android事件分发机制的实现原理

android中的事件处理，以及解决滑动冲突问题都离不开事件分发机制，android中的事件流，即motionevent都会经历一个从分发，拦截到处理的一个过程。即dispatchtouchevent(),oninterceptevent()到ontouchevent()的一个过程，在dispatchtouchevent()负责了事件的分发过程，在dispatchtouchevent()中会调用oninterceptevent()与ontouchevent()，如果oninterceptevent()返回true，那么会调用到当前view的ontouchevent()方法，如果不拦截，事件就会下发到子view的dispatchtouchevent()中进行同样的操作。本文将带领大家从源码角度来分析android是如何进行事件分发的。

android中的事件分发流程最先从activity的dispatchtouchevent()开始:

public boolean dispatchtouchevent(motionevent ev) {
  if (ev.getaction() == motionevent.action_down) {
    onuserinteraction();
  }
  if (getwidow().superdispatchtouchevent(ev)) {
    return true;
  }
  return ontouchevent(ev);
}

这里调用了getwindow().superdispatchtouchevent(ev)，这里可以看出activity将motionevent传寄给了window。而window是一个抽象类，superdispatchtouchevent()也是一个抽象方法，这里用到的是window的子类phonewindow。

@override
public boolean superdispatchtouchevent(motionevent event) {
  return mdecor.superdispatchtouchevent(event);
} 

从这里可以看出，event事件被传到了decorview,也就是我们的顶层view.我们继续跟踪：

public boolean superdispatchtouchevent(motionevent event) {
  return super.dispatchtouchevent(event);
}

这里调用到了父类的dispatchtouchevent()方法，而decorview是继承自framelayout，framelayout继承了viewgroup,所以这里会调用到viewgroup的dispatchtouchevent()方法。

所以整个事件流从activity开始，传递到window，最后再到我们的view(viewgroup也是继承自view)中，而view才是我们整个事件处理的核心阶段。

我们来看一下viewgroup的dispatchtouchevent()中的实现：

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();
    }

这是dispatchtouchevent()开始时截取的一段代码，我们来看一下，首先，当我们手指按下view时，会调用到resettouchstate()方法，在resettouchstate()中：

private void resettouchstate() {
  cleartouchtargets();
  resetcancelnextupflag(this);
  mgroupflags &= ~flag_disallow_intercept;
  mnestedscrollaxes = scroll_axis_none;
}

我们继续跟踪cleartouchtargets()方法：

private void cleartouchtargets() {
  touchtarget target = mfirsttouchtarget;
  if (target != null) {
    do {
      touchtarget next = target.next;
      target.recycle();
      target = next;
    } while (target != null);
    mfirsttouchtarget = null;
  }
}

在cleartouchtargets()方法中，我们最终将mfirsttouchtarget赋值为null，我们继续回到dispatchtouchevent()中，接着执行了下段代码：

// check for interception.
final boolean intercepted;
    if (actionmasked == motionevent.action_down
        || mfirsttouchtarget != null) {
      final boolean disallowintercept = (mgroupflags & flag_disallow_intercept) != 0;
      if (!disallowintercept) {
        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;
    }

当view被按下或mfirsttouchtarget != null 的时候，从前面可以知道，当每次view被按下时，也就是重新开始一次事件流的处理时，mfirsttouchtarget都会被设置成null，一会我们看mfirsttouchtarget是什么时候被赋值的。

从disallowintercept属性我们大概能猜到是用来判断是否需要坐拦截处理，而我们知道可以通过调用父view的requestdisallowintercepttouchevent(true)可以让我们的父view不能对事件进行拦截，我们先来看看requestdisallowintercepttouchevent()方法中的实现：

@override
public void requestdisallowintercepttouchevent(boolean disallowintercept) {

  if (disallowintercept == ((mgroupflags & flag_disallow_intercept) != 0)) {
    // we're already in this state, assume our ancestors are too
    return;
  }

  if (disallowintercept) {
    mgroupflags |= flag_disallow_intercept;
  } else {
    mgroupflags &= ~flag_disallow_intercept;
  }

  // pass it up to our parent
  if (mparent != null) {
    mparent.requestdisallowintercepttouchevent(disallowintercept);
  }
}

