【起航计划 036】2015 起航计划 Android APIDemo的魔鬼步伐 35 App->Service->Messenger Service Messenger实现进程间通信

前面LocalService 主要是提供同一Application中组件来使用，如果希望支持不同应用或进程使用Service。可以通过Messenger。使用Messgener可以用来支持进程间通信而无需使用AIDL。

 下面步骤说明里Messenger的使用方法：

• 在Service中定义一个Handler来处理来自Client的请求。
• 使用这个Handler创建一个Messenger (含有对Handler的引用).
• Messenger创建一个IBinder对象返回给Client( onBind方法）。
• Client 使用从Service返回的IBinder重新构造一个Messenger 对象，提供这个Messenger对象可以给Service 发送消息。
• Service提供Handler接受来自Client的消息Message. 提供handleMessage来处理消息。

在这种方式下，Service没有定义可以供Client直接调用的方法。而是通过”Message”来传递信息。

本例Messenger Service 涉及到两个类 MessengerServiceActivities 和 MessengerService.

首先看看Service的定义，在MessengerService定义了一个IncomingHandler ，用于处理来自Client的消息。

    /**
     * Handler of incoming messages from clients.
     */
    class IncomingHandler extends Handler {
        @Override
        public void handleMessage(Message msg) {
            switch (msg.what) {
                case MSG_REGISTER_CLIENT:
                    mClients.add(msg.replyTo);
                    break;
                case MSG_UNREGISTER_CLIENT:
                    mClients.remove(msg.replyTo);
                    break;
                case MSG_SET_VALUE:
                    mValue = msg.arg1;
                    for (int i=mClients.size()-1; i>=0; i--) {
                        try {
                            mClients.get(i).send(Message.obtain(null,
                                    MSG_SET_VALUE, mValue, 0));
                        } catch (RemoteException e) {
                            // The client is dead.  Remove it from the list;
                            // we are going through the list from back to front
                            // so this is safe to do inside the loop.
                            mClients.remove(i);
                        }
                    }
                    break;
                default:
                    super.handleMessage(msg);
            }
        }
    }

然后使用这个IncomingHandler定义一个Messenger：

    /**
     * Target we publish for clients to send messages to IncomingHandler.
     */
    final Messenger mMessenger = new Messenger(new IncomingHandler());

应为这种方法采用的“Bound” Service模式，onBind 需要返回一个IBind对象, 可以通过mMessenger.getBinder()返回与这个Messenger关联的IBinder对象，Client可以通过这个IBinder 对象重新构造一个Messenger对象，从而建立起与Service之间的通信链路。

    /**
     * When binding to the service, we return an interface to our messenger
     * for sending messages to the service.
     */
    @Override
    public IBinder onBind(Intent intent) {
        return mMessenger.getBinder();
    }

再看看Client 的代码MessengerServiceActivities 在 ServiceConnection的 onServiceConnected的定义，这个方法返回MessengerService 的onBind 定义的IBinder对象：

            public void onServiceConnected(ComponentName className,
                    IBinder service) {
                // This is called when the connection with the service has been
                // established, giving us the service object we can use to
                // interact with the service.  We are communicating with our
                // service through an IDL interface, so get a client-side
                // representation of that from the raw service object.
                mService = new Messenger(service);
                mCallbackText.setText("Attached.");

                // We want to monitor the service for as long as we are
                // connected to it.
                try {
                    Message msg = Message.obtain(null,
                            MessengerService.MSG_REGISTER_CLIENT);
                    msg.replyTo = mMessenger;
                    mService.send(msg);
                    
                    // Give it some value as an example.
                    msg = Message.obtain(null,
                            MessengerService.MSG_SET_VALUE, this.hashCode(), 0);
                    mService.send(msg);
                } catch (RemoteException e) {
                    // In this case the service has crashed before we could even
                    // do anything with it; we can count on soon being
                    // disconnected (and then reconnected if it can be restarted)
                    // so there is no need to do anything here.
                }
                
                // As part of the sample, tell the user what happened.
                Toast.makeText(Binding.this, R.string.remote_service_connected,
                        Toast.LENGTH_SHORT).show();
            }

本例实现了Client与Service 之间的双向通信，因此在Client也定义了一个Messenger对象mMessenger，用于处理来自Service的消息。

有了 mService对象，就可以使用send向Service发送消息，如过需要Service 返回信息，可以定义message.replyTo 对象。