Android equivalent to NSNotificationCenter

Question

In the process of porting an iPhone application over to android, I am looking for the best way to communicate within the app. Intents seem to be the way to go, is this the b

Answer 1

Here is something similar to @Shiki answer, but from the angle of iOS developers and Notification center.

First create some kind of NotificationCenter service:

public class NotificationCenter {

 public static void addObserver(Context context, NotificationType notification, BroadcastReceiver responseHandler) {
    LocalBroadcastManager.getInstance(context).registerReceiver(responseHandler, new IntentFilter(notification.name()));
 }

 public static void removeObserver(Context context, BroadcastReceiver responseHandler) {
    LocalBroadcastManager.getInstance(context).unregisterReceiver(responseHandler);
 }

 public static void postNotification(Context context, NotificationType notification, HashMap<String, String> params) {
    Intent intent = new Intent(notification.name());
    // insert parameters if needed
    for(Map.Entry<String, String> entry : params.entrySet()) {
        String key = entry.getKey();
        String value = entry.getValue();
        intent.putExtra(key, value);
    }
    LocalBroadcastManager.getInstance(context).sendBroadcast(intent);
 }
}

Then, you will also need some enum type to be secure of mistakes in coding with strings - (NotificationType):

public enum NotificationType {

   LoginResponse;
   // Others

}

Here is usage(add/remove observers) for example in activities:

public class LoginActivity extends AppCompatActivity{

    private BroadcastReceiver loginResponseReceiver = new BroadcastReceiver() {
        @Override
        public void onReceive(Context context, Intent intent) {
           // do what you need to do with parameters that you sent with notification

           //here is example how to get parameter "isSuccess" that is sent with notification
           Boolean result = Boolean.valueOf(intent.getStringExtra("isSuccess"));
        }
    };
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_login);

        //subscribe to notifications listener in onCreate of activity
        NotificationCenter.addObserver(this, NotificationType.LoginResponse, loginResponseReceiver);
    }

    @Override
    protected void onDestroy() {
        // Don't forget to unsubscribe from notifications listener
        NotificationCenter.removeObserver(this, loginResponseReceiver);
        super.onDestroy();
    }
}

and here is finally how we post notification to NotificationCenter from some callback or rest service or whatever:

public void loginService(final Context context, String username, String password) {
    //do some async work, or rest call etc.
    //...

    //on response, when we want to trigger and send notification that our job is finished
    HashMap<String,String> params = new HashMap<String, String>();          
    params.put("isSuccess", String.valueOf(false));
    NotificationCenter.postNotification(context, NotificationType.LoginResponse, params);
}

that's it, cheers!

Answer 2

You could try this: http://developer.android.com/reference/java/util/Observer.html

Answer 3

The best equivalent I found is LocalBroadcastManager which is part of the Android Support Package.

From the LocalBroadcastManager documentation:

Helper to register for and send broadcasts of Intents to local objects within your process. This is has a number of advantages over sending global broadcasts with sendBroadcast(Intent):

• You know that the data you are broadcasting won't leave your app, so don't need to worry about leaking private data.
• It is not possible for other applications to send these broadcasts to your app, so you don't need to worry about having security holes they can exploit.
• It is more efficient than sending a global broadcast through the system.

When using this, you can say that an Intent is an equivalent to an NSNotification. Here is an example:

ReceiverActivity.java

An activity that watches for notifications for the event named "custom-event-name".

@Override
public void onCreate(Bundle savedInstanceState) {

  ...
  
  // Register to receive messages.
  // This is just like [[NSNotificationCenter defaultCenter] addObserver:...]
  // We are registering an observer (mMessageReceiver) to receive Intents
  // with actions named "custom-event-name".
  LocalBroadcastManager.getInstance(this).registerReceiver(mMessageReceiver,
      new IntentFilter("custom-event-name"));
}

// Our handler for received Intents. This will be called whenever an Intent
// with an action named "custom-event-name" is broadcasted.
private BroadcastReceiver mMessageReceiver = new BroadcastReceiver() {
  @Override
  public void onReceive(Context context, Intent intent) {
    // Get extra data included in the Intent
    String message = intent.getStringExtra("message");
    Log.d("receiver", "Got message: " + message);
  }
};

@Override
protected void onDestroy() {
  // Unregister since the activity is about to be closed.
  // This is somewhat like [[NSNotificationCenter defaultCenter] removeObserver:name:object:] 
  LocalBroadcastManager.getInstance(this).unregisterReceiver(mMessageReceiver);
  super.onDestroy();
}

SenderActivity.java

The second activity that sends/broadcasts notifications.

