouchEvent，而onClick是在onTouchEvent内部通过performClick触发的。简单来说，事件如果被ViewGroup拦截或者子View的onTouchEvent都返回了false，则事件最终由ViewGroup处理。 

3.无人处理的点击事件 

如果一个点击事件，子View的onTouchEvent返回了false，则父View的onTouchEvent会被直接调用，以此类推。如果所有的View都不处理，则最终会由Activity来处理，这个时候，Activity的onTouchEvent会被调用。这个问题已经在1和2中做了说明。

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