大家好，又见面了，我是你们的朋友全栈君。

最近查看Framework源码的时候，读到了AudioService处理音量的流程，在这里碰到了IBinder的linkToDeath()这个知识点，比较感兴趣，所以记录下来，并自己写demo尝试了一下。

我们简单来看下AudioService处理静音这一块。
/frameworks/base/media/java/android/media/AudioManager.java

public class AudioManager { 
   
    ......
    /** * {@hide} */
     private final IBinder mICallBack = new Binder();

    public void setStreamMute(int streamType, boolean state) {
        IAudioService service = getService();
        try {
            service.setStreamMute(streamType, state, mICallBack);
        } catch (RemoteException e) {
            Log.e(TAG, "Dead object in setStreamMute", e);
        }
    }
    ......
}

service是一个IAudioService,它的实现类是AudioService,AudioManager.setStreamMute()中会调用AudioService.setStreamMute(streamType, state, mICallBack);其中mICallBack是一个Binder用来记录申请静音的客户端。

/frameworks/base/media/java/android/media/AudioService.java

private VolumeStreamState[] mStreamStates;
//第1步
/** @see AudioManager#setStreamMute(int, boolean) */
public void setStreamMute(int streamType, boolean state, IBinder cb) {
if (isStreamAffectedByMute(streamType)) {
mStreamStates[streamType].mute(cb, state);
}
}
public class VolumeStreamState { 

private ArrayList<VolumeDeathHandler> mDeathHandlers; //handles mute/solo clients death
private VolumeStreamState(String settingName, int streamType) {
mVolumeIndexSettingName = settingName;
mLastAudibleVolumeIndexSettingName = settingName + System.APPEND_FOR_LAST_AUDIBLE;
mStreamType = streamType;
mIndexMax = MAX_STREAM_VOLUME[streamType];
AudioSystem.initStreamVolume(streamType, 0, mIndexMax);
mIndexMax *= 10;
// mDeathHandlers must be created before calling readSettings()
mDeathHandlers = new ArrayList<VolumeDeathHandler>();
readSettings();
}
}
//第2步
public synchronized void mute(IBinder cb, boolean state) {
VolumeDeathHandler handler = getDeathHandler(cb, state);
if (handler == null) {
Log.e(TAG, "Could not get client death handler for stream: "+mStreamType);
return;
}
handler.mute(state);
}
private class VolumeDeathHandler implements IBinder.DeathRecipient { 

private IBinder mICallback; // To be notified of client's death
private int mMuteCount; // Number of active mutes for this client
VolumeDeathHandler(IBinder cb) {
mICallback = cb;
}
//第3步
// must be called while synchronized on parent VolumeStreamState
public void mute(boolean state) {
if (state) {
if (mMuteCount == 0) {
// Register for client death notification
try {
// mICallback can be 0 if muted by AudioService
if (mICallback != null) {
mICallback.linkToDeath(this, 0);
}
mDeathHandlers.add(this);
// If the stream is not yet muted by any client, set level to 0
if (muteCount() == 0) {
Set set = mIndex.entrySet();
Iterator i = set.iterator();
while (i.hasNext()) {
Map.Entry entry = (Map.Entry)i.next();
int device = ((Integer)entry.getKey()).intValue();
setIndex(0, device, false /* lastAudible */);
}
sendMsg(mAudioHandler,
MSG_SET_ALL_VOLUMES,
SENDMSG_QUEUE,
0,
0,
VolumeStreamState.this, 0);
}
} catch (RemoteException e) {
// Client has died!
binderDied();
return;
}
} else {
Log.w(TAG, "stream: "+mStreamType+" was already muted by this client");
}
mMuteCount++;
} else {
if (mMuteCount == 0) {
Log.e(TAG, "unexpected unmute for stream: "+mStreamType);
} else {
mMuteCount--;
if (mMuteCount == 0) {
// Unregister from client death notification
mDeathHandlers.remove(this);
// mICallback can be 0 if muted by AudioService
if (mICallback != null) {
mICallback.unlinkToDeath(this, 0);
}
if (muteCount() == 0) {
// If the stream is not muted any more, restore its volume if
// ringer mode allows it
if (!isStreamAffectedByRingerMode(mStreamType) ||
mRingerMode == AudioManager.RINGER_MODE_NORMAL) {
Set set = mIndex.entrySet();
Iterator i = set.iterator();
while (i.hasNext()) {
Map.Entry entry = (Map.Entry)i.next();
int device = ((Integer)entry.getKey()).intValue();
setIndex(getIndex(device,
true  /* lastAudible */),
device,
false  /* lastAudible */);
}
sendMsg(mAudioHandler,
MSG_SET_ALL_VOLUMES,
SENDMSG_QUEUE,
0,
0,
VolumeStreamState.this, 0);
}
}
}
}
}
}
public void binderDied() {
Log.w(TAG, "Volume service client died for stream: "+mStreamType);
if (mMuteCount != 0) {
// Reset all active mute requests from this client.
mMuteCount = 1;
mute(false);
}
}
}
private synchronized int muteCount() {
int count = 0;
int size = mDeathHandlers.size();
for (int i = 0; i < size; i++) {
count += mDeathHandlers.get(i).mMuteCount;
}
return count;
}

