Messenger的使用

在Android编程中，要想实现IPC，有如下几种方法：

1，使用AIDL，在需要实现多线程和并发处理时使用

2，继承Binder类来实现

3，使用Messenger

这一节我们研究Messenger的使用。以后有机会研究前两种。

首先，通过查看Messenger的源代码，我们可以看到它有两个构造函数：

    /**     * Create a new Messenger pointing to the given Handler.  Any Message     * objects sent through this Messenger will appear in the Handler as if     * {@link Handler#sendMessage(Message) Handler.sendMessage(Message)} had     * been called directly.     *      * @param target The Handler that will receive sent messages.     */    public Messenger(Handler target) {        mTarget = target.getIMessenger();    }

和

    /**     * Create a Messenger from a raw IBinder, which had previously been     * retrieved with {@link #getBinder}.     *      * @param target The IBinder this Messenger should communicate with.     */    public Messenger(IBinder target) {        mTarget = IMessenger.Stub.asInterface(target);    }

前者用来给服务端调用创建Messenger，后者给客户端调用。

使用方法

• 首先，服务端实现一个Handler ,用来接收客户端的信息
• 使用上述Handler创建一个 Messenger 对象（它是Handler类中的一个引用，Handler类的成员变量mMessenger指向这个新创建的对象）。
• 上述Messenger对象创建一个IBinder 对象, 服务端的onBind()返回该对象给客户端
• 客户端使用上述IBinder对象初始化Messenger对象(该对象引用服务端的Handler),然后服务端就可以使用它来发生 Message 对象给服务端。
• 服务端在它的Handler对象中收到上述Message对象，使用 handleMessage() 方法来处理。

使用这种方式，不存在客户端调用服务端的“方法”，而是客户端发送“信息”（即Message对象），服务端在它的Handler中接收并处理。

如下是使用Messager的服务端的代码：

public class MessengerService extends Service {    /** Command to the service to display a message */    static final int MSG_SAY_HELLO = 1;    /**     * Handler of incoming messages from clients.     */    class IncomingHandler extends Handler {        @Override        public void handleMessage(Message msg) {            switch (msg.what) {                case MSG_SAY_HELLO:                    Toast.makeText(getApplicationContext(), "hello!", Toast.LENGTH_SHORT).show();                    break;                default:                    super.handleMessage(msg);            }        }    }    /**     * Target we publish for clients to send messages to IncomingHandler.     */    final Messenger mMessenger = new Messenger(new IncomingHandler());    /**     * When binding to the service, we return an interface to our messenger     * for sending messages to the service.     */    @Override    public IBinder onBind(Intent intent) {        Toast.makeText(getApplicationContext(), "binding", Toast.LENGTH_SHORT).show();        return mMessenger.getBinder();    }}

上述 handleMessage() 方法就是服务端接收消息并决定如何处理的地方。

客户端只需要使用服务端返回的IBinder创建Messenger对象，然后使用该对象的send()方法发生消息即可。代码如下（先绑定服务，然后发送MSG_SAY_HELLO 消息给服务端：

public class ActivityMessenger extends Activity {    /** Messenger for communicating with the service. */    Messenger mService = null;    /** Flag indicating whether we have called bind on the service. */    boolean mBound;    /**     * Class for interacting with the main interface of the service.     */    private ServiceConnection mConnection = new ServiceConnection() {        public void onServiceConnected(ComponentName className, IBinder service) {            // This is called when the connection with the service has been            // established, giving us the object we can use to            // interact with the service.  We are communicating with the            // service using a Messenger, so here we get a client-side            // representation of that from the raw IBinder object.            mService = new Messenger(service);            mBound = true;        }        public void onServiceDisconnected(ComponentName className) {            // This is called when the connection with the service has been            // unexpectedly disconnected -- that is, its process crashed.            mService = null;            mBound = false;        }    };    public void sayHello(View v) {        if (!mBound) return;        // Create and send a message to the service, using a supported 'what' value        Message msg = Message.obtain(null, MessengerService.MSG_SAY_HELLO, 0, 0);        try {            mService.send(msg);        } catch (RemoteException e) {            e.printStackTrace();        }    }    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.main);    }    @Override    protected void onStart() {        super.onStart();        // Bind to the service        bindService(new Intent(this, MessengerService.class), mConnection,            Context.BIND_AUTO_CREATE);    }    @Override    protected void onStop() {        super.onStop();        // Unbind from the service        if (mBound) {            unbindService(mConnection);            mBound = false;        }    }}

上述示例没有显示服务端如何响应客户端。如果需要服务端响应的话，需要将客户端创建的Messenger对象赋值给的Message的replyTo参数，如下：

服务端：

public class MessengerService extends Service {    public static final int MSG_TAG_REMOTE = 0x110;    public static final int MSG_TAG_CLIENT = 0x111;    private int mCounter = 0;        class ServiceHandler extends Handler{        @Override        public void handleMessage(Message msg) {            switch (msg.what) {                case  MSG_TAG_REMOTE:                    Messenger client = msg.replyTo;                    try {                        client.send(Message.obtain(null, MSG_TAG_CLIENT, ++mCounter,0));                    } catch (RemoteException e) {                          e.printStackTrace();                    }                                }                            super.handleMessage(msg);        }            }    final Messenger mMessenger = new Messenger(new ServiceHandler());    /* (non-Javadoc)     * @see android.app.Service#onBind(android.content.Intent)     */    @Override    public IBinder onBind(Intent intent) {        return (mMessenger == null) ? null : mMessenger.getBinder();    }     }

客户端：

public class MessengerServceActivity extends Activity {    private TextView mTextView;    private Messenger mServiceMessenger;    final Messenger mClientMessenger = new Messenger(new ClientHandler());    ServiceConnection mServiceConn = new ServiceConnection(){        @Override        public void onServiceConnected(ComponentName name, IBinder service) {            mServiceMessenger = new Messenger(service);            Message msg = Message.obtain(null, MessengerService.MSG_TAG_REMOTE);            msg.replyTo = mClientMessenger;            try {                mServiceMessenger.send(msg);            } catch (RemoteException e) {                e.printStackTrace();            }                    }        @Override        public void onServiceDisconnected(ComponentName name) {            mServiceMessenger = null;                    }            };    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.one_textview);        mTextView = (TextView) findViewById(R.id.content_show);        bindService(new Intent(this, MessengerService.class), mServiceConn, BIND_AUTO_CREATE);    }        class ClientHandler extends Handler{        @Override        public void handleMessage(Message msg) {            switch (msg.what) {                case MessengerService.MSG_TAG_CLIENT:                    if (mTextView != null) {                        mTextView.setText(msg.arg1+"");                    }                    break;                default:                        super.handleMessage(msg);                    break;            }                    }            }}