Android事件分发机制

准备工作

对于Android事件处理分为两类：

1. 对于Activity和View： 只有两种事件：
   分发: dispatchTouchEvent函数
   消费: onTouchEvent函数

2. 对于ViewGroup： 全部三种事件：
   分发: dispatchTouchEvent函数
   拦截：onInterceptTouchEvent函数
   消费: onTouchEvent函数

重写一个ViewGroup和一个View，代码如下
EventViewGroup.java

public class EventViewGroup extends LinearLayout {
    public EventViewGroup(Context context) {
        super(context);
    }

    public EventViewGroup(Context context, @Nullable AttributeSet attrs) {
        super(context, attrs);
    }

    @Override
    public boolean dispatchTouchEvent(MotionEvent ev) {
        Log.e("event","----------->dispatchTouchEvent:::ViewGroup----" + ev.getAction());
        return super.dispatchTouchEvent(ev);
    }

    @Override
    public boolean onInterceptTouchEvent(MotionEvent ev) {
        Log.e("event","----------->onInterceptTouchEvent:::ViewGroup----" + ev.getAction());
        return super.onInterceptTouchEvent(ev);
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        Log.e("event","----------->onTouchEvent:::ViewGroup----" + event.getAction());
        return super.onTouchEvent(event);
    }
}

EventView.java

public class EventView extends View {

    public EventView(Context context) {
        super(context);
    }

    public EventView(Context context, @Nullable AttributeSet attrs) {
        super(context, attrs);
    }

    @Override
    public boolean dispatchTouchEvent(MotionEvent ev) {
        Log.e("event", "----------->dispatchTouchEvent:::View----" + ev.getAction());
        return super.dispatchTouchEvent(ev);
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        Log.e("event", "----------->onTouchEvent:::View----" + event.getAction());
        return super.onTouchEvent(event);
    }
}

MainActivity.java

package com.gerry.dispatchevent;

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.util.Log;
import android.view.MotionEvent;

import com.gerry.dispatchevent.view.EventView;
import com.gerry.dispatchevent.view.EventViewB;
import com.gerry.dispatchevent.viewgroup.EventViewGroup;
import com.gerry.dispatchevent.viewgroup.EventViewGroupB;

public class MainActivity extends AppCompatActivity {
    private EventViewGroup evg;
    private EventView ev;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        evgA= findViewById(R.id.evg);
        evA= findViewById(R.id.ev);
    }

    @Override
    public boolean dispatchTouchEvent(MotionEvent ev) {
        Log.e("event", "----------->dispatchTouchEvent:::MainActivity----" + ev.getAction());
        return super.dispatchTouchEvent(ev);
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        Log.e("event", "----------->onTouchEvent:::MainActivity----" + event.getAction());
        return super.onTouchEvent(event);
    }
}

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<FrameLayout 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"
    android:orientation="vertical"
    tools:context=".MainActivity">

     <com.gerry.dispatchevent.viewgroup.EventViewGroup
        android:id="@+id/evg"
        android:layout_width="match_parent"
        android:layout_height="500px"
        android:layout_marginTop="250px"
        android:background="@android:color/holo_green_dark"
        android:gravity="center"
        android:orientation="vertical">

        <com.gerry.dispatchevent.view.EventView
            android:id="@+id/ev"
            android:layout_width="200px"
            android:layout_height="150px"
            android:background="@android:color/holo_blue_light" />
    </com.gerry.dispatchevent.viewgroup.EventViewGroup>
</FrameLayout>

开始分析

0为按下动作，1为抬起动作。

1. 对事件不进行任何修改

• 触摸Activity

E/event: ----------->dispatchTouchEvent:::MainActivity----0
E/event: ----------->onTouchEvent:::MainActivity----0
E/event: ----------->dispatchTouchEvent:::MainActivity----1
E/event: ----------->onTouchEvent:::MainActivity----1

• 触摸ViewGroup

E/event: ----------->dispatchTouchEvent:::MainActivity----0
E/event: ----------->dispatchTouchEvent:::ViewGroup----0
E/event: ----------->onInterceptTouchEvent:::ViewGroup----0
E/event: ----------->onTouchEvent:::ViewGroup----0
E/event: ----------->onTouchEvent:::MainActivity----0
E/event: ----------->dispatchTouchEvent:::MainActivity----1
E/event: ----------->onTouchEvent:::MainActivity----1

• 触摸View

E/event: ----------->dispatchTouchEvent:::MainActivity----0
E/event: ----------->dispatchTouchEvent:::ViewGroup----0
E/event: ----------->onInterceptTouchEvent:::ViewGroup----0
E/event: ----------->dispatchTouchEvent:::View----0
E/event: ----------->onTouchEvent:::View----0
E/event: ----------->onTouchEvent:::ViewGroup----0
E/event: ----------->onTouchEvent:::MainActivity----0
E/event: ----------->dispatchTouchEvent:::MainActivity----1
E/event: ----------->onTouchEvent:::MainActivity----1

