android viewgroup事件分发机制

今天给大家代码ViewGroup事件分发的源码解析~~凡是自定义ViewGroup实现各种滑动效果的，不可避免的会出现很多事件的冲突，对ViewGroup事件分发机制的了解，也有益于大家了解冲突产生的原因，以及对冲突进行处理~

1、案例

首先我们接着上一篇的代码，在代码中添加一个自定义的LinearLayout：

[java]
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package com.example.zhy_event03;

import android.content.Context;
import android.util.AttributeSet;
import android.util.Log;
import android.view.MotionEvent;
import android.widget.LinearLayout;

public class MyLinearLayout extends LinearLayout
{
private static final String TAG = MyLinearLayout.class.getSimpleName();

public MyLinearLayout(Context context, AttributeSet attrs)
{
super(context, attrs);
}

@Override
public boolean dispatchTouchEvent(MotionEvent ev)
{
int action = ev.getAction();
switch (action)
{
case MotionEvent.ACTION_DOWN:
Log.e(TAG, "dispatchTouchEvent ACTION_DOWN");
break;
case MotionEvent.ACTION_MOVE:
Log.e(TAG, "dispatchTouchEvent ACTION_MOVE");
break;
case MotionEvent.ACTION_UP:
Log.e(TAG, "dispatchTouchEvent ACTION_UP");
break;

default:
break;
}
return super.dispatchTouchEvent(ev);
}

@Override
public boolean onTouchEvent(MotionEvent event)
{

int action = event.getAction();

switch (action)
{
case MotionEvent.ACTION_DOWN:
Log.e(TAG, "onTouchEvent ACTION_DOWN");
break;
case MotionEvent.ACTION_MOVE:
Log.e(TAG, "onTouchEvent ACTION_MOVE");
break;
case MotionEvent.ACTION_UP:
Log.e(TAG, "onTouchEvent ACTION_UP");
break;

default:
break;
}

return super.onTouchEvent(event);
}

@Override
public boolean onInterceptTouchEvent(MotionEvent ev)
{

int action = ev.getAction();
switch (action)
{
case MotionEvent.ACTION_DOWN:
Log.e(TAG, "onInterceptTouchEvent ACTION_DOWN");
break;
case MotionEvent.ACTION_MOVE:
Log.e(TAG, "onInterceptTouchEvent ACTION_MOVE");
break;
case MotionEvent.ACTION_UP:
Log.e(TAG, "onInterceptTouchEvent ACTION_UP");
break;

default:
break;
}

return super.onInterceptTouchEvent(ev);
}

@Override
public void requestDisallowInterceptTouchEvent(boolean disallowIntercept)
{
Log.e(TAG, "requestDisallowInterceptTouchEvent ");
super.requestDisallowInterceptTouchEvent(disallowIntercept);
}

}

继承LinearLayout，然后复写了与事件分发机制有关的代码，添加上了日志的打印~
然后看我们的布局文件：

[html]
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<com.example.zhy_event03.MyLinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".MainActivity" >

<com.example.zhy_event03.MyButton
android:id="@+id/id_btn"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="click me" />

</com.example.zhy_event03.MyLinearLayout>

MyLinearLayout中包含一个MyButton，MyButton都上篇博客中已经出现过，这里就不再贴代码了，不清楚可以去查看~
然后MainActivity就是直接加载布局，没有任何代码~~~

直接运行我们的代码，然后点击我们的Button，依然是有意的MOVE一下，不然不会触发MOVE事件，看一下日志的输出：

[html]
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09-06 09:57:27.287: E/MyLinearLayout(959): dispatchTouchEvent ACTION_DOWN
09-06 09:57:27.287: E/MyLinearLayout(959): onInterceptTouchEvent ACTION_DOWN
09-06 09:57:27.287: E/MyButton(959): dispatchTouchEvent ACTION_DOWN
09-06 09:57:27.297: E/MyButton(959): onTouchEvent ACTION_DOWN
09-06 09:57:27.297: E/MyButton(959): onTouchEvent ACTION_MOVE
09-06 09:57:27.327: E/MyLinearLayout(959): dispatchTouchEvent ACTION_MOVE
09-06 09:57:27.327: E/MyLinearLayout(959): onInterceptTouchEvent ACTION_MOVE
09-06 09:57:27.337: E/MyButton(959): dispatchTouchEvent ACTION_MOVE
09-06 09:57:27.337: E/MyButton(959): onTouchEvent ACTION_MOVE
09-06 09:57:27.457: E/MyLinearLayout(959): dispatchTouchEvent ACTION_UP
09-06 09:57:27.457: E/MyLinearLayout(959): onInterceptTouchEvent ACTION_UP
09-06 09:57:27.457: E/MyButton(959): dispatchTouchEvent ACTION_UP
09-06 09:57:27.457: E/MyButton(959): onTouchEvent ACTION_UP

