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)

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;  }}

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;  }}

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。

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。

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:

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:

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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