它们的流程图如下（markdown画的图，不怎么美观）

通过上面的代码我们可以得知VolumeDeathHandler继承IBinder.DeathRecipient.它可以监听申请静音的客户端的存活状态变化。
好吧，下面进入主题。

IBinder.DeathRecipient

/** * Interface for receiving a callback when the process hosting an IBinder has gone away. * * @see #linkToDeath */
public interface DeathRecipient { 

public void binderDied();
}

它是IBinder的内部接口，接口方法void binderDied()。注释的意思大概是这是一个接受Binder所在的宿主进程消失时的回调，并且建议我们去查看linkToDeath。

/** * Register the recipient for a notification if this binder * goes away. If this binder object unexpectedly goes away * (typically because its hosting process has been killed), * then the given {@link DeathRecipient}'s * {@link DeathRecipient#binderDied DeathRecipient.binderDied()} method * will be called. * * <p>You will only receive death notifications for remote binders, * as local binders by definition can't die without you dying as well. * * @throws Throws {@link RemoteException} if the target IBinder's * process has already died. * * @see #unlinkToDeath */
public void linkToDeath(DeathRecipient recipient, int flags)
throws RemoteException;
/** * Remove a previously registered death notification. * The recipient will no longer be called if this object * dies. * * @return Returns true if the <var>recipient</var> is successfully * unlinked, assuring you that its * {@link DeathRecipient#binderDied DeathRecipient.binderDied()} method * will not be called. Returns false if the target IBinder has already * died, meaning the method has been (or soon will be) called. * * @throws Throws {@link java.util.NoSuchElementException} if the given * <var>recipient</var> has not been registered with the IBinder, and * the IBinder is still alive. Note that if the <var>recipient</var> * was never registered, but the IBinder has already died, then this * exception will <em>not</em> be thrown, and you will receive a false * return value instead. */
public boolean unlinkToDeath(DeathRecipient recipient, int flags);

注释说的很清楚，通过一个IBinder.linkToDeath()可以监听这个Binder本身的消失，并调用回调DeathRecipient.binderDied().IBinder.unlinkToDeath()可以取消监听。

Android的c/s服务架构中，难免会发生服务端或者客户端异常终止的情况，而通过IBinder.DeathRecipient可以很好处理这种情况，当IBinder对象异常终止时可以做一些资源释放的处理。

实战 情景模拟

之前说过在Framework代码中AudioService出现过IBinder.DeathRecipient，但是我内心蠢蠢欲动，我就是想自己实践看看效果。有没有方法呢？自然有。

