您的位置:首页 > 移动开发 > Android开发

Android源码分析-点击事件派发机制

2014-04-10 18:14 555 查看
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

[java] view
plaincopy





/**

* 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

[java] view
plaincopy





/**

* 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

[java] view
plaincopy





@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

[java] view
plaincopy





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

[java] view
plaincopy





@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

[java] view
plaincopy





/**

* 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

[java] view
plaincopy





/**

* 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。
内容来自用户分享和网络整理,不保证内容的准确性,如有侵权内容,可联系管理员处理 点击这里给我发消息
标签: