Android事件分发机制

一、Android分发机制概述：

             Android如此受欢迎，就在于其优秀的交互性，这其中，Android优秀的事件分发机制功不可没，那么，作为一个优秀的程序员，要想做一个具有良好交互性的应用，必须透彻理解Android的事件分发机制。  要想充分理解android的分发机制，需要先对以下几个知识点有所了解：

       1、View和ViewGroup是什么？

      2、事件。

      3、View事件的分发机制。

      4、ViewGroup事件的分发机制。

二、View和ViewGroup：

      Android的UI界面都是由View和ViewGroup及其派生类组合而成的，其中View是所有UI组件的基类，而ViewGroup是容纳这些组件的容器，其本身也是从ViewGroup中派生出来的，也就是说ViewGroup的父类就是View。通常来说，Button、ImageView、TextView等控价都是继承父类View来实现的。RelativeLayout、LinearLayout、FrameLayout等布局都是继承父类ViewGroup来实现的。

三、事件：

    1、 当手指触摸到View或ViewGroup派生的控件后，将会触发一系列触发响应事件，如：

     onTouchEvent、onClick、onLongClick等。每个View都有自己处理事件的回调方法，开发人员只需要重写这些回调方法，就可以实现需要的响应事件。

    2、  而事件通常重要的有如下三种：

      MotionEvent.ACTION_DOWN       按下View，是所有事件的开始
　  MotionEvent.ACTION_MOVE        滑动事件
　  MotionEvent.ACTION_UP              与down对应，表示抬起

   3、 事件的响应原理：

     在Android开发设计模式中，最广泛的应用就是监听、回调、进而形成了事件响应的过程。

      以Button的OnClick为例，因为Button也是一个View，所以它也拥有View父类的方法,在View中源码如下：

/**定义接口成员变量*/

protected OnClickListener mOnClickListener;

/**

 * Interface definition for a callback to be invoked when a view is clicked.

 */

public interface OnClickListener {

    /**

     * Called when a view has been clicked.

     *

     * @param v The view that was clicked.

     */

    void onClick(View v);

}

/**

 * 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(OnClickListener l) {

    if (!isClickable()) {

        setClickable(true);

    }

    mOnClickListener = l;

}



/**

 * Call this view's OnClickListener, if it is defined.

 *

 * @return True there was an assigned OnClickListener that was called, false

 *         otherwise is returned.

 */

public boolean performClick() {

    sendAccessibilityEvent(AccessibilityEvent.TYPE_VIEW_CLICKED);



    if (mOnClickListener != null) {

        playSoundEffect(SoundEffectConstants.CLICK);

        mOnClickListener.onClick(this);

        return true;

    }



    return false;

}

/**触摸了屏幕后，实现并调用的方法*/

public boolean onTouchEvent(MotionEvent event) {

    …..

    if (mPerformClick == null) {

        mPerformClick = new PerformClick();

    }

    if (!post(mPerformClick)) {

        performClick();

    }

    …..

    ｝

    以上是View源码中关键代码行，以Button为例，假设需要在一个布局上添加一个按钮，并实现它的OnClick事件，需要如下步骤。

   1、OnClickListener类是一个当控件被点击后进行回调的一个接口，他完成被点击后的回调通知。

    2、创建一个按钮Button，并设置监听事件，对这个Button进行setOnCklickListener操作。

    3、当手指触摸到Button按钮，通过一系列方法，触发并执行到OntouchEvent方法并执行mPerformClick方法，在mPerformClick方法中，首先会判断注册的mOnClickListener是否为空，若不为空，他就会回调之前注册的OnClick方法，进而执行用户自定义代码。

   事件响应机制，简单来说上面的例子就已经基本诠释了

    1、注册一个监听对象

    2、实现监听对象的监听事件

    3、当某一触发事件到来，在触发事件中通过注册过的监听对象，回调注册对象的响应事件，来完成用户自定义实现。

   但凡明白了这一个简单的事件响应过程，就离事件驱动开发的整个过程就不远了。

四、View事件的分发机制：

     通过上面的例子，我们初步的接触了View的事件分发机制，在进一步了解，首先，我们要熟悉dispatchTouchEvent和onTouchEvent两个函数，这两个函数都是View的函数，要理解view事件的分发机制，只要清楚这两个函数就基本清楚了。

    在这里先提醒一句，这里的“分发”是指一个触摸或点击事件发生，分发给当前触摸控件所监听的事件（如Click、onTouch等）,进而来决定是控件的那个函数来响应此事件。