结论：

在默认情况下，Activity会把事件向下依次按照层级分发到触摸的最后一个ViewGroup或者View，然后会执行最后一个ViewGroup或者View的onTouchEvent()方法，然后又向上依次按照层级去触发onTouchEvent()事件，中途若没有被消费，就会传递直到activity，整个流程呈U形，隧道式向下分发，然后冒泡式向上处理。

2. 修改ViewGroup的dispatchTouchEvent方法，返回false。

• 触摸ViewGroup

E/event: ----------->dispatchTouchEvent:::MainActivity----0
E/event: ----------->dispatchTouchEvent:::ViewGroup----0
E/event: ----------->onTouchEvent:::MainActivity----0
E/event: ----------->dispatchTouchEvent:::MainActivity----1
E/event: ----------->onTouchEvent:::MainActivity----1

• 触摸View

E/event: ----------->dispatchTouchEvent:::MainActivity----0
E/event: ----------->dispatchTouchEvent:::ViewGroup----0
E/event: ----------->onTouchEvent:::MainActivity----0
E/event: ----------->dispatchTouchEvent:::MainActivity----1
E/event: ----------->onTouchEvent:::MainActivity----1

3. 修改View的dispatchTouchEvent方法，返回false。

• 触摸ViewGroup

E/event: ----------->dispatchTouchEvent:::MainActivity----0
E/event: ----------->dispatchTouchEvent:::ViewGroup----0
E/event: ----------->onInterceptTouchEvent:::ViewGroup----0
E/event: ----------->onTouchEvent:::ViewGroup----0
E/event: ----------->onTouchEvent:::MainActivity----0
E/event: ----------->dispatchTouchEvent:::MainActivity----1
E/event: ----------->onTouchEvent:::MainActivity----1

• 触摸View

E/event: ----------->dispatchTouchEvent:::MainActivity----0
E/event: ----------->dispatchTouchEvent:::ViewGroup----0
E/event: ----------->onInterceptTouchEvent:::ViewGroup----0
E/event: ----------->dispatchTouchEvent:::View----0
E/event: ----------->onTouchEvent:::ViewGroup----0
E/event: ----------->onTouchEvent:::MainActivity----0
E/event: ----------->dispatchTouchEvent:::MainActivity----1
E/event: ----------->onTouchEvent:::MainActivity----1

4. 修改ViewGroup的dispatchTouchEvent方法，返回true。

• 触摸ViewGroup

E/event: ----------->dispatchTouchEvent:::MainActivity----0
E/event: ----------->dispatchTouchEvent:::ViewGroup----0
E/event: ----------->dispatchTouchEvent:::MainActivity----1
E/event: ----------->dispatchTouchEvent:::ViewGroup----1

• 触摸View

E/event: ----------->dispatchTouchEvent:::MainActivity----0
E/event: ----------->dispatchTouchEvent:::ViewGroup----0
E/event: ----------->dispatchTouchEvent:::MainActivity----1
E/event: ----------->dispatchTouchEvent:::ViewGroup----1

5. 修改View的dispatchTouchEvent方法，返回true。

• 触摸ViewGroup

E/event: ----------->dispatchTouchEvent:::MainActivity----0
E/event: ----------->dispatchTouchEvent:::ViewGroup----0
E/event: ----------->onInterceptTouchEvent:::ViewGroup----0
E/event: ----------->onTouchEvent:::ViewGroup----0
E/event: ----------->onTouchEvent:::MainActivity----0
E/event: ----------->dispatchTouchEvent:::MainActivity----1
E/event: ----------->onTouchEvent:::MainActivity----1

• 触摸View

E/event: ----------->dispatchTouchEvent:::MainActivity----0
E/event: ----------->dispatchTouchEvent:::ViewGroup----0
E/event: ----------->onInterceptTouchEvent:::ViewGroup----0
E/event: ----------->dispatchTouchEvent:::View----0
E/event: ----------->dispatchTouchEvent:::MainActivity----1
E/event: ----------->dispatchTouchEvent:::ViewGroup----1
E/event: ----------->onInterceptTouchEvent:::ViewGroup----1
E/event: ----------->dispatchTouchEvent:::View----1