可以看到大体的事件流程为：
MyLinearLayout的dispatchTouchEvent -> MyLinearLayout的onInterceptTouchEvent -> MyButton的dispatchTouchEvent ->Mybutton的onTouchEvent

可以看出，在View上触发事件，最先捕获到事件的为View所在的ViewGroup，然后才会到View自身~

下面我们按照日志的输出，进入源码~

2、源码分析

ViewGroup - dispatchTouchEvent

1、ViewGroup - dispatchTouchEvent - ACTION_DOWN

首先是ViewGroup的dispatchTouchEvent方法：

[java]
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@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
if (!onFilterTouchEventForSecurity(ev)) {
return false;
}

final int action = ev.getAction();
final float xf = ev.getX();
final float yf = ev.getY();
final float scrolledXFloat = xf + mScrollX;
final float scrolledYFloat = yf + mScrollY;
final Rect frame = mTempRect;

boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;

if (action == MotionEvent.ACTION_DOWN) {
if (mMotionTarget != null) {
// this is weird, we got a pen down, but we thought it was
// already down!
// XXX: We should probably send an ACTION_UP to the current
// target.
mMotionTarget = null;
}
// If we're disallowing intercept or if we're allowing and we didn't
// intercept
if (disallowIntercept || !onInterceptTouchEvent(ev)) {
// reset this event's action (just to protect ourselves)
ev.setAction(MotionEvent.ACTION_DOWN);
// We know we want to dispatch the event down, find a child
// who can handle it, start with the front-most child.
final int scrolledXInt = (int) scrolledXFloat;
final int scrolledYInt = (int) scrolledYFloat;
final View[] children = mChildren;
final int count = mChildrenCount;

for (int i = count - 1; i >= 0; i--) {
final View child = children[i];
if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE
|| child.getAnimation() != null) {
child.getHitRect(frame);
if (frame.contains(scrolledXInt, scrolledYInt)) {
// offset the event to the view's coordinate system
final float xc = scrolledXFloat - child.mLeft;
final float yc = scrolledYFloat - child.mTop;
ev.setLocation(xc, yc);
child.mPrivateFlags &= ~CANCEL_NEXT_UP_EVENT;
if (child.dispatchTouchEvent(ev)) {
// Event handled, we have a target now.
mMotionTarget = child;
return true;
}
// The event didn't get handled, try the next view.
// Don't reset the event's location, it's not
// necessary here.
}
}
}
}
} ....//other code omitted

代码比较长，决定分段贴出，首先贴出的是ACTION_DOWN事件相关的代码。
16行：进入ACTION_DOWN的处理

17-23行：将mMotionTarget置为null

26行：进行判断：if(disallowIntercept || !onInterceptTouchEvent(ev))

两种可能会进入IF代码段

1、当前不允许拦截，即disallowIntercept =true，

2、当前允许拦截但是不拦截，即disallowIntercept =false,但是onInterceptTouchEvent(ev)返回false ;

注：disallowIntercept 可以通过viewGroup.requestDisallowInterceptTouchEvent(boolean);进行设置，后面会详细说；而onInterceptTouchEvent(ev)可以进行复写。

36-57行：开始遍历所有的子View

41行：进行判断当前的x,y坐标是否落在子View身上，如果在，47行，执行child.dispatchTouchEvent(ev)，就进入了View的dispatchTouchEvent代码中了，如果不了解请参考：Android
View的事件分发机制，当child.dispatchTouchEvent(ev)返回true，则为mMotionTarget=child;然后return
true;

ViewGroup的ACTION_DOWN分析结束，总结一下：

ViewGroup实现捕获到DOWN事件，如果代码中不做TOUCH事件拦截，则开始查找当前x,y是否在某个子View的区域内，如果在，则把事件分发下去。

按照日志，接下来到达ACTION_MOVE

2、ViewGroup - dispatchTouchEvent - ACTION_MOVE

首先我们源码进行删减，只留下MOVE相关的代码：

[java]
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@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
final int action = ev.getAction();
final float xf = ev.getX();
final float yf = ev.getY();
final float scrolledXFloat = xf + mScrollX;
final float scrolledYFloat = yf + mScrollY;
final Rect frame = mTempRect;

boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;

//...ACTION_DOWN

//...ACTIN_UP or ACTION_CANCEL

// The event wasn't an ACTION_DOWN, dispatch it to our target if
// we have one.
final View target = mMotionTarget;