     1、dispatchTouchEvent：此函数负责事件的分发，你只需要记住当触摸一个View控件，首先会调用这个函数就行，在这个函数体里决定将事件分发给谁来处理。

      2、onTouchEvent：此函数负责执行事件的处理，负责处理事件，主要处理MotionEvent.ACTION_DOWN、
MotionEvent.ACTION_MOVE 、MotionEvent.ACTION_UP这三个事件。

      3、 public boolean onTouchEvent (MotionEvent event) ：参数event为手机屏幕触摸事件封装类的对象，其中封装了该事件的所有信息，例如触摸的位置，触摸的类型以及触摸的时间等。该对象会在用户触摸手机屏幕时被创建。

     那么他是如何执行这个流程的呢，我们以布局上的一个按钮为例，看看他是如何实现的。

     我们知道，View作为所有控件的父类，他本身定义了很多接口来监听触摸在View上的事件，如OnClickListener（点击）、OnLongClickListener（长按）、OnTouchListener（触摸监听）等。那么当手指触摸到View的时候，该响应“点击”还是“触摸”呢，就是根据dispatchTouchEvent和onTouchEvent这两个函数组合实现的，我们之下的讨论，仅对常用的“点击onclick”和“触摸onTouch”来讨论，顺藤摸瓜，找出主线，进而搞清楚view的事件分发机制。

   对于上面的那个按钮，点击他一下，我们期望两种结果，第一种，他响应一个点击事件，第二种，不响应点击事件。

     第一种结果的源码

public class MainActivity extends AppCompatActivity implements View.OnClickListener,View.OnTouchListener {

        private Button btnButton;
        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_main);
            btnButton= (Button) findViewById(R.id.btn);
            btnButton.setOnClickListener(this);
            btnButton.setOnTouchListener(this);

        }


        @Override
        public void onClick(View view) {
            switch (view.getId()) {
                case R.id.btn:
                   //该方法被执行
                     break;
                default:
                    break;
            }
        }

        @Override
        public boolean onTouch(View view, MotionEvent motionEvent) {
            return false;
        }
}

     第二种结果的源码

    

public class MainActivity extends AppCompatActivity implements View.OnClickListener,View.OnTouchListener {

        private Button btnButton;
        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_main);
            btnButton= (Button) findViewById(R.id.btn);
            btnButton.setOnClickListener(this);
            btnButton.setOnTouchListener(this);

        }


        @Override
        public void onClick(View view) {
            switch (view.getId()) {
                case R.id.btn:
                   //该方法未被执行
                     break;
                default:
                    break;
            }
        }

        @Override
        public boolean onTouch(View view, MotionEvent motionEvent) {
            return true;
        }
}

     结果分析：

    上面两处代码，第一种执行了onClick函数和onTouch函数，第二种执行了onTouch函数，并没有执行onClick函数，而且对两处代码进行比较，发现只有在onTouch处返回值true和false不同。当onTouch返回值为false时，onClick被执行，返回true时，onClick未被执行。

    为什么这样呢？我们只有深入源码才能分析出来。

    前面提到，触摸一个view就会执行dispatchTouchEvent方法去“分发事件”，既然触发的按钮是Button，那么我们就去查看Button源码，寻找dispatchTouchEvent方法，Button源码中没有dispatchTouchEvent方法，但知道Button继承TextView,寻找TextView，发现他也没有dispatchTouchEvent方法。继续查找TextView的父类View,发现View有dispatchTouchEvent方法，那我们就分析dispatchTouchEvent方法。

   代码如下：

public boolean dispatchTouchEvent(MotionEvent event) {

        if (onFilterTouchEventForSecurity(event)) {

             //noinspection SimplifiableIfStatement

            if (mOnTouchListener != null && (mViewFlags & ENABLED_MASK) == ENABLED &&

                     mOnTouchListener.onTouch(this, event)) {

                 return true;

             }

             if (onTouchEvent(event)) {

                return true;

             }

        }

        return false;

