android:使用Messenger进行进程间通信(一)

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

github地址:https://github.com/zhangbz/MusicPlayer