DevTut

# Otto Event Bus

# Passing an event

This example describes passing an event using the Otto Event Bus (opens new window).

To use the Otto Event Bus in Android Studio you have to insert the following statement in your modules gradle file:

dependencies {
    compile 'com.squareup:otto:1.3.8'
}

The event we'd like to pass is a simple Java object:

public class DatabaseContentChangedEvent {
    public String message;

    public DatabaseContentChangedEvent(String message) {
        this.message = message;
    }
}

We need a Bus to send events. This is typically a singleton:

import com.squareup.otto.Bus;

public final class BusProvider {
    private static final Bus mBus = new Bus();

    public static Bus getInstance() {
        return mBus;
    }

    private BusProvider() {
    }
}

To send an event we only need our BusProvider and it's post method. Here we send an event if the action of an AsyncTask is completed:

public abstract class ContentChangingTask extends AsyncTask<Object, Void, Void> {

    ...

    @Override
    protected void onPostExecute(Void param) {
        BusProvider.getInstance().post(
            new DatabaseContentChangedEvent("Content changed")
        );
    }
}

# Receiving an event

To receive an event it is necessary to implement a method with the event type as parameter and annotate it using @Subscribe. Furthermore you have to register/unregister the instance of your object at the BusProvider (see example Sending an event):

public class MyFragment extends Fragment {
    private final static String TAG = "MyFragment";

    ...

    @Override
    public void onResume() {
        super.onResume();
        BusProvider.getInstance().register(this);
    }

    @Override
    public void onPause() {
        super.onPause();
        BusProvider.getInstance().unregister(this);
    }

    @Subscribe
    public void onDatabaseContentChanged(DatabaseContentChangedEvent event) {
        Log.i(TAG, "onDatabaseContentChanged: "+event.message);
    }
}

Important: In order to receive that event an instance of the class has to exist. This is usually not the case when you want to send a result from one activity to another activity. So check your use case for the event bus.

# Remarks

Otto is deprecated (opens new window) in favor of RxJava and RxAndroid. These projects permit the same event-driven programming model as Otto, but they’re more capable and offer better control of threading.

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