 }

     分析：
    先来看dispatchTouchEvent函数返回值，如果返回true，表明事件被处理了，反之，表明事件未被处理。
    这个判定很重要，mOnTouchListener != null，判断该控件是否注册了OnTouchListener对象的监听，(mViewFlags & ENABLED_MASK) == ENABLED，判断当前的控件是否能被点击（比如Button默认可以点击，ImageView默认不许点击，看到这里就了然了），mOnTouchListener.onTouch(this, event)这个是关键，这个调用，就是回调你注册在这个View上的mOnTouchListener对象的onTouch方法，如果你在onTouch方法里返回false，那么这个判断语句就跳出，去执行下面的程序，否则，当前2个都返回了true，自定义onTouch方法也返回true，条件成立，就直接返回了，不再执行下面的程序。接下来，if (onTouchEvent(event)) 这个判断很重要，能否回调OnClickListener接口的onClick函数，关键在于此，可以肯定的是，如果上面if (mOnTouchListener != null && (mViewFlags & ENABLED_MASK) == ENABLED &&mOnTouchListener.onTouch(this, event))返回true，那么就不会执行并回调OnClickListener接口的onClick函数。接下来，我们看onTouchEvent这个函数，看它是如何响应点击事件的。

主要代码如下：


  public boolean onTouchEvent(MotionEvent event) {
 
         final int viewFlags = mViewFlags;
 
 
 
          if ((viewFlags & ENABLED_MASK) == DISABLED) {
 
             if (event.getAction() == MotionEvent.ACTION_UP && (mPrivateFlags & PRESSED) != 0) {

                 mPrivateFlags &= ~PRESSED;
 
                  refreshDrawableState();
 
            }
 
           // 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));
 
          }
 
  
 
          if (mTouchDelegate != null) {
 
              if (mTouchDelegate.onTouchEvent(event)) {
 
                 return true;
 
             }
 
          }
 
  
 
         if (((viewFlags & CLICKABLE) == CLICKABLE ||
 
                  (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE)) {
 
              switch (event.getAction()) {
 
                  case MotionEvent.ACTION_UP:
 
                     boolean prepressed = (mPrivateFlags & PREPRESSED) != 0;
 
                      if ((mPrivateFlags & 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.
 
                           mPrivateFlags |= PRESSED;

                             refreshDrawableState();
 
                         }
 
  
 
                          if (!mHasPerformedLongPress) {
 
                              // 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();
 
                     }
 
                     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 |= PREPRESSED;
                         if (mPendingCheckForTap == null) {
 
                            mPendingCheckForTap = new CheckForTap();
 
                        }

                         postDelayed(mPendingCheckForTap, ViewConfiguration.getTapTimeout());
 
                    } else {
 
                         // Not inside a scrolling container, so show the feedback right away
 
                         mPrivateFlags |= PRESSED;

                        refreshDrawableState();
 
                         checkForLongClick(0);
 
                     }
 
                     break;

 
 
                 case MotionEvent.ACTION_CANCEL:
 
                     mPrivateFlags &= ~PRESSED;
 
                     refreshDrawableState();
 
                     removeTapCallback();
 
                     break;
 
 
 
                 case MotionEvent.ACTION_MOVE:
 
                    final int x = (int) event.getX();
 
                     final int y = (int) event.getY();
 
 
                     // Be lenient about moving outside of buttons
 
                     if (!pointInView(x, y, mTouchSlop)) {
 
                         // Outside button
 
                        removeTapCallback();
 
                         if ((mPrivateFlags & PRESSED) != 0) {
 
                             // Remove any future long press/tap checks
 
                             removeLongPressCallback();
  
 
                             // Need to switch from pressed to not pressed
 
                             mPrivateFlags &= ~PRESSED;

                             refreshDrawableState();
 
                         }

                     }

                    break;
 
             }
 
             return true;
 
         }
 
 
 
         return false;
 
 }

     public boolean performClick() {
 
         sendAccessibilityEvent(AccessibilityEvent.TYPE_VIEW_CLICKED);
 

        if (mOnClickListener != null) {
 
             playSoundEffect(SoundEffectConstants.CLICK);
 
             mOnClickListener.onClick(this);
 
             return true;
 
         }
 
 
 
         return false;
 
     }

复制代码

 

      代码量太大了，不过不要紧，我们通过主要代码分析一下。

 
复制代码

   public boolean onTouchEvent(MotionEvent event) {
  
       
  
          //控件不能被点击
 
          if ((viewFlags & ENABLED_MASK) == DISABLED) {
  
                …
 
          }
 
  //委托代理别的View去实现
 
          if (mTouchDelegate != null) {
 
              if (mTouchDelegate.onTouchEvent(event)) {
 
                  return true;
 
              }
 
          }
 
          //控件能够点击或者长按
 
          if (((viewFlags & CLICKABLE) == CLICKABLE ||
 
                  (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE)) {
 
              switch (event.getAction()) {
 
              //抬起事件
 
                  case MotionEvent.ACTION_UP:
 
                            …...
 
