【起航計劃 036】2015 起航計劃 Android APIDemo的魔鬼步伐 35 App->Service->Messenger Service Messenger實現處理序間通訊

標籤：

前面LocalService 主要是提供同一Application中組件來使用，如果希望支援不同應用或進程使用Service。可以通過Messenger。使用Messgener可以用來支援處理序間通訊而無需使用AIDL。

 下面步驟說明裡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重新構造一個Messenger 對象，提供這個Messenger對象可以給Service 發送訊息。
• Service提供Handler接受來自Client的訊息Message. 提供handleMessage來處理訊息。

在這種方式下，Service沒有定義可以供Client直接調用的方法。而是通過”Message”來傳遞資訊。

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的訊息。

    /**     * 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());

 

應為這種方法採用的“Bound” Service模式，onBind 需要返回一個IBind對象, 可以通過mMessenger.getBinder()返回與這個Messenger關聯的IBinder對象，Client可以通過這個IBinder 對象重新構造一個Messenger對象，從而建立起與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，因此在Client也定義了一個Messenger對象mMessenger，用於處理來自Service的訊息。

有了 mService對象，就可以使用send向Service發送訊息，如過需要Service 返回資訊，可以定義message.replyTo 對象。

 

【起航計劃 036】2015 起航計劃 Android APIDemo的魔鬼步伐 35 App->Service->Messenger Service Messenger實現處理序間通訊