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Android ViewGroup事件分发机制

2017-03-20 17:44 295 查看
转载请标明出处:http://blog.csdn.net/lmj623565791/article/details/39102591,本文出自【张鸿洋的博客】

上一篇已经完整的解析了Android View的事件分发机制,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] view
plain copy

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] view
plain copy

<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] view
plain copy

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] view
plain copy

@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] view
plain copy

@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
plain copy

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] view
plain copy

@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] view
plain copy

@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] view
plain copy

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
plain copy

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
plain copy

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
plain copy

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~~
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