A bound Service 提供了一种CS模式的Service。
A bound service allows components (such as activities) to bind to the service, send requests, receive responses, and even perform interprocess communication

Server--Extend a Service， 然后实现onBind()方法，返回IBinder对象。
Client--bindService(),然后必须实现ServiceConnection接口，其具有两个方法，onServiceConnected(),可通知连接服务的状态，并返回IBinder对象。
其中onBinder只在第一次服务被连接时才会被调用，之后将直接返回IBinder对象。
当最后一个client unbind时，系统将destroy 该Service.

Creating a Bound Service

• 继承Binder类
  然后将它的一个实例作为onBind()返回值。适用于Server和Client在同一进程内。

创建Server

public class LocalService extends Service {
    // Binder given to clients
    private final IBinder mBinder = new LocalBinder();
    // Random number generator
    private final Random mGenerator = new Random();

    /**
     * Class used for the client Binder.  Because we know this service always
     * runs in the same process as its clients, we don't need to deal with IPC.
     */
    public class LocalBinder extends Binder {
        LocalService getService() {
            // Return this instance of LocalService so clients can call public methods
            return LocalService.this;
        }
    }

    @Override
    public IBinder onBind(Intent intent) {
        return mBinder;
    }

    /** method for clients */
    public int getRandomNumber() {
      return mGenerator.nextInt(100);
    }
}

Tip: 在LocalBinder中提供了getService()方法，使得客户端可以获得Service的实例，这样就可以调用Service的公共方法。

Client调用：

public class BindingActivity extends Activity {
    LocalService mService;
    boolean mBound = false;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
    }

    @Override
    protected void onStart() {
        super.onStart();
        // Bind to LocalService
        Intent intent = new Intent(this, LocalService.class);
        bindService(intent, mConnection, Context.BIND_AUTO_CREATE);
    }

    @Override
    protected void onStop() {
        super.onStop();
        // Unbind from the service
        if (mBound) {
            unbindService(mConnection);
            mBound = false;
        }
    }

    /** Called when a button is clicked (the button in the layout file attaches to
      * this method with the android:onClick attribute) */
    public void onButtonClick(View v) {
        if (mBound) {
            // Call a method from the LocalService.
            // However, if this call were something that might hang, then this request should
            // occur in a separate thread to avoid slowing down the activity performance.
            int num = mService.getRandomNumber();
            Toast.makeText(this, "number: " + num, Toast.LENGTH_SHORT).show();
        }
    }

    /** Defines callbacks for service binding, passed to bindService() */
    private ServiceConnection mConnection = new ServiceConnection() {

        @Override
        public void onServiceConnected(ComponentName className,
                IBinder service) {
            // We've bound to LocalService, cast the IBinder and get LocalService instance
            LocalBinder binder = (LocalBinder) service;
            mService = binder.getService();
            mBound = true;
        }

        @Override
        public void onServiceDisconnected(ComponentName arg0) {
            mBound = false;
        }
    };
}

可注意到Service的bind，使用以及unBind()的使用。

• 使用Messenger
  在Service中Create 一个Handler，然后可以接收和处理不同的Message type，同时Messenger 也可返回一个IBinder对象给客户端。此外，客户端也可定义自己的Messenger，然后Service可以发送Message以便返回结果。最简单的实现IPC通信。适用于跨进程。

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();
    }
}

以下是Messenger的定义

Reference to a Handler, which others can use to send messages to it. This allows for the implementation of message-based communication across processes, by creating a Messenger pointing to a Handler in one process, and handing that Messenger to another process.

意思是它是对Handler的引用，其他组件可通过它发送消息到handler。允许基于message的handle IPC。它是对Binder的简单包装。

Client 调用

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;
        }
    }
}

就是一个事件传递。

• 使用AIDL
  上个使用Messenger的方式，底层结构也是使用AIDL。但Messenger则只是依次处理。但若想同时处理，直接使用AIDL。适用于跨进程。

Tip: 大部分应用无需使用AIDL来创建bound service, 它可能需要多线程处理能力以及更复杂的实现。

Caution:Only activities, services, and content providers can bind to a service—you cannot bind to a service from a broadcast receiver.

bindService(intent, mConnection, Context.BIND_AUTO_CREATE);
第三个参数BIND_AUTO_CREATE指create service if its not already alive. 还可以是BIND_DEBUG_UNBIND, BIND_NOT_FOREGROUD， 或者0。

建议：you should bind during onStart() and unbind during onStop().如果希望在App可见时。
若想在activity接收响应在stopped时候，可bind during onCreate() and unbind during onDestroy().