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深入透析Android事件分发机制

时间:2015-11-07 16:12:51      阅读:411      评论:0      收藏:0      [点我收藏+]

标签:android   事件分发   

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向,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。

三、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。

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,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)
技术分享
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;

  }

}

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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;

  }

}

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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。

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;

}

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。

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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。

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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:

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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:

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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。

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http://www.cnblogs.com/duoduohuakai/p/3996385.html**

深入透析Android事件分发机制

标签:android   事件分发   

原文地址:http://blog.csdn.net/taoolee/article/details/49702511

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