RxJava extension for Gcm which acts as an architectural approach to easily satisfy the requirements of an android app when dealing with push notifications.

• Remove android boilerplate code (not need for Manifest or Service(s) configuration).
• Decouple presentation responsibilities from data responsibilities when receiving notifications.
• Deploy a targeting strategy to aim for the desired Activity/Fragment when receiving notifications.

Add RxGcm dependency and Google Services plugin to project level build.gradle.

apply plugin: 'com.google.gms.google-services'

dependencies {
    compile 'com.github.VictorAlbertos:RxGcm:0.3.0'
    compile 'io.reactivex:rxjava:1.1.5'
}

Add Google Services to classpath and jitpack repository to root level build.gradle.

dependencies {
     classpath 'com.google.gms:google-services:2.1.0'
}

allprojects {
    repositories {
        //..
        maven { url "https://jitpack.io" }
    }
}

There is, thought, one step behind which RxGcm can't do for you. You need to create a google-services.json configuration file and place it in your Android application module. (You can create and download it from here)

GcmReceiverData implementation should be responsible for updating the data models. The onNotification method requires to return an instance of the observable supplied as argument, after applying doOnNext operator to perform the update action:

public class AppGcmReceiverData implements GcmReceiverData {

  
 
 	@Override public Observable<Message> onNotification(Observable<Message> oMessage) {
        
 		return oMessage.doOnNext(message -> {});
    
 	}

 	
 }

The observable type is an instance of Message, which holds a reference to the android Application instance, the Bundle notification and a method called target(), which returns the key associated with this notification.

public class AppGcmReceiverData implements GcmReceiverData {

    @Override public Observable<Message> onNotification(Observable<Message> oMessage) {
        return oMessage.doOnNext(message -> {
            Bundle payload = message.payload();

            String title = payload.getString("title");
            String body = payload.getString("body");

            if (message.target().equals("issues")) SimpleCache.addIssue(new Notification(title, body));
            else if (message.target().equals("supplies")) SimpleCache.addSupply(new Notification(title, body));
        });
    }
    
} 

To RxGcm be able to return a not null string value when calling target() method, you need to add the key rx_gcm_key_target to the payload of the push notification:

{ 
  "data": {
    "title":"A title 4",
    "body":"A body 4",
    "rx_gcm_key_target":"supplies"
  },
  "to":"token_device"
  }
}

If rx_gcm_key_target is not added to the json payload, you will get a null value when calling the target() method. So, you can ignore this, but you would be missing the benefits of the targeting strategy.

Both of them will be called only after GcmReceiverData observable has reached onCompleted() state. This way it’s safe to assume that any operation related to updating the data model has been successfully achieved, and now it’s time to reflect these updates in the presentation layer.

GcmReceiverUIBackground implementation will be called when a notification is received and the application is in the background. Probably the implementation class will be responsable for building and showing system notifications.

public class AppGcmReceiverUIBackground implements GcmReceiverUIBackground {

 
   
	@Override public void onNotification(Observable<Message> oMessage) {
        
		oMessage.subscribe(message -> buildAndShowNotification(message));
    
	}
	
}

GcmReceiverUIForeground implementation will be called when a notification is received and the application is in the foreground. The implementation class must be an Activity or an android.support.v4.app.Fragment. GcmReceiverUIForeground exposes a method called matchesTarget(), which receives an string (the value of the rx_gcm_key_target node payload notification) and forces to the implementation class to return a boolean.

If the current Activity or visible Fragment matchesTarget() method returns true, onTargetNotification() method will be called, otherwise onMismatchTargetNotification() method will be called.

public abstract class BaseFragment extends android.support.v4.app.Fragment implements GcmReceiverUIForeground {

    
	
    @Override public void onMismatchTargetNotification(Observable<Message> oMessage) {
        oMessage.subscribe(message -> {
            showAlert(message);
        });
    }  
	 

}

public class FragmentIssues extends BaseFragment {

    
	
    @Override public void onTargetNotification(Observable<Message> oMessage) {
        oMessage.subscribe(message -> {
            notificationAdapter.notifyDataSetChanged();
        });
    }   
	 
    @Override public boolean matchesTarget(String key) {
        return "issues".equals(key);
    }
	 

}

public class FragmentSupplies extends android.support.v4.app.Fragment implements GcmReceiverUIForeground {

    
	
    @Override public void onTargetNotification(Observable<Message> oMessage) {
        oMessage.subscribe(message -> {
            notificationAdapter.notifyDataSetChanged();
        });
    }     
	 

	@Override public boolean matchesTarget(String key) {
        return "supplies".equals(key);
    }
}

Limitation:: Your fragments need to extend from android.support.v4.app.Fragment instead of android.app.Fragment, otherwise they won't be notified.

GcmRefreshTokenReceiver implementation will be called when the token has been updated. As the documentation points out, the token device may need to be refreshed for some particular reason.

public class RefreshTokenReceiver implements GcmRefreshTokenReceiver {
    
    @Override public void onTokenReceive(Observable<TokenUpdate> oTokenUpdate) {
        oTokenUpdate.subscribe(tokenUpdate -> {}, error -> {});
    }
    
}

If at some point you need to retrieve the gcm token device -e.g for updating the value on your server, you could do it easily calling RxGcm.Notifications.currentToken:

    RxGcm.Notifications.currentToken().subscribe(token -> {}, error -> {});

Once you have implemented GcmReceiverData and GcmReceiverUIBackground interfaces is time to register them in your Android Application class calling RxGcm.Notifications.register. Plus, register RefreshTokenReceiver implementation too at this point.

public class RxSampleApp extends Application {

    @Override public void onCreate() {
        super.onCreate();

        RxGcm.Notifications.register(this, AppGcmReceiverData.class, AppGcmReceiverUIBackground.class)
                .subscribe(token -> {}, error -> {});   
                
        RxGcm.Notifications.onRefreshToken(RefreshTokenReceiver.class);
    }

}

Important: The observable returned by RxGcm.Notifications.register()method will not emit the token twice. It means that it will be emit the token only the first time RxGgm asks to Google for a token. But if the token is already cached, the observable will complete without emitting the item.

RxGcm.Notifications.register method uses internally RxAndroid. Thanks to this, the observable provided observes on the Main Thread and subscribes on an IO thread. This means you do not need to worry about threading and sync. But if you need to change this behaviour, you can do it easily setting in which scheduler the observable needs to observe and subscribe.

There is a complete example of RxGcm in the app module. Plus, it has an integration test managed by Espresso test kit which show several uses cases.

You can easily send http post request to Google Cloud Messaging server using Postman or Advanced Rest Client. Or you can send directly push notifications using this page.

Víctor Albertos

• https://twitter.com/_victorAlbertos
• https://linkedin.com/in/victoralbertos
• https://github.com/VictorAlbertos

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