从2，3，4，5可以得出的结论：

1. 当dispatchTouchEvent返回false时，事件不会继续往下分发，无论是ViewGroup还是View，都会从上一级的onTouchEvent事件向上传递，不会执行自己的onTouchEvent()方法
2. 当dispatchTouchEvent返回true时，表示事件直接被消费，这个事件也就停止分发且不会逆向向上传递，直接结束了。
3. 返回super：事件将会继续向下分发，直到事件被消费为止。

6. 修改ViewGroup的onInterceptTouchEvent方法，返回false。

• 触摸ViewGroup

E/event: ----------->dispatchTouchEvent:::MainActivity----0
E/event: ----------->dispatchTouchEvent:::ViewGroup----0
E/event: ----------->onInterceptTouchEvent:::ViewGroup----0
E/event: ----------->onTouchEvent:::ViewGroup----0
E/event: ----------->onTouchEvent:::MainActivity----0
E/event: ----------->dispatchTouchEvent:::MainActivity----1
E/event: ----------->onTouchEvent:::MainActivity----1

• 触摸View

E/event: ----------->dispatchTouchEvent:::MainActivity----0
E/event: ----------->dispatchTouchEvent:::ViewGroup----0
E/event: ----------->onInterceptTouchEvent:::ViewGroup----0
E/event: ----------->dispatchTouchEvent:::View----0
E/event: ----------->onTouchEvent:::View----0
E/event: ----------->onTouchEvent:::ViewGroup----0
E/event: ----------->onTouchEvent:::MainActivity----0
E/event: ----------->dispatchTouchEvent:::MainActivity----1
E/event: ----------->onTouchEvent:::MainActivity----1

7. 修改ViewGroup的onInterceptTouchEvent方法，返回true。

• 触摸ViewGroup

E/event: ----------->dispatchTouchEvent:::MainActivity----0
E/event: ----------->dispatchTouchEvent:::ViewGroup----0
E/event: ----------->onInterceptTouchEvent:::ViewGroup----0
E/event: ----------->onTouchEvent:::ViewGroup----0
E/event: ----------->onTouchEvent:::MainActivity----0
E/event: ----------->dispatchTouchEvent:::MainActivity----1
E/event: ----------->onTouchEvent:::MainActivity----1

• 触摸View

E/event: ----------->dispatchTouchEvent:::MainActivity----0
E/event: ----------->dispatchTouchEvent:::ViewGroup----0
E/event: ----------->onInterceptTouchEvent:::ViewGroup----0
E/event: ----------->onTouchEvent:::ViewGroup----0
E/event: ----------->onTouchEvent:::MainActivity----0
E/event: ----------->dispatchTouchEvent:::MainActivity----1
E/event: ----------->onTouchEvent:::MainActivity----1

从6，7可以得出的结论：

1. 当onInterceptTouchEvnt返回false/super（默认）时，事件将会正常向下分发，分发至下级的dispatchTouchEvent方法 再次判断是否分发事件。
2. 当onInterceptTouchEvent返回true时，表示ViewGroup容器拦截后续事件，会执行该ViewGroup的onTouchEvent()方法然后停止向下分发转而通过onTouchEvent()向上传递，直到最终被消费

8. 修改ViewGroup的onTouchEvent方法，返回false。

• 触摸ViewGroup

E/event: ----------->dispatchTouchEvent:::MainActivity----0
E/event: ----------->dispatchTouchEvent:::ViewGroup----0
E/event: ----------->onInterceptTouchEvent:::ViewGroup----0
E/event: ----------->onTouchEvent:::ViewGroup----0
E/event: ----------->onTouchEvent:::MainActivity----0
E/event: ----------->dispatchTouchEvent:::MainActivity----1
E/event: ----------->onTouchEvent:::MainActivity----1

• 触摸View

E/event: ----------->dispatchTouchEvent:::MainActivity----0
E/event: ----------->dispatchTouchEvent:::ViewGroup----0
E/event: ----------->onInterceptTouchEvent:::ViewGroup----0
E/event: ----------->dispatchTouchEvent:::View----0
E/event: ----------->onTouchEvent:::View----0
E/event: ----------->onTouchEvent:::ViewGroup----0
E/event: ----------->onTouchEvent:::MainActivity----0
E/event: ----------->dispatchTouchEvent:::MainActivity----1
E/event: ----------->onTouchEvent:::MainActivity----1

9. 修改ViewGroup的onTouchEvent方法，返回true。

• 触摸ViewGroup

E/event: ----------->dispatchTouchEvent:::MainActivity----0
E/event: ----------->dispatchTouchEvent:::ViewGroup----0
E/event: ----------->onInterceptTouchEvent:::ViewGroup----0
E/event: ----------->onTouchEvent:::ViewGroup----0
E/event: ----------->dispatchTouchEvent:::MainActivity----1
E/event: ----------->dispatchTouchEvent:::ViewGroup----1
E/event: ----------->onTouchEvent:::ViewGroup----1