接下来的Demo中我会创建两个App应用，一个作为服务端，一个作为客户端。客户端通过IBinder.DeathRecipient来监听服务端的异常终止情况。
服务端
ITest.aidl

package com.example.deathrecipientdemo;
interface ITest{
void test();
}

TestService.java

package com.example.deathrecipientdemo;
import android.app.Service;
import android.content.Intent;
import android.os.Binder;
import android.os.IBinder;
import android.os.RemoteException;
import android.util.Log;
public class TestService extends Service { 

private static final String TAG = "frank";
private Binder mBinder = new ITest.Stub() {
@Override
public void test() throws RemoteException {
// TODO Auto-generated method stub
Log.i(TAG, "server");
}
};
@Override
public IBinder onBind(Intent intent) {
// TODO Auto-generated method stub
Log.i(TAG, "onBind");
new Thread(new Runnable() {
@Override
public void run() {
// TODO Auto-generated method stub
try {
Thread.sleep(10000);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
Log.i(TAG,e.toString());
}
//结束自己
android.os.Process.killProcess(android.os.Process.myPid());
//TestService.this.stopSelf();
//Log.i("test", "stopSelf");
}
}).start();
return mBinder;
}
}

可以看到，它会在被启动时过10秒后自杀。

<service android:name="com.example.deathrecipientdemo.TestService" android:enabled="true" android:exported="true">
<intent-filter >
<action android:name="com.frank.test"/>
</intent-filter>
</service>

好了，以上是服务器App的部分代码。
接下我新建一个客户端的App

package com.example.testdemo;
public class MainActivity extends Activity { 

private static final String TAG = "frank";
private ServiceConnection mCon;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Intent intent = new Intent();
intent.setComponent(new ComponentName("com.example.deathrecipientdemo","com.example.deathrecipientdemo.TestService"));
intent.setAction("com.frank.test");
final DeathRecipient deathHandle = new DeathRecipient() {
@Override
public void binderDied() {
// TODO Auto-generated method stub
Log.i(TAG, "binder is died");
}
};
mCon = new ServiceConnection() {
@Override
public void onServiceDisconnected(ComponentName name) {
// TODO Auto-generated method stub
Log.i(TAG, "onServiceDisconnected "+name.toShortString());
}
@Override
public void onServiceConnected(ComponentName name, IBinder service) {
// TODO Auto-generated method stub
try {
Log.i(TAG, "onServiceConnected "+name.toShortString()+" "+service.getInterfaceDescriptor());
service.linkToDeath(deathHandle, 0);
} catch (RemoteException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
};
bindService(intent,mCon,Context.BIND_AUTO_CREATE);
}
@Override
protected void onDestroy() {
// TODO Auto-generated method stub
super.onDestroy();
unbindService(mCon);
}
}

接下来就是验证时间，把两个App都装进了手机，先启动服务App,再启动客户端App,然后查看Log。我习惯于打开CMD,然后adb logcat | findstr frank。因为我在demo中将Log的TAG都设置为了frank.

按照理想中的预期，服务端被启动后10秒就会自杀，而自杀时客户端能够监听得到，真实情况是不是这样的呢？看log

I/frank   (17732): onBind
I/frank   (17475): onServiceConnected {com.example.deathrecipientdemo/com.example.deathrecipientdemo
.TestService}  com.example.deathrecipientdemo.ITest
D/ActivityThread(17732): SVC-BIND_SERVICE handled : 0 / BindServiceData{token=android.os.BinderProxy
@5ebc3b5 intent=Intent { act=com.frank.test cmp=com.example.deathrecipientdemo/.TestService }}
I/frank   (17475): binder is died
I/frank   (17475): onServiceDisconnected {com.example.deathrecipientdemo/com.example.deathrecipientd
emo.TestService}

I/frank (17475): binder is died
这个被成功打印，所以代表我们实现成功了，我们能够准确运用IBinder.DeathRecipient接口去监听服务端的消失变动。当然实际开发中，我们要根据业务是否存在这样的需求而去编码，这里只作为学习探讨之用，反正我觉得涉及到服务之间的交互这个功能是很有用武之地的。