How to get started with EventBus in 3 steps

Contents

The EventBus API is as easy as 1-2-3.

Before we get started make sure to add EventBus as a dependency to your project.

Step 1: Define events

Events are POJO (plain old Java object) without any specific requirements.

public class MessageEvent {

    public final String message;

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

public class MessageEvent {

public final String message;

public MessageEvent(String message) {

this.message = message;

}

Step 2: Prepare subscribers

Subscribers implement event handling methods (also called “subscriber methods”) that will be called when an event is posted. These are defined with the @Subscribe annotation.
Note that with EventBus 3 the method name can be chosen freely (no naming conventions like in EventBus 2).

// This method will be called when a MessageEvent is posted (in the UI thread for Toast)
@Subscribe(threadMode = ThreadMode.MAIN)
public void onMessageEvent(MessageEvent event) {
    Toast.makeText(getActivity(), event.message, Toast.LENGTH_SHORT).show();
}

// This method will be called when a SomeOtherEvent is posted
@Subscribe
public void handleSomethingElse(SomeOtherEvent event) {
    doSomethingWith(event);
}

// This method will be called when a MessageEvent is posted (in the UI thread for Toast)

@Subscribe(threadMode = ThreadMode.MAIN)

public void onMessageEvent(MessageEvent event) {

Toast.makeText(getActivity(), event.message, Toast.LENGTH_SHORT).show();

}

// This method will be called when a SomeOtherEvent is posted

@Subscribe

public void handleSomethingElse(SomeOtherEvent event) {

doSomethingWith(event);

}

Subscribers also need to register themselves to and unregister from the bus. Only while subscribers are registered, they will receive events. In Android, in activities and fragments you should usually register according to their life cycle. For most cases onStart/onStop works fine:

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

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

@Override

public void onStart() {

super.onStart();

EventBus.getDefault().register(this);

}

@Override

public void onStop() {

EventBus.getDefault().unregister(this);

super.onStop();

}

Step 3: Post events

Post an event from any part of your code. All currently registered subscribers matching the event type will receive it.

EventBus.getDefault().post(new MessageEvent("Hello everyone!"));

1	EventBus.getDefault().post(new MessageEvent("Hello everyone!"));

Learn more

Have a look at the full documentation to learn about all features of EventBus.

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