我们先来写个测试应用，主要文件如下：

1. MainActivity.java

package com.test.keyevent;import android.app.Activity;
import android.os.Bundle;
import android.util.Log;
import android.view.KeyEvent;public class MainActivity extends Activity {@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.activity_main);}@Overridepublic boolean onKeyDown(int keyCode, KeyEvent event) {Log.d("KEYEVENT", "MainActivity:onKeyDown");return super.onKeyDown(keyCode, event);}@Overridepublic boolean onKeyUp(int keyCode, KeyEvent event) {Log.d("KEYEVENT", "MainActivity:onKeyUp");return super.onKeyUp(keyCode, event);}@Overridepublic void onUserInteraction() {Log.d("KEYEVENT", "MainActivity:onUserInteraction");super.onUserInteraction();}@Overridepublic boolean dispatchKeyEvent(KeyEvent event) {Log.d("KEYEVENT", "MainActivity:dispatchKeyEvent");return super.dispatchKeyEvent(event);}}

2. MyFrameLayout.java

package com.test.keyevent;import android.content.Context;
import android.util.AttributeSet;
import android.util.Log;
import android.view.KeyEvent;
import android.widget.FrameLayout;public class MyFrameLayout extends FrameLayout {public MyFrameLayout(Context context) {super(context, null);}public MyFrameLayout(Context context, AttributeSet attrs) {super(context, attrs, 0);}public MyFrameLayout(Context context, AttributeSet attrs, int defStyle) {super(context, attrs, defStyle);}@Overridepublic boolean dispatchKeyEvent(KeyEvent event) {Log.d("KEYEVENT", "MyFrameLayout:dispatchKeyEvent");return super.dispatchKeyEvent(event);}@Overridepublic boolean onKeyDown(int keyCode, KeyEvent event) {// TODO Auto-generated method stubLog.d("KEYEVENT", "MyFrameLayout:onKeyDown");return super.onKeyDown(keyCode, event);}@Overridepublic boolean onKeyUp(int keyCode, KeyEvent event) {// TODO Auto-generated method stubLog.d("KEYEVENT", "MyFrameLayout:onKeyUp");return super.onKeyUp(keyCode, event);}}

3. MyEditText.java

package com.test.keyevent;import android.content.Context;
import android.util.AttributeSet;
import android.util.Log;
import android.view.KeyEvent;
import android.widget.EditText;public class MyEditText extends EditText {public MyEditText(Context context) {super(context, null);// TODO Auto-generated constructor stub}public MyEditText(Context context, AttributeSet attrs) {super(context, attrs, 0);// TODO Auto-generated constructor stub}public MyEditText(Context context, AttributeSet attrs, int defStyle) {super(context, attrs, defStyle);// TODO Auto-generated constructor stub}@Overridepublic boolean onKeyDown(int keyCode, KeyEvent event) {Log.d("KEYEVENT", "MyEditText:onKeyDown");return super.onKeyDown(keyCode, event);}@Overridepublic boolean onKeyUp(int keyCode, KeyEvent event) {Log.d("KEYEVENT", "MyEditText:onKeyUp");return super.onKeyUp(keyCode, event);}@Overridepublic boolean dispatchKeyEvent(KeyEvent event) {Log.d("KEYEVENT", "MyEditText:dispatchKeyEvent");return super.dispatchKeyEvent(event);}}

4. activity_main.xml

<com.test.keyevent.MyFrameLayout 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:background="@android:color/black"><com.test.keyevent.MyEditTextandroid:layout_width="wrap_content"android:layout_height="wrap_content"android:layout_gravity="center"android:background="@android:color/white"android:textColor="@android:color/black"android:hint="请输入..." />
</com.test.keyevent.MyFrameLayout>

安装并打开应用。

在adb shell中，输入input keyevent 29 往系统注入一个KeyEvent（29是a的键值，可以在系统源码KeyEvent.java中查到），我们得到如下日志：

03-02 20:18:48.991: D/KEYEVENT(5840): MainActivity:dispatchKeyEvent

03-02 20:18:48.991: D/KEYEVENT(5840): MainActivity:onUserInteraction

03-02 20:18:48.991: D/KEYEVENT(5840): MyFrameLayout:dispatchKeyEvent

03-02 20:18:48.991: D/KEYEVENT(5840): MyEditText:dispatchKeyEvent

03-02 20:18:48.992: D/KEYEVENT(5840): MyEditText:onKeyDown

03-02 20:18:49.040: D/KEYEVENT(5840): MainActivity:dispatchKeyEvent

03-02 20:18:49.040: D/KEYEVENT(5840): MainActivity:onUserInteraction

03-02 20:18:49.040: D/KEYEVENT(5840): MyFrameLayout:dispatchKeyEvent

03-02 20:18:49.040: D/KEYEVENT(5840): MyEditText:dispatchKeyEvent

03-02 20:18:49.040: D/KEYEVENT(5840): MyEditText:onKeyUp

03-02 20:18:49.040: D/KEYEVENT(5840): MainActivity:onKeyUp

下面我们一步一步分析。

1. MainActivity:dispatchKeyEvent

首先被调用的是MainActivity的dispatchKeyEvent的函数。KeyEvent是如何传递到Acitivity中的，这是比较复杂的，可以参看http://blog.csdn.net/luoshengyang/article/details/6882903这篇博文。但要注意，这篇文章是更具android2.3来写的，对于我们常见的4.2以上的系统，经过我分析比较，是有一些不同的。

