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

   /**
   * 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。

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