这里也是通过设置标志位做判断处理，所以这里是通过改变mgroupflags标志，然后在dispatchtouchevent()刚发中变更disallowintercept的值判断是否拦截，当为true时，即需要拦截，这个时候便会跳过onintercepttouchevent()拦截判断，并标记为不拦截，即intercepted = false，我们继续看viewgroup的onintercepttouchevent()处理：

public boolean onintercepttouchevent(motionevent ev) {
  if (ev.isfromsource(inputdevice.source_mouse)
      && ev.getaction() == motionevent.action_down
      && ev.isbuttonpressed(motionevent.button_primary)
      && isonscrollbarthumb(ev.getx(), ev.gety())) {
    return true;
  }
  return false;
}

即默认情况下，只有在action_down时，viewgroup才会表现为拦截。

我们继续往下看：

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 arraylist<view> preorderedlist = buildtouchdispatchchildlist();
   final boolean customorder = preorderedlist == null
              && ischildrendrawingorderenabled();
   final view[] children = mchildren;
   for (int i = childrencount - 1; i >= 0; i--) {
      final int childindex = getandverifypreorderedindex(
                childrencount, i, customorder);
            final view child = getandverifypreorderedview(
                preorderedlist, children, childindex);

            // if there is a view that has accessibility focus we want it
            // to get the event first and if not handled we will perform a
            // normal dispatch. we may do a double iteration but this is
            // safer given the timeframe.
            if (childwithaccessibilityfocus != null) {
              if (childwithaccessibilityfocus != child) {
                continue;
              }
              childwithaccessibilityfocus = null;
              i = childrencount - 1;
            }

            if (!canviewreceivepointerevents(child)
                || !istransformedtouchpointinview(x, y, child, null)) {
              ev.settargetaccessibilityfocus(false);
              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();
              if (preorderedlist != null) {
                // childindex points into presorted list, find original index
                for (int j = 0; j < childrencount; j++) {
                  if (children[childindex] == mchildren[j]) {
                    mlasttouchdownindex = j;
                    break;
                  }
                }
              } else {
                mlasttouchdownindex = childindex;
              }
              mlasttouchdownx = ev.getx();
              mlasttouchdowny = ev.gety();
              newtouchtarget = addtouchtarget(child, idbitstoassign);
              alreadydispatchedtonewtouchtarget = true;
              break;
            }

            // the accessibility focus didn't handle the event, so clear
            // the flag and do a normal dispatch to all children.
            ev.settargetaccessibilityfocus(false);
          }
          if (preorderedlist != null) preorderedlist.clear();
        }

这段代码首先会通过一个循环去遍历所有的子view，最终会调用到dispatchtransformedtouchevent()方法，我们继续看dispatchtransformedtouchevent()的实现：

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) {
      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;
}

这段代码就比较明显了，如果child不为null，始终会调用到child.dispatchtouchevent();否则调用super.dispatchtouchevent();

如果child不为null时，事件就会向下传递，如果子view处理了事件，即dispatchtransformedtouchevent()即返回true。继续向下执行到addtouchtarget()方法，我们继续看addtouchtarget()方法的执行结果：

private touchtarget addtouchtarget(@nonnull view child, int pointeridbits) {
  final touchtarget target = touchtarget.obtain(child, pointeridbits);
  target.next = mfirsttouchtarget;
  mfirsttouchtarget = target;
  return target;
}

这个时候我们发现mfirsttouchtarget又出现了，这时候会给mfirsttouchtarget重新赋值，即mfirsttouchtarget不为null。也就是说，如果事件被当前view或子view消费了，那么在接下来的action_move或action_up事件中，mfirsttouchtarget就不为null。但如果我们继承了该viewgroup，并在onintercepttouchevent()的action_move中拦截了事件，那么后续事件将不会下发，将由该viewgroup直接处理，从下面代码我们可以得到：

// 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;
      }

当存在子view并且事件被子view消费时，即在action_down阶段mfirsttouchtarget会被赋值，即在接下来的action_move事件中，由于intercepted为true，所以将action_cancel 事件传递过去，从dispatchtransformedtouchevent()中可以看到：

if (cancel || oldaction == motionevent.action_cancel) {
    event.setaction(motionevent.action_cancel);
    if (child == null) {
      handled = super.dispatchtouchevent(event);
    } else {
      handled = child.dispatchtouchevent(event);
    }
    event.setaction(oldaction);
    return handled;
  }

并将mfirsttouchtarget 最终赋值为 next，而此时mfirsttouchtarget位于touchtarget链表尾部，所以mfirsttouchtarget会赋值为null，那么接下来的事件将不会进入到onintercepttouchevent()中。也就会直接交由该view处理。

如果我们没有进行事件的拦截，而是交由子view去处理，由于viewgroup的onintercepttouchevent()默认并不会拦截除了action_down以外的事件，所以后续事件将继续交由子view去处理，如果存在子view且事件位于子view内部区域的话。