// if have a target, see if we're allowed to and want to intercept its
// events
if (!disallowIntercept && onInterceptTouchEvent(ev)) {
//....
}
// finally offset the event to the target's coordinate system and
// dispatch the event.
final float xc = scrolledXFloat - (float) target.mLeft;
final float yc = scrolledYFloat - (float) target.mTop;
ev.setLocation(xc, yc);

return target.dispatchTouchEvent(ev);
}

18行：把ACTION_DOWN时赋值的mMotionTarget，付给target ;

23行：if (!disallowIntercept && onInterceptTouchEvent(ev)) 当前允许拦截且拦截了，才进入IF体，当然了默认是不会拦截的~这里执行了onInterceptTouchEvent(ev)

28-30行：把坐标系统转化为子View的坐标系统

32行：直接return target.dispatchTouchEvent(ev);

可以看到，正常流程下，ACTION_MOVE在检测完是否拦截以后，直接调用了子View.dispatchTouchEvent，事件分发下去；

最后就是ACTION_UP了

3、ViewGroup
- dispatchTouchEvent - ACTION_UP

[java]
view plaincopy

public boolean dispatchTouchEvent(MotionEvent ev) {
if (!onFilterTouchEventForSecurity(ev)) {
return false;
}

final int action = ev.getAction();
final float xf = ev.getX();
final float yf = ev.getY();
final float scrolledXFloat = xf + mScrollX;
final float scrolledYFloat = yf + mScrollY;
final Rect frame = mTempRect;

boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;

if (action == MotionEvent.ACTION_DOWN) {...}

boolean isUpOrCancel = (action == MotionEvent.ACTION_UP) ||
(action == MotionEvent.ACTION_CANCEL);

if (isUpOrCancel) {
mGroupFlags &= ~FLAG_DISALLOW_INTERCEPT;
}
final View target = mMotionTarget;
if(target ==null ){...}
if (!disallowIntercept && onInterceptTouchEvent(ev)) {...}

if (isUpOrCancel) {
mMotionTarget = null;
}

// finally offset the event to the target's coordinate system and
// dispatch the event.
final float xc = scrolledXFloat - (float) target.mLeft;
final float yc = scrolledYFloat - (float) target.mTop;
ev.setLocation(xc, yc);

return target.dispatchTouchEvent(ev);
}

17行：判断当前是否是ACTION_UP

21，28行：分别重置拦截标志位以及将DOWN赋值的mMotionTarget置为null，都UP了，当然置为null，下一次DOWN还会再赋值的~

最后，修改坐标系统，然后调用target.dispatchTouchEvent(ev);

正常情况下，即我们上例整个代码的流程我们已经走完了：

1、ACTION_DOWN中，ViewGroup捕获到事件，然后判断是否拦截，如果没有拦截，则找到包含当前x,y坐标的子View，赋值给mMotionTarget，然后调用
mMotionTarget.dispatchTouchEvent

2、ACTION_MOVE中，ViewGroup捕获到事件，然后判断是否拦截，如果没有拦截，则直接调用mMotionTarget.dispatchTouchEvent(ev)

3、ACTION_UP中，ViewGroup捕获到事件，然后判断是否拦截，如果没有拦截，则直接调用mMotionTarget.dispatchTouchEvent(ev)

当然了在分发之前都会修改下坐标系统，把当前的x，y分别减去child.left 和 child.top ，然后传给child;

3、关于拦截

1、如何拦截

上面的总结都是基于：如果没有拦截；那么如何拦截呢？

复写ViewGroup的onInterceptTouchEvent方法：

[java]
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@Override
public boolean onInterceptTouchEvent(MotionEvent ev)
{
int action = ev.getAction();
switch (action)
{
case MotionEvent.ACTION_DOWN:
//如果你觉得需要拦截
return true ;
case MotionEvent.ACTION_MOVE:
//如果你觉得需要拦截
return true ;
case MotionEvent.ACTION_UP:
//如果你觉得需要拦截
return true ;
}

return false;
}

默认是不拦截的，即返回false；如果你需要拦截，只要return true就行了，这要该事件就不会往子View传递了，并且如果你在DOWN retrun true ，则DOWN,MOVE,UP子View都不会捕获事件；如果你在MOVE return true , 则子View在MOVE和UP都不会捕获事件。
原因很简单，当onInterceptTouchEvent(ev) return true的时候，会把mMotionTarget 置为null ;

2、如何不被拦截

如果ViewGroup的onInterceptTouchEvent(ev) 当ACTION_MOVE时return true ，即拦截了子View的MOVE以及UP事件；

此时子View希望依然能够响应MOVE和UP时该咋办呢？

Android给我们提供了一个方法：requestDisallowInterceptTouchEvent(boolean) 用于设置是否允许拦截，我们在子View的dispatchTouchEvent中直接这么写：