4.2系统中，native层InputManager不是在windowManagerService中创建的，而是在SystemServer中创建了个InputManagerService，并在里面创建初始化了InputManager。InputManager的启动也是由SystemServer来启动的。并且InputReaderThread的loopOnce()支持一次读取多条event，优化了性能。还有很多其他的不同的地方，但是大体逻辑上还是一样的。

这块好应用开发关系不太大，就不细说了。最终，Activity的ViewRootImpl的deliverKeyEventPostIme方法中会调用       DecorView的dispatchKeyEvent方法，而DecorView的dispatchKeyEvent方法，可以看下源码，它回调了Activity的       dispatchKeyEvent方法，于是，我们看到了这行日志。

2.MainActivity:onUserInteraction

我们看Activity的dispatchKeyEvent方法,

    public boolean dispatchKeyEvent(KeyEvent event) {onUserInteraction();Window win = getWindow();if (win.superDispatchKeyEvent(event)) {return true;}View decor = mDecor;if (decor == null) decor = win.getDecorView();return event.dispatch(this, decor != null? decor.getKeyDispatcherState() : null, this);}

里面调用了onUserInteraction方法。我们可以覆写这个方法，这样就可以在KeyEvent被派发之前，做一些操作，默认是什么都不做。

3. MyFrameLayout:dispatchKeyEvent

从日志看出，事件从Activity又回到了我们的View中。为什么要从Activity中走一下，这样给Activity一个机会，可以控制KeyEvent的派发。

如何回去的，我们接着看Acitivity.dispatchKeyEvent方法

        Window win = getWindow();if (win.superDispatchKeyEvent(event)) {return true;}

win就是Activity的mWindow对象，是一个PhoneWindow。我们看PhoneWindow的superDispatchKeyEvent(KeyEvent event)方法：

    public boolean superDispatchKeyEvent(KeyEvent event) {return mDecor.superDispatchKeyEvent(event);}

mDecor就是我们熟悉的DecorView对象啦，看它的superDispatchKeyEvent：

        public boolean superDispatchKeyEvent(KeyEvent event) {if (super.dispatchKeyEvent(event)) {return true;}......}

因为DecorView是FrameLayout子类，FrameLayout是ViewGroup的子类，所以super.dispatchKeyEvent(event)会调用到ViewGroup的dispatchKeyEvent方法:

  public boolean dispatchKeyEvent(KeyEvent event) {if (mInputEventConsistencyVerifier != null) {mInputEventConsistencyVerifier.onKeyEvent(event, 1);}if ((mPrivateFlags & (PFLAG_FOCUSED | PFLAG_HAS_BOUNDS))== (PFLAG_FOCUSED | PFLAG_HAS_BOUNDS)) {// 如果自己有焦点，则调用父类View的dispatchKeyEventif (super.dispatchKeyEvent(event)) {return true;}} else if (mFocused != null && (mFocused.mPrivateFlags & PFLAG_HAS_BOUNDS)== PFLAG_HAS_BOUNDS) {// 子视图有焦点，则调用子视图if (mFocused.dispatchKeyEvent(event)) {return true;}}if (mInputEventConsistencyVerifier != null) {mInputEventConsistencyVerifier.onUnhandledEvent(event, 1);}return false;}

焦点在MyEditText上，而MyEditText包含于MyFrameLayout中，所以最终mFocused.dispatchKeyEvent(event)会调用到MyFrameLayout.dispatchKeyEvent方法。

4. MyEditText:dispatchKeyEvent

MyFrameLayout也是FrameLayout子类，所以MyFrameLayout.dispatchKeyEvent和上面一样，会调用ViewGroup的dispatchKeyEvent。这次mFocused就是MyEditText,调用它的dispatchKeyEvent方法。

5.MyEditText:onKeyDown

MyEditText是View子类，会调用View的dispatchKeyEvent方法：

  public boolean dispatchKeyEvent(KeyEvent event) {......if (event.dispatch(this, mAttachInfo != null? mAttachInfo.mKeyDispatchState : null, this)) {return true;}......return false;}

View.dispatchKeyEvent会调用KeyEvent的dispatch方法：

    public final boolean dispatch(Callback receiver, DispatcherState state,Object target) {switch (mAction) {case ACTION_DOWN: {......boolean res = receiver.onKeyDown(mKeyCode, this);......}}

receiver就是之前的View，也就是MyEditText，也就回调了MyEditText的onKeyDown方法。

至此，ACTION_DOWN的KeyEvent分析完毕。

ACTION_UP的KeyEvent和Down的基本一样，只是应为View的onKeyUp方法返回了false，所以最终会调用到Acitivity的onKeyUp方法，所以才会有最后一条日志的输出。