【起航計劃 036】2015 起航計劃 Android APIDemo的魔鬼步伐 35 App->Service->Messenger Service Messenger實現進程間通訊

前面LocalService 主要是提供同一Application中組件來使用，若是但願支持不一樣應用或進程使用Service。能夠經過Messenger。使用Messgener能夠用來支持進程間通訊而無需使用AIDL。

 下面步驟說明裏Messenger的使用方法：

• 在Service中定義一個Handler來處理來自Client的請求。
• 使用這個Handler建立一個Messenger (含有對Handler的引用).
• Messenger建立一個IBinder對象返回給Client( onBind方法）。
• Client 使用從Service返回的IBinder從新構造一個Messenger 對象，提供這個Messenger對象能夠給Service 發送消息。
• Service提供Handler接受來自Client的消息Message. 提供handleMessage來處理消息。

在這種方式下，Service沒有定義能夠供Client直接調用的方法。而是經過」Message」來傳遞信息。

本例Messenger Service 涉及到兩個類 MessengerServiceActivities 和 MessengerService.

首先看看Service的定義，在MessengerService定義了一個IncomingHandler ，用於處理來自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);
            }
        }
    }

 

而後使用這個IncomingHandler定義一個Messenger：

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

 

應爲這種方法採用的「Bound」 Service模式，onBind 須要返回一個IBind對象, 能夠經過mMessenger.getBinder()返回與這個Messenger關聯的IBinder對象，Client能夠經過這個IBinder 對象從新構造一個Messenger對象，從而創建起與Service之間的通訊鏈路。

    /**
     * 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 的代碼MessengerServiceActivities 在 ServiceConnection的 onServiceConnected的定義，這個方法返回MessengerService 的onBind 定義的IBinder對象：

            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();
            }

 

本例實現了Client與Service 之間的雙向通訊，所以在Client也定義了一個Messenger對象mMessenger，用於處理來自Service的消息。

有了 mService對象，就能夠使用send向Service發送消息，如過須要Service 返回信息，能夠定義message.replyTo 對象。