所以无论是否进行拦截，事件流都会交由view的dispatchtouchevent()中进行处理，我们接下来跟踪一下view中的dispatchtouchevent()处理过程：

if (actionmasked == motionevent.action_down) {
    // defensive cleanup for new gesture
    stopnestedscroll();
  }

  if (onfiltertoucheventforsecurity(event)) {
    if ((mviewflags & enabled_mask) == enabled && handlescrollbardragging(event)) {
      result = true;
    }
    //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;
    }
  }

当被按下时，即action_down时，view会停止内部的滚动，如果view没有被覆盖或遮挡时，首先会进行mlistenerinfo是否为空的判断，我们看下mlistenerinfo是在哪里初始化的：

listenerinfo getlistenerinfo() {
  if (mlistenerinfo != null) {
    return mlistenerinfo;
  }
  mlistenerinfo = new listenerinfo();
  return mlistenerinfo;
}

这里可以看出，mlistenerinfo一般不会是null，知道在我们使用它时调用过这段代码，而当view被加入window中的时候，会调用下面这段代码，从注释中也可以看出来：

/**
 * add a listener for attach state changes.
 *
 * this listener will be called whenever this view is attached or detached
 * from a window. remove the listener using
 * {@link #removeonattachstatechangelistener(onattachstatechangelistener)}.
 *
 * @param listener listener to attach
 * @see #removeonattachstatechangelistener(onattachstatechangelistener)
 */
public void addonattachstatechangelistener(onattachstatechangelistener listener) {
  listenerinfo li = getlistenerinfo();
  if (li.monattachstatechangelisteners == null) {
    li.monattachstatechangelisteners
        = new copyonwritearraylist<onattachstatechangelistener>();
  }
  li.monattachstatechangelisteners.add(listener);
}

到这里我们就知道，mlistenerinfo一开始就是被初始化好了的，所以li不可能为null，li.montouchlistener != null即当设置了touchlistener时不为null，并且view是enabled状态，一般情况view都是enable的。这个时候会调用到ontouch()事件，当ontouch()返回true时，这个时候result会赋值true。而当result为true时，ontouchevent()将不会被调用。

从这里可以看出，ontouch()会优先ontouchevent()调用；
当view设置touch监听并返回true时，那么它的ontouchevent()将被屏蔽。否则会调用ontouchevent()处理。

那么让我们继续来看看ontouchevent()中的事件处理：

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);
  }

首先，当view状态是disabled时，只要view是clickable或long_clickable或context_clickable，都会返回true,而button默认是clickable的，textview默认不是clickable的，而view一般默认都不是long_clickable的。

我们继续向下看：

if (mtouchdelegate != null) {
    if (mtouchdelegate.ontouchevent(event)) {
      return true;
    }
  }

如果有代理事件，仍然会返回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, x, y);
        }
        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;
  }

当view是clickable或long_clickable或context_clickable状态时，当手指抬起时，如果设置了click监听，最终会调用到performclick()，触发click()事件。这点从performclick()方法中可以看出：

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;
}

从这里我们也可以得出，click事件会在ontouchevent()中被调用，如果view设置了ontouch()监听并返回true，那么click事件也会被屏蔽掉，不过我们可以在ontouch()中通过调用view的performclick()继续执行click()事件，这个就看我们的业务中的需求了。

从这里我们可以看出，如果事件没有被当前view或子view处理，即返回false，那么事件就会交由外层view继续处理，直到被消费。

如果事件一直没有被处理，会最终传递到activity的ontouchevent()中。

到这里我们总结一下：

事件是从activity->window->view(viewgroup)的一个传递流程；

如果事件没有被中途拦截，那么它会一直传到最内层的view控件；

如果事件被某一层拦截，那么事件将不会向下传递，交由该view处理。如果该view消费了事件，那么接下来的事件也会交由该view处理；如果该view没有消费该事件，那么事件会交由外层view处理，...并最终调用到activity的ontouchevent()中，除非某一层消费了该事件；

一个事件只能交由一个view处理；

dispatchtouchevent()总是会被调用，而且最先被调用，onintercepttouchevent()和ontouchevent()在dispatchtouchevent()内部调用；

子view不能干扰viewgroup对action_down事件的处理；

子view可以通过requestdisallowintercepttouchevent(true)控制父view不对事件进行拦截，跳过onintercepttouchevent()方法的执行。

以上就是本文的全部内容，希望对大家的学习有所帮助，也希望大家多多支持。