                              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)) {
 
                         //这里就是去执行回调注册的onClick函数，实现点击
 
                                      performClick();
 
                                  }
 
                              }
 
                              ……
 
                      break;
 
             //按下事件
 
                  case MotionEvent.ACTION_DOWN:
 
                      
 
                      ……
 
                      break;
 
  
 
                 ……
 
            //移动事件
 
                  case MotionEvent.ACTION_MOVE:
 
                       ……
 
                     break;
 
             }
 
       
 
              return true;

          }
         return false;
 
 }

 

复制代码

 

 

 

      从上面主要代码可以看出onTouchEvent传参MotionEvent类型，它封装了触摸的活动事件，其中就有MotionEvent.ACTION_DOWN、MotionEvent.ACTION_MOVE、MotionEvent.ACTION_UP三个事件。我们在来看看onTouchEvent的返回值，因为onTouchEvent是在dispatchTouchEvent事件分发处理中调用的，
复制代码

  public boolean dispatchTouchEvent(MotionEvent event) {
 
           ……
 
              if (onTouchEvent(event)) {
 
                  return true;
 
              }
 
 return fasle;
 
         }

复制代码

 

      如果onTouchEvent返回true，dispatchTouchEvent就返回true，表明事件被处理了，反之，事件未被处理。

 

程序的关键在  if (((viewFlags & CLICKABLE) == CLICKABLE ||

                (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE))的判断里，我们发现无论switch的分支在什么地方跳出，返回都是true。这就表明，无论是三个事件中的哪一个，都会返回true。

      参照下图，结合上述，不难理解View的分发机制了。

 

                                      （图④）

四、      ViewGroup事件分发机制：

      ViewGroup事件分发机制较View的稍微复杂一些，不过对View的机制只要精确的理解后，仔细看过这一节，睡几觉起来，估计也就悟出来了，学习就是这么奇怪，当下理解不了或模糊的地方，只要脑子有印象，忽然一夜好像就懂了。

      先来看下面的一个简单布局，我们将通过例子，了解ViewGroup+View的android事件处理机制。

                  

                                           （图⑤）

      上图由：黑色为线性布局LinearLayout，紫色为相对布局RelativeLayout，按钮Button三部分组成。RelativeLayout为LinearLayout的子布局，Button为RelativeLayout的子布局。以下RelativeLayout简称（R），LinearLayout简称（L），Button简称（B）。

      经过前面讲解，我们首先知道这样两件事情。

1、（R）和（L）的父类是ViewGroup，（B）的父类是View。

2、dispatchTouchEvent这个函数很重要，不论是ViewGroup还是View，都由它来处理事件的消费和传递。

      下面，我们通过横向和纵向两个维度，通过源码和图解的方式，充分理解事件的传递机制。

      先来看整体的事件传递过程：

                                                             （图⑥）

      当手指点击按钮B时，事件传递的顺序是从底向上传递的，也就是按照L->R->B的顺序由下往上逐层传递，响应正好相反，是自上而下。

      L首先接收到点击事件，L的父类是ViewGroup类，并将事件传递给dispatchTouchEvent方法，dispatchTouchEvent函数中判断该控件L是否重载了onInterceptTouchEvent方法进行事件拦截，onInterceptTouchEvent默认返回false不拦截，那么dispatchTouchEvent方法将事件传递给R去处理（进入第2流程处理），如果返回true表示当前L控件拦截了事件向其它控件的传递，交给它自己父类View的dispatchTouchEvent去处理，在父方法的dispatchTouchEvent中，将会按照前面讲的View的事件处理机制去判断，比如判断L是否重载了onTouch方法，是否可点击，是否做了监听等事件。

      R也是ViewGroup的子类，因此与第1流程基本相似，如果onInterceptTouchEvent返回了false，表示事件将不拦截继续传递给B。

      B是View的子类，它没有onInterceptTouchEvent方法，直接交给自己父类View的dispatchTouchEvent去处理，流程同不再敷述。

      总结：

      onInterceptTouchEvent只有ViewGroup才有，当一个控件是继承自ViewGroup而来的，那么它就可能会有子控件，因此，才有可能传递给子控件，而继承自View的控件，不会有子控件，也就没有onInterceptTouchEvent函数了。

      通过dispatchTouchEvent分发的控件返回值True和false，表示当前控件是否消费了传递过来的事件，如果消费了，返回True，反之false。消费了，就不再继续传递了，没有消费，如果有子控件将继续传递。

      啰嗦点，如果想再深层次了解一下，再次从源码ViewGroup来分析一个L控件的事件传递过程，请看下图：

                                                      （图⑦）

      结合上面的图例，下面列出ViewGroup源码来分析一下，我们只需要分析ViewGroup的dispatchTouchEvent、onInterceptTouchEvent、dispatchTransformedTouchEvent三个方法即可。
复制代码

   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) {
 
                      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;
 
              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 (childrenCount != 0) {
 
                         // Find a child that can receive the event.
 
