Android源码分析-点击事件派发机制

原文出处：点击打开链接

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

 /**
  * Called to process touch screen events.  You can override this to
  * intercept all touch screen events before they are dispatched to the
  * window.  Be sure to call this implementation for touch screen events
  * that should be handled normally.
  *
  * @param ev The touch screen event.
  *
  * @return boolean Return true if this event was consumed.
  */
 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

 /**
  * Used by custom windows, such as Dialog, to pass the touch screen event
  * further down the view hierarchy. Application developers should
  * not need to implement or call this.
  *
  */
 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

 /**
  * Transforms a motion event into the coordinate space of a particular child view,
  * filters out irrelevant pointer ids, and overrides its action if necessary.
  * If child is null, assumes the MotionEvent will be sent to this ViewGroup instead.
  */
 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

 /**
   * 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 (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。