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*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*view了，注意：*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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