本文会详细分析Android KeyEvent分发过程，以及相关api的作用

一、背景

Activity和View有三个override的方法dispatchKeyEvent、onKeyUp、onKeyDown，其中View还有一个setOnKeyListener方法。只知道这几个方法是设置处理key event的，但是一直不太理解这几个方法的作用和相互影响。所以准备写个demo彻底搞清楚这几个方法的作用和原理。

二、测试准备

新建三个类

public class KeyDispatchDemo extends Activity {

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

    @Override
    public boolean dispatchKeyEvent(KeyEvent event) {
        Log.d("Activity", "dispatchKeyEvent: " + event.getAction() + " | " + event.getKeyCode());
        return super.dispatchKeyEvent(event);
    }

    @Override
    public boolean onKeyUp(int keyCode, KeyEvent event) {
        Log.d("Activity", "onKeyUp: " + keyCode);
        return super.onKeyUp(keyCode, event);
    }

    @Override
    public boolean onKeyDown(int keyCode, KeyEvent event) {
        Log.d("Activity", "onKeyDown: " + keyCode);
        return super.onKeyDown(keyCode, event);
    }
}

public class KeyDispatchLinearLayout extends LinearLayout {

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

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

    public KeyDispatchLinearLayout(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
    }

    @Override
    public boolean dispatchKeyEvent(KeyEvent event) {
        Log.d("ViewGroup", "dispatchKeyEvent: " + event.getAction() + " | " + event.getKeyCode());
        return super.dispatchKeyEvent(event);
    }

    @Override
    public boolean onKeyUp(int keyCode, KeyEvent event) {
        Log.d("ViewGroup", "onKeyUp: " + keyCode);
        return super.onKeyUp(keyCode, event);
    }

    @Override
    public boolean onKeyDown(int keyCode, KeyEvent event) {
        Log.d("ViewGroup", "onKeyDown: " + keyCode);
        return super.onKeyDown(keyCode, event);
    }
}

public class KeyDispatchView extends View {

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

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

    public KeyDispatchView(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
    }

    @Override
    public boolean dispatchKeyEvent(KeyEvent event) {
        Log.d("View", "dispatchKeyEvent: " + event.getAction() + " | " + event.getKeyCode());
        return super.dispatchKeyEvent(event);
    }

    @Override
    public boolean onKeyUp(int keyCode, KeyEvent event) {
        Log.d("View", "onKeyUp: " + keyCode);
        return super.onKeyUp(keyCode, event);
    }

    @Override
    public boolean onKeyDown(int keyCode, KeyEvent event) {
        Log.d("View", "onKeyDown: " + keyCode);
        return super.onKeyDown(keyCode, event);
    }
}

布局文件：

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <com.lhl.common.keyevent.KeyDispatchLinearLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent">

        <com.lhl.common.keyevent.KeyDispatchView
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:background="@color/blue"
            android:focusable="true" />
    </com.lhl.common.keyevent.KeyDispatchLinearLayout>

</LinearLayout>

三、Key event分发流程分析

首先在关键方法上都打上断点，按下键盘，发出一个key event事件。

1、dispatchKeyEvent的执行流程

dispatchKeyEvent的分发主要是：DecorView——>Activity——>ViewGroup——>View

一个Key event是首先传到DecorView中，DecorView通过下面的代码分发给Activity：

if (!mWindow.isDestroyed()) {
    final Window.Callback cb = mWindow.getCallback();
    final boolean handled = cb != null && mFeatureId < 0 ? cb.dispatchKeyEvent(event)
            : super.dispatchKeyEvent(event);
    if (handled) {
        return true;
    }
}