• 触摸View

E/event: ----------->dispatchTouchEvent:::MainActivity----0
E/event: ----------->dispatchTouchEvent:::ViewGroup----0
E/event: ----------->onInterceptTouchEvent:::ViewGroup----0
E/event: ----------->dispatchTouchEvent:::View----0
E/event: ----------->onTouchEvent:::View----0
E/event: ----------->onTouchEvent:::ViewGroup----0
E/event: ----------->dispatchTouchEvent:::MainActivity----1
E/event: ----------->dispatchTouchEvent:::ViewGroup----1
E/event: ----------->onTouchEvent:::ViewGroup----1

10. 修改View的onTouchEvent方法，返回false。

• 触摸ViewGroup

E/event: ----------->dispatchTouchEvent:::MainActivity----0
E/event: ----------->dispatchTouchEvent:::ViewGroup----0
E/event: ----------->onInterceptTouchEvent:::ViewGroup----0
E/event: ----------->onTouchEvent:::ViewGroup----0
E/event: ----------->onTouchEvent:::MainActivity----0
E/event: ----------->dispatchTouchEvent:::MainActivity----1
E/event: ----------->onTouchEvent:::MainActivity----1

• 触摸View

E/event: ----------->dispatchTouchEvent:::MainActivity----0
E/event: ----------->dispatchTouchEvent:::ViewGroup----0
E/event: ----------->onInterceptTouchEvent:::ViewGroup----0
E/event: ----------->dispatchTouchEvent:::View----0
E/event: ----------->onTouchEvent:::View----0
E/event: ----------->onTouchEvent:::ViewGroup----0
E/event: ----------->onTouchEvent:::MainActivity----0
E/event: ----------->dispatchTouchEvent:::MainActivity----1
E/event: ----------->onTouchEvent:::MainActivity----1

11. 修改View的onTouchEvent方法，返回true。

• 触摸ViewGroup

E/event: ----------->dispatchTouchEvent:::MainActivity----0
E/event: ----------->dispatchTouchEvent:::ViewGroup----0
E/event: ----------->onInterceptTouchEvent:::ViewGroup----0
E/event: ----------->onTouchEvent:::ViewGroup----0
E/event: ----------->onTouchEvent:::MainActivity----0
E/event: ----------->dispatchTouchEvent:::MainActivity----1
E/event: ----------->onTouchEvent:::MainActivity----1

• 触摸View

E/event: ----------->dispatchTouchEvent:::MainActivity----0
E/event: ----------->dispatchTouchEvent:::ViewGroup----0
E/event: ----------->onInterceptTouchEvent:::ViewGroup----0
E/event: ----------->dispatchTouchEvent:::View----0
E/event: ----------->onTouchEvent:::View----0
E/event: ----------->dispatchTouchEvent:::MainActivity----1
E/event: ----------->dispatchTouchEvent:::ViewGroup----1
E/event: ----------->onInterceptTouchEvent:::ViewGroup----1
E/event: ----------->dispatchTouchEvent:::View----1
E/event: ----------->onTouchEvent:::View----1

从8，9，10，11可以得出的结论：

1. 当onTouchEvent返回false/super（默认）时，不消费掉此次事件，事件将会层层向上传递，直到被消费。
2. 当onTouchEvent返回true时，立即消费掉事件，事件将不会向上传递，事件到此终止。

最终结论

dispatchTouchEvent 分发

1. 当dispatchTouchEvent返回false时，事件不会继续往下分发，无论是ViewGroup还是View，都会从上一级的onTouchEvent事件向上传递，不会执行自己的onTouchEvent()方法
2. 当dispatchTouchEvent返回true时，表示事件直接被消费，这个事件也就停止分发且不会逆向向上传递，直接结束了。
3. 返回super：事件将会继续向下分发，直到事件被消费为止。

onInterceptTouchEvent 拦截

1. 当onInterceptTouchEvnt返回false/super（默认）时，事件将会正常向下分发，分发至下级的dispatchTouchEvent方法 再次判断是否分发事件。
2. 当onInterceptTouchEvent返回true时，表示ViewGroup容器拦截后续事件，会执行该ViewGroup的onTouchEvent()方法然后停止向下分发转而通过onTouchEvent()向上传递，直到最终被消费

onTouchEvent 消费

1. 当onTouchEvent返回false/super（默认）时，不消费掉此次事件，事件将会层层向上传递，直到被消费。
2. 当onTouchEvent返回true时，立即消费掉事件，事件将不会向上传递，事件到此终止。

参考链接：
https://www.jianshu.com/p/fc0590afb1bf
https://www.jianshu.com/p/66a80ff0133b