                         // Scan children from front to back.

                        final View[] children = mChildren;
 
                         final float x = ev.getX(actionIndex);
 
                         final float y = ev.getY(actionIndex);
 
 

                        for (int i = childrenCount - 1; i >= 0; i--) {

                             final View child = children[i];
 
                             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 = i;
 
                                 mLastTouchDownX = ev.getX();
 
                                 mLastTouchDownY = ev.getY();
 
                                 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.
 
             if (mFirstTouchTarget == null) {
 
                // No touch targets so treat this as an ordinary view.
 
                 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;
 
 }
 
   public boolean onInterceptTouchEvent(MotionEvent ev) {
 
         return false;
 
     }
 
   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;
 
     }
 
 

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      代码量比较大，我们先概述一下各个函数的主要作用。

      dispatchTouchEvent主要用来分发事件，函数主要作用是来决定当前的事件是交由自己消费处理，还是交由子控件处理。

      onInterceptTouchEvent主要来决定当前控件是否需要拦截传递给子控件，如果返回True表示该控件拦截，并交由自己父类的dispatchTouchEvent处理消费，如果返回false表示不拦截，允许传递给子控件处理。

      dispatchTransformedTouchEvent主要根据传来的子控件，决定是自身处理消费，还是交由子控件处理消费。

我们主要来分析一下dispatchTouchEvent函数：

 
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     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;
 
             }

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      这段代码，如果当前传递的事件是Down（按下）或者当前触摸链表不为空，那么它调用onInterceptTouchEvent函数，判断是否进行事件拦截处理，通过返回值来决定intercepted变量的值。

接下来if (!canceled && !intercepted){} 这个括号内的代码需要注意了，只有当intercepted返回值为false的时候，才满足这个条件进入代码段。因此，我们结合onInterceptTouchEvent源码，发现它默认值返回的是false，也就说如果你不重载onInterceptTouchEvent方法并令其返回True，它一定是返回false，并能够执行花括号内的代码。

      我们分析一下花括号中的代码，if (actionMasked == MotionEvent.ACTION_DOWN

                        || (split && actionMasked == MotionEvent.ACTION_POINTER_DOWN)

                        || actionMasked == MotionEvent.ACTION_HOVER_MOVE) {}判断当前的事件是否是ACTION_DOWN、ACTION_POINTER_DOWN（多点触摸）、ACTION_HOVER_MOVE(悬停)，如果是，执行花括号内代码，

final int childrenCount = mChildrenCount;

if (childrenCount != 0) {}判断当前控件是否有子控件，如果大于0，执行花括号内代码，

  for (int i = childrenCount - 1; i >= 0; i--)遍历子控件，

if (!canViewReceivePointerEvents(child)

      判断当前的down、POINTER_DOWN、HOVER_MOVE三个事件的坐标点是否落在了子控件上，如果落在子控件上，

if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign))

      通过dispatchTransformedTouchEvent传递事件，交由子控件判断是否传递或自己消费处理。如果dispatchTransformedTouchEvent返回true，表示子控件已消费处理，并添加此子控件View到触摸链表，并放置链表头，并结束遍历子控件。newTouchTarget = addTouchTarget(child, idBitsToAssign);false表示未处理。

      接着分析

 
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   if (mFirstTouchTarget == null) {
 
                  handled = dispatchTransformedTouchEvent(ev, canceled, null,
 
                         TouchTarget.ALL_POINTER_IDS);
 
    } else {
 
        ……
 
 }

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      mFirstTouchTarget什么时候为空呢？从前面的代码可以看到，如果onInterceptTouchEvent返回为false（也就是不拦截），mFirstTouchTarget就为空，直接交给自己父View执行dispatchTouchEvent去了。如果mFirstTouchTarget不为空，它就取出触摸链表，逐个遍历判断处理，如果前面比如Down事件处理过了，就不再处理了。