文章目录
  1. 1. 概述
  2. 2. Android点击事件的派发机制
    1. 2.1. 1. 从Activity传递到底层View
    2. 2.2. 2.底层View对事件的分发过程
    3. 2.3. 3.无人处理的点击事件
转载请注明出处:http://blog.csdn.net/singwhatiwanna/article/details/17339857

概述

一直想写篇关于Android事件派发机制的文章,却一直没写,这两天刚好是周末,有时间了,想想写一篇吧,不然总是只停留在会用的层次上但是无法了解其内部机制。我用的是4.4源码,打开看看,挺复杂的,尤其是事件是怎么从Activity派发出来的,太费解了。了解Windows消息机制的人会发现,觉得Android的事件派发机制和Windows的消息派发机制挺像的,其实这是一种典型的消息“冒泡”机制,很多平台采用这个机制,消息最先到达最底层View,然后它先进行判断是不是它所需要的,否则就将消息传递给它的子View,这样一来,消息就从水底的气泡一样向上浮了一点距离,以此类推,气泡达到顶部和空气接触,破了(消息被处理了),当然也有气泡浮出到顶层了,还没破(消息无人处理),这个消息将由系统来处理,对于Android来说,会由Activity来处理。

Android点击事件的派发机制

1. 从Activity传递到底层View

点击事件用MotionEvent来表示,当一个点击操作发生时,事件最先传递给当前Activity,由Activity的dispatchTouchEvent来进行事件派发,具体的工作是由Activity内部的Window来完成的,Window会将事件传递给decor view,decor view一般就是当前界面的底层容器(即setContentView所设置的View的父容器),通过Activity.getWindow.getDecorView()可以获得。另外,看下面代码的的时候,主要看我注释的地方,代码很多很复杂,我无法一一说明,但是我注释的地方都是关键点,是博主仔细读代码总结出来的。

源码解读:

事件是由哪里传递给Activity的,这个我还不清楚,但是不要紧,我们从activity开始分析,已经足够我们了解它的内部实现了。

Code:Activity#dispatchTouchEvent

/** 
 * Called to process touch screen events.  You can override this to 
 * intercept all touch screen events before they are dispatched to the 
 * window.  Be sure to call this implementation for touch screen events 
 * that should be handled normally. 
 *  
 * @param ev The touch screen event. 
 *  
 * @return boolean Return true if this event was consumed. 
 */  
public boolean dispatchTouchEvent(MotionEvent ev) {  
    if (ev.getAction() == MotionEvent.ACTION_DOWN) {  
        //这个函数其实是个空函数,啥也没干,如果你没重写的话,不用关心  
        onUserInteraction();  
    }  
    //这里事件开始交给Activity所附属的Window进行派发,如果返回true,整个事件循环就结束了  
    //返回false意味着事件没人处理,所有人的onTouchEvent都返回了false,那么Activity就要来做最后的收场。  
    if (getWindow().superDispatchTouchEvent(ev)) {  
        return true;  
    }  
    //这里,Activity来收场了,Activity的onTouchEvent被调用  
    return onTouchEvent(ev);  
}  

Window是如何将事件传递给ViewGroup的

Code:Window#superDispatchTouchEvent

/** 
 * Used by custom windows, such as Dialog, to pass the touch screen event 
 * further down the view hierarchy. Application developers should 
 * not need to implement or call this. 
 * 
 */  
public abstract boolean superDispatchTouchEvent(MotionEvent event);  

这竟然是一个抽象函数,还注明了应用开发者不要实现它或者调用它,这是什么情况?再看看如下类的说明,大意是说:这个类可以控制顶级View的外观和行为策略,而且还说这个类的唯一一个实现位于android.policy.PhoneWindow,当你要实例化这个Window类的时候,你并不知道它的细节,因为这个类会被重构,只有一个工厂方法可以使用。好吧,还是很模糊啊,不太懂,不过我们可以看一下android.policy.PhoneWindow这个类,尽管实例化的时候此类会被重构,但是重构而已,功能是类似的。

Abstract base class for a top-level window look and behavior policy. An instance of this class should be used as the top-level view added to the window manager. It provides standard UI policies such as a background, title area, default key processing, etc.

The only existing implementation of this abstract class is android.policy.PhoneWindow, which you should instantiate when needing a Window. Eventually that class will be refactored and a factory method added for creating Window instances without knowing about a particular implementation.
Code:PhoneWindow#superDispatchTouchEvent

@Override  
public boolean superDispatchTouchEvent(MotionEvent event) {  
    return mDecor.superDispatchTouchEvent(event);  
}  

这个逻辑很清晰了,PhoneWindow将事件传递给DecorView了,这个DecorView是啥呢,请看下面

private final class DecorView extends FrameLayout implements RootViewSurfaceTaker  

// This is the top-level view of the window, containing the window decor.  
private DecorView mDecor;  

@Override  
public final View getDecorView() {  
    if (mDecor == null) {  
        installDecor();  
    }  
    return mDecor;  
}  

顺便说一下,平时Window用的最多的就是((ViewGroup)getWindow().getDecorView().findViewById(android.R.id.content)).getChildAt(0)即通过Activity来得到内部的View。这个mDecor显然就是getWindow().getDecorView()返回的View,而我们通过setContentView设置的View是它的一个子View。目前事件传递到了DecorView 这里,由于DecorView 继承自FrameLayout且是我们的父View,所以最终事件会传递给我们的View,原因先不管了,换句话来说,事件肯定会传递到我们的View,不然我们的应用如何响应点击事件呢。不过这不是我们的重点,重点是事件到了我们的View以后应该如何传递,这是对我们更有用的。从这里开始,事件已经传递到我们的顶级View了,注意:顶级View实际上是最底层View,也叫根View。

