Android Messenger原理

前言

Messenger中文意思是送信者，通过Messenger我们可以在不同进程之间传递Message对象，其底层也是通过上文讲到的AIDL实现的，先来看看基本用法

一、基本用法

以客户端发送消息给服务端，服务端回复一个消息给客户端为例。

// 运行在其它进程
class MessengerService : Service() {
    class MessengerHandler : Handler() {
        val TAG = "MessengerHandler"
        override fun handleMessage(msg: Message?) {
            when (msg?.what) {
                ADD_BOOK -> {
                    // 必须设置classLoader不然会抛出异常
                    msg.data.classLoader = Thread.currentThread().contextClassLoader
                    Log.d(TAG, "do add book ${msg.data.get("book")}")
                    val replyMessage = Message.obtain(null, REPLY_ADD_BOOK)
                    val bundle = Bundle()
                    bundle.putString("reply", "我收到了响应")
                    replyMessage.data = bundle
                    msg.replyTo.send(replyMessage)
                }
                ALL_BOOKS -> {
                    Log.d(TAG, "do all books")
                }
            }
        }
    }
    override fun onBind(intent: Intent?): IBinder? {
        val messenger = Messenger(MessengerHandler())
        return messenger.binder
    }
}
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接着在MainActivity中绑定服务，代码如下

class MainActivity : AppCompatActivity() {
 
    private lateinit var connection: ServiceConnection
    private lateinit var replyMessenger: Messenger
    private var messenger: Messenger? = null
    
    class GetReplyHandler : Handler() {
        private var TAG = "MainActivity"
        override fun handleMessage(msg: Message?) {
            when (msg?.what) {
                REPLY_ADD_BOOK -> {
                    Log.d(TAG, msg.data.getString("reply"))
                }
            }
        }
    }
 
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        connection = object : ServiceConnection {
            override fun onServiceConnected(name: ComponentName?, service: IBinder?) {
                messenger = Messenger(service)
            }
            override fun onServiceDisconnected(name: ComponentName?) {
            }
        }
        replyMessenger = Messenger(GetReplyHandler())
        val intent = Intent(this, MessengerService::class.java)
        bindService(intent, connection, Context.BIND_AUTO_CREATE)
    }
 
    fun send(v: View) {
        val msg = Message.obtain(null, ADD_BOOK)
    //    msg.obj = Book(0, "第0本书") 不能使用obj跨进程传递自定义的Parcelable对象，使用Bundle因为其可以设置classLoader
        val msg = Message.obtain(null, ADD_BOOK)
        val bundle = Bundle()
        bundle.putParcelable("book", Book(0, "第0本书"))
        msg.data = bundle
        msg.replyTo = replyMessenger
        messenger?.send(msg)
    }
    
    override fun destroy() {
        unbindService(connection)
    }
}
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注意send方法内部发送的Parcelable类(这里是Book)在服务端必须也要存在，这样当调用MainActivity的send方法时就会服务进程就会打印出do add book，下面就来看看该过程的源码

二、源码分析

• 客户端发送消息给服务端

首先我们在MessengerService的onBind中创建了一个Messenger实例，所以我们就从Messenger构造器开始说起

public Messenger(Handler target) {
    mTarget = target.getIMessenger();
}
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继续看看Handler的getIMessager

final IMessenger getIMessenger() {
    synchronized (mQueue) {
        if (mMessenger != null) {
            return mMessenger;
        }
        mMessenger = new MessengerImpl();
        return mMessenger;
    }
}
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刚开始mMessenger肯定为null，继续看看MessengerImpl

private final class MessengerImpl extends IMessenger.Stub {
    public void send(Message msg) {
        msg.sendingUid = Binder.getCallingUid();
        Handler.this.sendMessage(msg);
    }
}
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这里居然有个Stub！瞬间想起了AIDL，于是就去找了找有没有IMessenger.aidl这个文件，最终找到了该文件，文件内容如下

oneway interface IMessenger {
    void send(in Message msg);
}
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这里的oneway表示调用send方法并不会挂起当前线程等待服务端执行，而是会立即返回，send方法实现为将收到的消息发送给创建Messenger时的入参，至此服务端的Messenger使用分析完毕接着看看客户端中Messenger的使用，在onServiceConnected中通过传入的Binder对象构造了Messenger对象。

public Messenger(IBinder target) {
    mTarget = IMessenger.Stub.asInterface(target);
}
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调用asInterface(上篇文章有讲到)拿到IMessenger.Stub.Proxy实例赋值给mTarget，最后客户端通过调用Messengr.send发送消息

public void send(Message message) throws RemoteException {
    mTarget.send(message);
}
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不管这里send是个直接调用还是IPC调用都会调用到以下MessengerImpl的send方法，该方法又把消息发送到了对应的Handler，因此服务端的Handler就能收到消息了

• 服务端收到消息回复客户端

注意客户端将replyMessenger设置给了Message.replyTo然后发送消息，这个过程中会调用Message.writeToParcel

public void writeToParcel(Parcel dest, int flags) {
    if (callback != null) {
        throw new RuntimeException(
            "Can't marshal callbacks across processes.");
    }
    dest.writeInt(what);
    dest.writeInt(arg1);
    dest.writeInt(arg2);
    if (obj != null) {
        try {
            Parcelable p = (Parcelable)obj;
            dest.writeInt(1);
            dest.writeParcelable(p, flags);
        } catch (ClassCastException e) {
            throw new RuntimeException(
                "Can't marshal non-Parcelable objects across processes.");
        }
    } else {
        dest.writeInt(0);
    }
    dest.writeLong(when);
    dest.writeBundle(data);
    Messenger.writeMessengerOrNullToParcel(replyTo, dest);
    dest.writeInt(sendingUid);
}
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又会调用到Messenger.writeMessengerOrNullToParcel

public static void writeMessengerOrNullToParcel(Messenger messenger,
        Parcel out) {
    out.writeStrongBinder(messenger != null ? messenger.mTarget.asBinder()
            : null);
}
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向Parcel中写入了一个MessengerImpl实例(Binder)，然后在IPC结束后会调用Message.readFromParcel

private void readFromParcel(Parcel source) {
    what = source.readInt();
    arg1 = source.readInt();
    arg2 = source.readInt();
    if (source.readInt() != 0) {
        obj = source.readParcelable(getClass().getClassLoader());
    }
    when = source.readLong();
    data = source.readBundle();
    replyTo = Messenger.readMessengerOrNullFromParcel(source);
    sendingUid = source.readInt();
}
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又调用到了Messenger.readMessengerOrNullFromParcel

public static Messenger readMessengerOrNullFromParcel(Parcel in) {
    IBinder b = in.readStrongBinder();
    return b != null ? new Messenger(b) : null;
}
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如果是跨进程传递那么读取的会是一个BinderProxy对象，通过该BinderProxy构造Messenger对象其内部的mTarget就会是一个IMessenger.Stub.Proxy实例，因此服务端就可以调用客户端的对应方法了