Messenger简介
Messenger和AIDL是实现进程间通信(interprocess communication)的两种方式.
实际上,Messenger的实现其实是对AIDL的封装.
Messenger适合于多进程单线程,AIDL适合于多进程多线程,需要开发者自己实现线程安全.
google官方文档指出对于大部分的程序,service不需要执行多线程,所以应该首先考虑使用Messenger.
为什么需要进程间通信?
因为不同进程之间的数据是不共享的.
实践(以音乐播放器demo为例)
关键词: bind + messenger + handler
step1 (AndroidManifest.xml)在AndroidManifest.xml中注册service,并设置为其android:process属性赋值
注意① 忘记注册service的话,程序不会响应,但是也不会崩溃的...
注意② 此处设置该service为一个全局进程,意即不同应用程序可以共享该进程,若process以":"开头,则表示其为该应用程序的私有进程.
<service android:name=".MusicService"
android:process="com.example.janiszhang.musicplayer.service.process"/>
step2 (MusicService)在service中实现一个继承Handler的子类(),用于处理由activity发来的message
class IncomingHandler extends Handler {
@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
mActivityMessenger = msg.replyTo;
switch (msg.what) {
case 0:
mMediaPlayer.stop();
i = msg.arg1;
mMediaPlayer = MediaPlayer.create(MusicService.this, mMusicDatas.get(i).getSrc());
if(isPlaying) {
mMediaPlayer.start();
}
//mRemoteViews.setTextViewText(R.id.music_name, mMusicDatas.get(i).getName());
//mRemoteViews.setTextViewText(R.id.singer_name, mMusicDatas.get(i).getSinger());
//mNotificationManager.notify(NOTIFICATION_ID, mBuilder.build());
break;
case 1:
if(msg.arg1==1){
isPlaying =false;
mMediaPlayer.pause();
//mRemoteViews.setImageViewResource(R.id.btn_play, R.drawable.note_btn_play);
//mNotificationManager.notify(NOTIFICATION_ID, mBuilder.build());
} else {
isPlaying = true;
i = msg.arg2;
mMediaPlayer = MediaPlayer.create(MusicService.this, mMusicDatas.get(i).getSrc());
mMediaPlayer.start();
//mRemoteViews.setImageViewResource(R.id.btn_play, R.drawable.note_btn_pause);
//mNotificationManager.notify(NOTIFICATION_ID, mBuilder.build());
}
break;
default:
break;
}
}
}
step3 (MusicService)实例化自定义的handler,作为参数,创建一个Messenger
Messenger mMessenger = new Messenger(new IncomingHandler());
step4 (MusicService)实现onBind()方法
@Nullable
@Override
public IBinder onBind(Intent intent) {
return mMessenger.getBinder();
}
step5 (MusicActivity) 创建一个ServiceConnection实例,在onServiceConnected方法中获取到service返回的messenger.
private ServiceConnection mServiceConnection = new ServiceConnection() {
@Override
public void onServiceConnected(ComponentName name, IBinder service) {
mMessenger = new Messenger(service);
}
@Override
public void onServiceDisconnected(ComponentName name) {
}
};
step6 (MusicActivity)bindservice
Intent intent = new Intent(this, MusicService.class);
bindService(intent, mServiceConnection, BIND_AUTO_CREATE);
step7 bind成功后,就可以获取到一个Messenger的实例,通过该实例activity可以向另外一个进程中的service发送message,例如:
mNextBtn.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
mIndex = (mIndex+1)>=mMusicDatas.size()? 0 : mIndex+1;
setMusicNameAndSingerName(mIndex);
if(mMessenger != null) {
Message message = Message.obtain(null, 0);
message.arg1 = mIndex;
message.replyTo = mActivityMessenger;
try {
mMessenger.send(message);
} catch (RemoteException e) {
e.printStackTrace();
}
}
}
});
至此,基于Messenger的由activity到service的进程间通信就是实现完成了.
遇到的问题:
当我试图Message实例的obj赋值为true时,会导致异常:
java.lang.RuntimeException: Can't marshal non-Parcelable objects across processes.
意思是说obj应该是一个实现了Parcelable接口的对象.
解决方案:对于这种情况,应该使用bundle来传递数据.
总结一下:
1.activity与service以bind的方式通信.
2.service中创建一个用于处理数据的handler,并以之为参数创建一个Messenger,通过onbind返回给activity.
3.activity使用service返回的Messenger实例send message