Android 事件分发笔记

记住的核心的三个方法：

• 事件分发boolean dispatchTouchEvent()
• 事件拦截boolean onInterceptTouchEvent()
• 事件消费boolean onTouchEvent()

Activity、ViewGroup和View三者，只有Activity和View没有事件拦截。

ViewGroup事件分发机制伪代码：

public boolean dispatchTouchEvent(MotionEvent ev) {
    boolean result = false;             // 默认状态为没有消费过

    if (!onInterceptTouchEvent(ev)) {   // 如果没有拦截交给子View
        result = child.dispatchTouchEvent(ev);
    }

    if (!result) {                      // 如果事件没有被消费,询问自身onTouchEvent
        result = onTouchEvent(ev);
    }

    return result;
}

示例代码：

MotionEventViewGroupA

public class MotionEventViewGroupA extends RelativeLayout {

  //折叠构造器代码
  ....

    @Override
    public boolean dispatchTouchEvent(MotionEvent ev) {
        Log.e("TAG", "MotionEventViewGroupA: dispatchTouchEvent");
        boolean isDispatchTouchEvent = super.dispatchTouchEvent(ev);
        Log.e("TAG", "MotionEventViewGroupA: dispatchTouchEvent:" + isDispatchTouchEvent);
        return isDispatchTouchEvent;
    }

    @Override
    public boolean onInterceptTouchEvent(MotionEvent ev) {
        Log.e("TAG", "MotionEventViewGroupA: onInterceptTouchEvent");
        boolean isInterceptTouchEvent = super.onInterceptTouchEvent(ev);
        Log.e("TAG", "MotionEventViewGroupA: onInterceptTouchEvent : " + isInterceptTouchEvent);
        return isInterceptTouchEvent;
    }


    @Override
    public boolean onTouchEvent(MotionEvent event) {
        Log.e("TAG", "MotionEventViewGroupA: onTouchEvent");
        boolean isOnTOuchEvent = super.onTouchEvent(event);
        Log.e("TAG", "MotionEventViewGroupA: onTouchEvent : " + isOnTOuchEvent);
        return isOnTOuchEvent;
    }
}

MotionEventViewGroupB

public class MotionEventViewGroupB extends RelativeLayout {

    //折叠构造器代码
    ....

    @Override
    public boolean dispatchTouchEvent(MotionEvent ev) {
        Log.e("TAG", "MotionEventViewGroupB: dispatchTouchEvent");
        boolean isDispatchTouchEvent = super.dispatchTouchEvent(ev);
        Log.e("TAG", "MotionEventViewGroupB: dispatchTouchEvent:" + isDispatchTouchEvent);
        return isDispatchTouchEvent;
    }

    @Override
    public boolean onInterceptTouchEvent(MotionEvent ev) {
        Log.e("TAG", "MotionEventViewGroupB: onInterceptTouchEvent");
        boolean isInterceptTouchEvent = super.onInterceptTouchEvent(ev);
        Log.e("TAG", "MotionEventViewGroupB: onInterceptTouchEvent : " + isInterceptTouchEvent);
        return isInterceptTouchEvent;
    }


    @Override
    public boolean onTouchEvent(MotionEvent event) {
        Log.e("TAG", "MotionEventViewGroupB: onTouchEvent");
        boolean isOnTOuchEvent = super.onTouchEvent(event);
        Log.e("TAG", "MotionEventViewGroupB: onTouchEvent : " + isOnTOuchEvent);
        return isOnTOuchEvent;
    }
}

MotionEventViewC

public class MotionEventViewC extends View {

  //折叠构造器代码
  ....

    @Override
    public boolean dispatchTouchEvent(MotionEvent ev) {
        Log.e("TAG", "MotionEventViewC: dispatchTouchEvent");
        boolean isDispatchTouchEvent = super.dispatchTouchEvent(ev);
        Log.e("TAG", "MotionEventViewC: dispatchTouchEvent:" + isDispatchTouchEvent);
        return isDispatchTouchEvent;
    }


    @Override
    public boolean onTouchEvent(MotionEvent event) {
        Log.e("TAG", "MotionEventViewC: onTouchEvent");
        boolean isOnTOuchEvent = super.onTouchEvent(event);
        Log.e("TAG", "MotionEventViewC: onTouchEvent : " + isOnTOuchEvent);
        return isOnTOuchEvent;
    }
}

布局文件：

<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.linwei.traindemo.ui.EventDemoActivity">


    <com.linwei.traindemo.eventDemo.MotionEventViewGroupA
        android:layout_width="match_parent"
        android:background="@color/colorPrimary"
        android:layout_height="400dp">

        <com.linwei.traindemo.eventDemo.MotionEventViewGroupB
            android:layout_width="200dp"
            android:layout_height="200dp"
            android:background="@color/colorAccent"
            android:layout_centerInParent="true">

            <com.linwei.traindemo.eventDemo.MotionEventViewC
                android:layout_width="100dp"
                android:layout_height="100dp"
                android:background="@android:color/white"
                android:layout_centerInParent="true" />
        </com.linwei.traindemo.eventDemo.MotionEventViewGroupB>
    </com.linwei.traindemo.eventDemo.MotionEventViewGroupA>

</android.support.constraint.ConstraintLayout>

当点击ViewC的时候打印：

MotionEventViewGroupA: dispatchTouchEvent
MotionEventViewGroupA: onInterceptTouchEvent
MotionEventViewGroupA: onInterceptTouchEvent : false
MotionEventViewGroupB: dispatchTouchEvent
MotionEventViewGroupB: onInterceptTouchEvent
MotionEventViewGroupB: onInterceptTouchEvent : false
MotionEventViewC: dispatchTouchEvent
MotionEventViewC: onTouchEvent
MotionEventViewC: onTouchEvent : false
MotionEventViewC: dispatchTouchEvent:false
MotionEventViewGroupB: onTouchEvent
MotionEventViewGroupB: onTouchEvent : false
MotionEventViewGroupB: dispatchTouchEvent:false
MotionEventViewGroupA: onTouchEvent
MotionEventViewGroupA: onTouchEvent : false
MotionEventViewGroupA: dispatchTouchEvent:false

分析下ViewGroup和View的事件分发的顺序：

发生触摸事件的时候，最外层的View会先收到事件，此时最外层ViewGroupA的dispatchTouchEvent被调用，
dispatchTouchEvent方法中调用本身的onInterceptTouchEvent方法。

判断onInterceptTouchEvent的返回值，如果，为true则表示自身拦截事件，会调用自身的onTouchEvent方法。

并且根据onTouchEvent返回值判断事件是否消费，并且将返回值传递给dispatchTouchEvent返回。
如果onTouchEvent返回值为true则事件消费，否则事件未消费，并且分发失败，往上一级View传递false。

为false则自身不拦截事件，并且遍历自身的View，知道最后一级View。

Summary:

只有最后onTouchEvent返回true才消费了事件。父View拦截并消费了事件，则不会再分发给子View。

PS:从网上看到很多关于事件分发机制介绍很好写的很好的，这篇笔记只是简单整理下自己的思路，具体大家可以了解下以下文章。这篇笔记也来自这些：

http://www.gcssloop.com/customview/dispatch-touchevent-theory

http://wuxiaolong.me/2015/12/19/MotionEvent/