@Override
public void onCreate(Bundle savedInstanceState) {
  
  ...
  
  // Every time a button is clicked, we want to broadcast a notification.
  findViewById(R.id.button_send).setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View v) {
      sendMessage();
    }
  });
}

// Send an Intent with an action named "custom-event-name". The Intent sent should 
// be received by the ReceiverActivity.
private void sendMessage() {
  Log.d("sender", "Broadcasting message");
  Intent intent = new Intent("custom-event-name");
  // You can also include some extra data.
  intent.putExtra("message", "This is my message!");
  LocalBroadcastManager.getInstance(this).sendBroadcast(intent);
}

With the code above, every time the button R.id.button_send is clicked, an Intent is broadcasted and is received by mMessageReceiver in ReceiverActivity.

The debug output should look like this:

01-16 10:35:42.413: D/sender(356): Broadcasting message
01-16 10:35:42.421: D/receiver(356): Got message: This is my message! 

Answer 4

Kotlin: Here's a @Shiki's version in Kotlin with a little bit refactor in a fragment.

1. Register the observer in Fragment.

Fragment.kt

class MyFragment : Fragment() {

    private var mContext: Context? = null

    private val mMessageReceiver = object: BroadcastReceiver() {
        override fun onReceive(context: Context?, intent: Intent?) {
            //Do something here after you get the notification
            myViewModel.reloadData()
        }
    }

    override fun onAttach(context: Context) {
        super.onAttach(context)

        mContext = context
    }

    override fun onStart() {
        super.onStart()
        registerSomeUpdate()
    }

    override fun onDestroy() {
        LocalBroadcastManager.getInstance(mContext!!).unregisterReceiver(mMessageReceiver)
        super.onDestroy()
    }

    private fun registerSomeUpdate() {
        LocalBroadcastManager.getInstance(mContext!!).registerReceiver(mMessageReceiver, IntentFilter(Constant.NOTIFICATION_SOMETHING_HAPPEN))
    }

}

2. Post notification anywhere. Only you need the context.

   LocalBroadcastManager.getInstance(context).sendBroadcast(Intent(Constant.NOTIFICATION_SOMETHING_HAPPEN))```
   

PS:

1. you can add a Constant.kt like me for well organize the notifications. Constant.kt

object Constant {
    const val NOTIFICATION_SOMETHING_HAPPEN = "notification_something_happened_locally"
}

2. For the context in a fragment, you can use activity (sometimes null) or conext like what I used.

Answer 5

I found that the usage of EventBus of Guava lib is the simplest way for publish-subscribe-style communication between components without requiring the components to explicitly register with one another

see their sample on https://code.google.com/p/guava-libraries/wiki/EventBusExplained

// Class is typically registered by the container.
class EventBusChangeRecorder {
  @Subscribe public void recordCustomerChange(ChangeEvent e) {
    recordChange(e.getChange());
  }

// somewhere during initialization
eventBus.register(this);

}

// much later
public void changeCustomer() {
  eventBus.post(new ChangeEvent("bla bla") );
} 

you can add this lib simply on Android Studio by adding a dependency to your build.gradle:

compile 'com.google.guava:guava:17.0'

Answer 6

You could use weak references.

This way you could manage the memory yourself and add and remove observers as you please.

When you addObserver add these parameters - cast that context from the activity you are adding it in to the empty interface, add a notification name, and call the method to run interface.

The method to run interface would have a function that is called run to return the data that you are passing something like this

public static interface Themethodtorun {
        void run(String notification_name, Object additional_data);
    }

Create a observation class that invokes a reference with a empty interface. Also construct your Themethodtorun interface from the context being passed in the addobserver.

Add the observation to a data structure.

To call it would be the same method however all you need to do is find the specific notification name in the data structure, use the Themethodtorun.run(notification_name, data).

This will send a callback to where ever you created an observer with a specific notification name. Dont forget to remove them when your done!

This is good reference for weak references.

http://learningviacode.blogspot.co.nz/2014/02/weak-references-in-java.html

I am in the process of uploading this code to github. Keep eyes open!