Android事件分发机制

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什么？

②   事件

③   View 事件的分发机制

④   ViewGroup事件的分发机制

下面，就让我们沿着大致方针，开始事件分发的探究之旅吧……

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View派生出来的，也就是说ViewGroup的父类就是View。

通常来说，Button、ImageView、TextView等控件都是继承父类View来实现的。RelativeLayout、LinearLayout、FrameLayout等布局都是继承父类ViewGroup来实现的。

事件：

当手指触摸到View或ViewGroup派生的控件后，将会触发一系列的触发响应事件，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。

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

事件的响应原理：

在android开发设计模式中，最广泛应用的就是监听、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，它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setOnClickListener操作

3、         当手指触摸到Button按钮，通过一系列方法（之后将会详细讲解，这里暂时忽略），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，它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上诠释了

注册一个监听对象

实现监听对象的监听事件

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的过程，就离事件驱动开发整个过程就不远了，大道至简，请完全理解了这个例子，再继续之后的学习，事半功倍。

三、      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，再进一步了解。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上清楚了。

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

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。

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。

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。

那么它是如何执行这个流程的呢？我们还以布局上的按钮为例，看看它是如何实现的。（看图①）

 

图①

我们知道，View做为所有控件的父类，它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。

对于上面的按钮，点击它一下，我们期望2种结果，第一种：它响应一个点击事件。第二种：不响应点击事件。

第一种源码：

public class MainActivity extends Activity implements OnClickListener ,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 v) {

       // TODO Auto-generated method stub

       switch (v.getId()) {

       case R.id.btn:

             Log.e("View", "onClick===========>");

             break;

       default:

             break;

       }

  }

 

  @Override

  public boolean onTouch(View v, MotionEvent event) {

       // TODO Auto-generated method stub

       Log.e("View", "onTouch..................................");

       return false;

  }

}

 

                                           （图②）

第二种源码：

public class MainActivity extends Activity implements OnClickListener ,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 v) {

       // TODO Auto-generated method stub

       switch (v.getId()) {

       case R.id.btn:

             Log.e("View", "onClick===========>");

             break;

       default:

             break;

       }

  }

 

  @Override

  public boolean onTouch(View v, MotionEvent event) {

       // TODO Auto-generated method stub

       Log.e("View", "onTouch..................................");

       return true;

  }

}

 

                                               （图③）

 

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false，onClick被执行了，返回true，onClick未被执行。

为什么会这样呢？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，  既然触摸的是按钮Button，那么我们就查看Button的源码，寻找dispatchTouchEvent方法，Button源码中没有dispatchTouchEvent方法，但知道Button继承自TextView，寻找TextView，发现它也没有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。

主要代码如下：

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。

if (onFilterTouchEventForSecurity(event))这个是事件安全过滤，与主题无关，继续看。

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

                    mOnTouchListener.onTouch(this, event)) {

                return true;

            }

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;

    }

代码量太大了，不过不要紧，我们通过主要代码分析一下。

 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;

}

 

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，

 

public boolean dispatchTouchEvent(MotionEvent event) {

         ……

            if (onTouchEvent(event)) {

                return true;

            }

return fasle;

        }

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。

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;

    }

 

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：

      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;

            }

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

  if (mFirstTouchTarget == null) {

                handled = dispatchTransformedTouchEvent(ev, canceled, null,

                        TouchTarget.ALL_POINTER_IDS);

   } else {

       ……

}

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。