[java]
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@Override
public boolean dispatchTouchEvent(MotionEvent event)
{
getParent().requestDisallowInterceptTouchEvent(true);
int action = event.getAction();

switch (action)
{
case MotionEvent.ACTION_DOWN:
Log.e(TAG, "dispatchTouchEvent ACTION_DOWN");
break;
case MotionEvent.ACTION_MOVE:
Log.e(TAG, "dispatchTouchEvent ACTION_MOVE");
break;
case MotionEvent.ACTION_UP:
Log.e(TAG, "dispatchTouchEvent ACTION_UP");
break;

default:
break;
}
return super.dispatchTouchEvent(event);
}

getParent().requestDisallowInterceptTouchEvent(true); 这样即使ViewGroup在MOVE的时候return true，子View依然可以捕获到MOVE以及UP事件。

从源码也可以解释：

ViewGroup MOVE和UP拦截的源码是这样的：

[java]
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if (!disallowIntercept && onInterceptTouchEvent(ev)) {
final float xc = scrolledXFloat - (float) target.mLeft;
final float yc = scrolledYFloat - (float) target.mTop;
mPrivateFlags &= ~CANCEL_NEXT_UP_EVENT;
ev.setAction(MotionEvent.ACTION_CANCEL);
ev.setLocation(xc, yc);
if (!target.dispatchTouchEvent(ev)) {
// target didn't handle ACTION_CANCEL. not much we can do
// but they should have.
}
// clear the target
mMotionTarget = null;
// Don't dispatch this event to our own view, because we already
// saw it when intercepting; we just want to give the following
// event to the normal onTouchEvent().
return true;
}

当我们把disallowIntercept设置为true时，!disallowIntercept直接为false，于是拦截的方法体就被跳过了~
注：如果ViewGroup在onInterceptTouchEvent(ev) ACTION_DOWN里面直接return true了，那么子View是木有办法的捕获事件的~~~

4、如果没有找到合适的子View

我们的实例，直接点击ViewGroup内的按钮，当然直接很顺利的走完整个流程；

但是有两种特殊情况

1、ACTION_DOWN的时候，子View.dispatchTouchEvent(ev)返回的为false ;

如果你仔细看了，你会注意到ViewGroup的dispatchTouchEvent(ev)的ACTION_DOWN代码是这样的

[java]
view plaincopy

if (child.dispatchTouchEvent(ev)) {
// Event handled, we have a target now.
mMotionTarget = child;
return true;
}

只有在child.dispatchTouchEvent(ev)返回true了，才会认为找到了能够处理当前事件的View，即mMotionTarget = child;
但是如果返回false,那么mMotionTarget 依然是null

mMotionTarget 为null会咋样呢？

其实ViewGroup也是View的子类，如果没有找到能够处理该事件的子View，或者干脆就没有子View；

那么，它作为一个View，就相当于View的事件转发了~~直接super.dispatchTouchEvent(ev);

源码是这样的：

[java]
view plaincopy

final View target = mMotionTarget;
if (target == null) {
// We don't have a target, this means we're handling the
// event as a regular view.
ev.setLocation(xf, yf);
if ((mPrivateFlags & CANCEL_NEXT_UP_EVENT) != 0) {
ev.setAction(MotionEvent.ACTION_CANCEL);
mPrivateFlags &= ~CANCEL_NEXT_UP_EVENT;
}
return super.dispatchTouchEvent(ev);
}

我们没有一个能够处理该事件的目标元素，意味着我们需要自己处理~~~就相当于传统的View~
2、那么什么时候子View.dispatchTouchEvent(ev)返回的为true

如果你仔细看了上篇博客，你会发现只要子View支持点击或者长按事件一定返回true~~

源码是这样的：

[java]
view plaincopy

if (((viewFlags & CLICKABLE) == CLICKABLE ||
(viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE)) {
return true ; }

5、总结

关于代码流程上面已经总结过了~

1、如果ViewGroup找到了能够处理该事件的View，则直接交给子View处理，自己的onTouchEvent不会被触发；

2、可以通过复写onInterceptTouchEvent(ev)方法，拦截子View的事件（即return true），把事件交给自己处理，则会执行自己对应的onTouchEvent方法

3、子View可以通过调用getParent().requestDisallowInterceptTouchEvent(true); 阻止ViewGroup对其MOVE或者UP事件进行拦截；

好了，那么实际应用中能解决哪些问题呢？

比如你需要写一个类似slidingmenu的左侧隐藏menu，主Activity上有个Button、ListView或者任何可以响应点击的View，你在当前View上死命的滑动，菜单栏也出不来；因为MOVE事件被子View处理了~
你需要这么做：在ViewGroup的dispatchTouchEvent中判断用户是不是想显示菜单，如果是，则在onInterceptTouchEvent(ev)拦截子View的事件；自己进行处理，这样自己的onTouchEvent就可以顺利展现出菜单栏了~~