详细分析Android中onTouch事件传递机制

ontach介绍

ontach是android系统中整个事件机制的基础。android中的其他事件，如onclick、onlongclick等都是以ontach为基础的。

ontach包括从手指按下到离开手机屏幕的整个过程，在微观形式上，具体表现为action_down、action_move和action_up等过程。

ontach两种主要定义形式如下：

1.在自定义控件中，常见的有重写ontouchevent(motionevent ev)方法。如在开发中经常可以看到重写的ontouchevent方法，

并且其中有针对不同的微观表现（action_down、action_move和action_up等）做出的相应判断，执行逻辑并可能返回不同的布尔值。

2.在代码中，直接对现有控件设置setontouchlistener监听器。并重写监听器的ontouch方法。ontouch回调函数中有view和motionevent

ontouch事件传递机制

大家都知道一般我们使用的ui控件都是继承自共同的父类——view。所以view这个类应该掌管着ontouch事件的相关处理。那就让我们去看看：在view中寻找touch相关的方法，其中一个很容易地引起了我们的注意： dispatchtouchevent(motionevent event) 。

根据方法名的意思应该是负责分发触摸事件的，下面给出了源码：

/**
 * 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 the event should be handled by accessibility focus first.
 if (event.istargetaccessibilityfocus()) {
 // we don't have focus or no virtual descendant has it, do not handle the event.
 if (!isaccessibilityfocusedvieworhost()) {
  return false;
 }
 // we have focus and got the event, then use normal event dispatch.
 event.settargetaccessibilityfocus(false);
 }

 boolean result = false;

 if (minputeventconsistencyverifier != null) {
 minputeventconsistencyverifier.ontouchevent(event, 0);
 }

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

 if (onfiltertoucheventforsecurity(event)) {
 //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;
 }
 }

 if (!result && minputeventconsistencyverifier != null) {
 minputeventconsistencyverifier.onunhandledevent(event, 0);
 }

 // clean up after nested scrolls if this is the end of a gesture;
 // also cancel it if we tried an action_down but we didn't want the rest
 // of the gesture.
 if (actionmasked == motionevent.action_up ||
  actionmasked == motionevent.action_cancel ||
  (actionmasked == motionevent.action_down && !result)) {
 stopnestedscroll();
 }

 return result;
}

源码有点长，但我们不必每一行都看。首先注意到dispatchtouchevent的返回值是boolean类型的，注释上的解释：@return true if the event was handled by the view, false otherwise.也就是说如果该触摸事件被这个view消费了就返回true，否则返回false。在方法中首先判断了该event是否是否得到了焦点，如果没有得到焦点直接返回false。然后让我们把目光转向if (li != null && li.montouchlistener != null&& (mviewflags & enabled_mask) == enabled&& li.montouchlistener.ontouch(this, event))这个片段，看到这里有一个名为li的局部变量，属于 listenerinfo 类，经 mlistenerinfo 赋值得到。listenerinfo只是一个包装类，里面封装了大量的监听器。

再在 view 类中去寻找 mlistenerinfo ，可以看到下面的代码：

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

因此我们可以知道mlistenerinfo是不为空的，所以li也不是空，第一个判断为true，然后看到li.montouchlistener，前面说过listenerinfo是一个监听器的封装类，所以我们同样去追踪montouchlistener：

/**
 * register a callback to be invoked when a touch event is sent to this view.
 * @param l the touch listener to attach to this view
 */
public void setontouchlistener(ontouchlistener l) {
 getlistenerinfo().montouchlistener = l;
}

正是通过上面的方法来设置 montouchlistener 的，我想上面的方法大家肯定都很熟悉吧，正是我们平时经常用的 xxx.setontouchlistener ，好了我们从中得知如果设置了ontouchlistener则第二个判断也为true，第三个判断为如果该view是否为enable，默认都是enable的，所以同样为true。还剩最后一个：li.montouchlistener.ontouch(this, event) ，显然是回调了第二个判断中监听器的ontouch()方法，如果ontouch()方法返回true,则上面四个判断全部为true,dispatchtouchevent()方法会返回true，并且不会执行if (!result && ontouchevent(event))这个判断；而在这个判断中我们又看到了一个熟悉的方法：ontouchevent() 。所以想要执行ontouchevent，则在上面的四个判断中必须至少有一个false。

那就假定我们在ontouch()方法中返回的是false，这样就顺利地执行了ontouchevent，那就看看ontouchevent的源码吧：

/**
 * implement this method to handle touch screen motion events.
 * <p>
 * if this method is used to detect click actions, it is recommended that
 * the actions be performed by implementing and calling
 * {@link #performclick()}. this will ensure consistent system behavior,
 * including:
 * <ul>
 * <li>obeying click sound preferences
 * <li>dispatching onclicklistener calls
 * <li>handling {@link accessibilitynodeinfo#action_click action_click} when
 * accessibility features are enabled
 * </ul>
 *
 * @param event the motion event.
 * @return true if the event was handled, false otherwise.
 */
public boolean ontouchevent(motionevent event) {
 final float x = event.getx();
 final float y = event.gety();
 final int viewflags = mviewflags;
 final int action = event.getaction();

 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);
 }

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

 return false;
}

这段源码比 dispatchtouchevent 的还要长，不过同样我们挑重点的看：
if (((viewflags & clickable) == clickable || (viewflags & long_clickable) == long_clickable) || (viewflags & context_clickable) == context_clickable)
看到这句话就大概知道了主要是判断该view是否是可点击的，如果可以点击则接着执行，否则直接返回false。可以看到if里面用switch来判断是哪种触摸事件，但在最后都是返回true的。

还有一点要注意：在 action_up 中会执行 performclick() 方法：

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;
}

可以看到上面的li.monclicklistener.onclick(this); ，没错，我们好像又有了新的发现。根据上面的经验，这句代码会去回调我们设置好的点击事件监听器。也就是我们平常用的xxx.setonclicklistener(listener);

/**
 * register a callback to be invoked when this view is clicked. if this view is not
 * clickable, it becomes clickable.
 *
 * @param l the callback that will run
 *
 * @see #setclickable(boolean)
 */
public void setonclicklistener(@nullable onclicklistener l) {
 if (!isclickable()) {
 setclickable(true);
 }
 getlistenerinfo().monclicklistener = l;
}

我们可以看到上面方法设置正是mlistenerinfo的点击监听器，验证了上面的猜想。到了这里ontouch事件的传递机制基本已经分析完成了,也算是告一段落了。

好了，这下我们可以解决开头的问题了，顺便我们再来小结一下：在dispatchtouchevent中，如果设置了ontouchlistener并且view是enable的，那么首先被执行的是ontouchlistener中的ontouch(view v, motionevent event) 。若ontouch返回true,则dispatchtouchevent不再往下执行并且返回true；不然会执行ontouchevent，在ontouchevent中若view是可点击的，则返回true，不然为false。还有在ontouchevent中若view是可点击以及当前触摸事件为action_up，会执行performclick() ，回调onclicklistener的onclick方法。

下面是我画的一张草图：

还有一点值得注意的地方是：假如当前事件是action_down，只有dispatchtouchevent返回true了之后该view才会接收到接下来的action_move,action_up事件，也就是说只有事件被消费了才能接收接下来的事件。

总结

以上就是关于android中ontouch事件传递机制的详细分析，希望对各位android开发者们的学习或者工作能有一定的帮助，如果有疑问大家可以留言交流。