GreenRobot EventBus

Passing a Simple Event

The first thing we need to do it add EventBus to our module's gradle file:

dependencies {
    ...
    compile 'org.greenrobot:eventbus:3.0.0'
    ...
}

Now we need to create a model for our event. It can contain anything we want to pass along. For now we'll just make an empty class.

public class DeviceConnectedEvent
{
}

Now we can add the code to our Activity that will register with EventBus and subscribe to the event.

public class MainActivity extends AppCompatActivity
{
    private EventBus _eventBus;

    @Override
    protected void onCreate (Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        _eventBus = EventBus.getDefault();
    }

    @Override
    protected void onStart ()
    {
        super.onStart();
        _eventBus.register(this);
    }

    @Override
    protected void onStop ()
    {
        _eventBus.unregister(this);
        super.onStop();
    }

    @Subscribe(threadMode = ThreadMode.MAIN)
    public void onDeviceConnected (final DeviceConnectedEvent event)
    {
        // Process event and update UI
    }
}

In this Activity we get an instance of EventBus in the onCreate() method. We register / unregister for events in onStart() / onStop(). It's important to remember to unregister when your listener loses scope or you could leak your Activity.

Finally we define the method that we want called with the event. The @Subscribe annotation tells EventBus which methods it can look for to handle events. You have to have at least one methods annotated with @Subscribe to register with EventBus or it will throw an exception. In the annotation we define the thread mode. This tells EventBus which thread to call the method on. It is a very handy way of passing information from a background thread to the UI thread! That's exactly what we're doing here. ThreadMode.MAIN means that this method will be called on Android's main UI thread so it's safe to do any UI manipulations here that you need. The name of the method doesn't matter. The only think, other that the @Subscribe annotation, that EventBus is looking for is the type of the argument. As long as the type matches it will be called when an event is posted.

The last thing we need to do it to post an event. This code will be in our Service.

EventBus.getDefault().post(new DeviceConnectedEvent());

That's all there is to it! EventBus will take that DeviceConnectedEvent and look through its registered listeners, look through the methods that they've subscribed and find the ones that take a DeviceConnectedEvent as an argument and call them on the thread that they want to be called on.

Receiving Events

For receiving events you need to register your class on the EventBus.

@Override
public void onStart() {
    super.onStart();
    EventBus.getDefault().register(this);
}

@Override
public void onStop() {
   EventBus.getDefault().unregister(this);
   super.onStop();
}

And then subscribe to the events.

@Subscribe(threadMode = ThreadMode.MAIN)
public void handleEvent(ArbitaryEvent event) {
    Toast.makeText(getActivity(), "Event type: "+event.getEventType(), Toast.LENGTH_SHORT).show();
}

Sending Events

Sending events is as easy as creating the Event object and then posting it.

EventBus.getDefault().post(new ArbitaryEvent(ArbitaryEvent.TYPE_1));

Creating an Event object

For sending and receiving events we first need an Event object. Event objects are actually simple POJOs.

public class ArbitaryEvent{
    public static final int TYPE_1 = 1;
    public static final int TYPE_2 = 2;
    private int eventType;
    public ArbitaryEvent(int eventType){
        this.eventType = eventType;
    }
    
    public int getEventType(){
        return eventType;
    }
}

Syntax

@Subscribe(threadMode = ThreadMode.POSTING) public void onEvent(EventClass event) { }

Parameters

Thread Mode	Description
ThreadMode.POSTING	Will be called on the same thread that the event was posted on. This is the default mode.
ThreadMode.MAIN	Will be called on the main UI thread.
ThreadMode.BACKGROUND	Will be called on a background thread. If the posting thread isn't the main thread it will be used. If posted on the main thread EventBus has a single background thread that it will use.
ThreadMode.ASYNC	Will be called on its own thread.