2.底层View对事件的分发过程

点击事件到底层View(一般是一个ViewGroup)以后,会调用ViewGroup的dispatchTouchEvent方法,然后的逻辑是这样的:如果底层ViewGroup拦截事件即onInterceptTouchEvent返回true,则事件由ViewGroup处理,这个时候,如果ViewGroup的mOnTouchListener被设置,则会onTouch会被调用,否则,onTouchEvent会被调用,也就是说,如果都提供的话,onTouch会屏蔽掉onTouchEvent。在onTouchEvent中,如果设置了mOnClickListener,则onClick会被调用。如果顶层ViewGroup不拦截事件,则事件会传递给它的在点击事件链上的子View,这个时候,子View的dispatchTouchEvent会被调用,到此为止,事件已经从最底层View传递给了上一层View,接下来的行为和其底层View一致,如此循环,完成整个事件派发。另外要说明的是,ViewGroup默认是不拦截点击事件的,其onInterceptTouchEvent返回false。

源码解读:

Code:ViewGroup#dispatchTouchEvent

@Override  
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) {  
          //这里判断是否拦截点击事件,如果拦截,则intercepted=true  
                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;  
         //这里面一大堆是派发事件到子View,如果intercepted是true,则直接跳过  
        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 (newTouchTarget == null && childrenCount != 0) {  
                    final float x = ev.getX(actionIndex);  
                    final float y = ev.getY(actionIndex);  
                    // Find a child that can receive the event.  
                    // Scan children from front to back.  
                    final View[] children = mChildren;  

                    final boolean customOrder = isChildrenDrawingOrderEnabled();  
                    for (int i = childrenCount - 1; i >= 0; i--) {  
                        final int childIndex = customOrder ?  
                                getChildDrawingOrder(childrenCount, i) : i;  
                        final View child = children[childIndex];  
                        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 = childIndex;  
                            mLastTouchDownX = ev.getX();  
                            mLastTouchDownY = ev.getY();  
                            //注意下面两句,如果有子View处理了点击事件,则newTouchTarget会被赋值,  
                            //同时alreadyDispatchedToNewTouchTarget也会为true,这两个变量是直接影响下面的代码逻辑的。  
                            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.  
     //这里如果当前ViewGroup拦截了事件,或者其子View的onTouchEvent都返回了false,则事件会由ViewGroup处理  
        if (mFirstTouchTarget == null) {  
            // No touch targets so treat this as an ordinary view.  
          //这里就是ViewGroup对点击事件的处理  
            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;  
}  

下面再看ViewGroup对点击事件的处理

Code:ViewGroup#dispatchTransformedTouchEvent

/** 
 * Transforms a motion event into the coordinate space of a particular child view, 
 * filters out irrelevant pointer ids, and overrides its action if necessary. 
 * If child is null, assumes the MotionEvent will be sent to this ViewGroup instead. 
 */  
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) {  
      //这里就是ViewGroup对点击事件的处理,其调用了View的dispatchTouchEvent方法  
            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;  
}  

再看

Code:View#dispatchTouchEvent

/** 
  * Pass the touch screen motion event down to the target view, or this 
  * view if it is the target. 
  * 
  * @param event The motion event to be dispatched. 
  * @return True if the event was handled by the view, false otherwise. 
  */  
 public boolean dispatchTouchEvent(MotionEvent event) {  
     if (mInputEventConsistencyVerifier != null) {  
         mInputEventConsistencyVerifier.onTouchEvent(event, 0);  
     }  

     if (onFilterTouchEventForSecurity(event)) {  
         //noinspection SimplifiableIfStatement  
         ListenerInfo li = mListenerInfo;  
         if (li != null && li.mOnTouchListener != null && (mViewFlags & ENABLED_MASK) == ENABLED  
                 && li.mOnTouchListener.onTouch(this, event)) {  
             return true;  
         }  

         if (onTouchEvent(event)) {  
             return true;  
         }  
     }  

     if (mInputEventConsistencyVerifier != null) {  
         mInputEventConsistencyVerifier.onUnhandledEvent(event, 0);  
     }  
     return false;  
 }  

这段代码比较简单,View对事件的处理是这样的:如果设置了OnTouchListener就调用onTouch,否则就直接调用onTouchEvent,而onClick是在onTouchEvent内部通过performClick触发的。简单来说,事件如果被ViewGroup拦截或者子View的onTouchEvent都返回了false,则事件最终由ViewGroup处理。

3.无人处理的点击事件

如果一个点击事件,子View的onTouchEvent返回了false,则父View的onTouchEvent会被直接调用,以此类推。如果所有的View都不处理,则最终会由Activity来处理,这个时候,Activity的onTouchEvent会被调用。这个问题已经在1和2中做了说明。


本文出处程序员头条:http://www.iswifting.com/2015/12/26/android-source-analysis/
转载请在开头注明本文出处。

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文章目录
  1. 1. 概述
  2. 2. Android点击事件的派发机制
    1. 2.1. 1. 从Activity传递到底层View
    2. 2.2. 2.底层View对事件的分发过程
    3. 2.3. 3.无人处理的点击事件