EventBus is a publish/subscribe event bus for Android that simplifies communication between components like activities, fragments, threads and services. It provides an alternative to traditional Android communication mechanisms like intents, handlers and broadcasts. With EventBus, components define events to publish, register as subscribers to receive events, and post events. This reduces the need for direct calls, callbacks and listener management between components.

1. EventBus for Android
Ting Cheng

3. Intent, Handler,
Broadcast

4. Status Quo in Android
Fragment Fragment
Activity
Service / Helper
Thread
Activity

5. Issues !
• Define interfaces
• Callbacks for async
• Listener management
• Propagation through all layers

6. EventBus
Fragment Fragment
Activity
Service / Helper
Thread
Activity
Event
Bus

7. Three things
• Event
• Subscriber
• Publisher

8. EventBus in 4 Steps
1. Define an event
public class MyEvent {}
2. Register subscriber
EventBus.getDefault().register(this);
3. Post an event
EventBus.getDefault().post(event);
4. Receive the event
public void onEvent(MyEvent event);

9. Publish / Subscribe
Publisher
Event
Bus
Subscriber
Event
post()
Event
onEvent()
Subscriber
Event
onEvent()

10. • EventBus instances
• It anywhere in your code
• Event handler methods
• Naming convention: onEvent
• public visibility
• No return value (void)
• Single parameter for the event to receive

11. Event Type is a Filter
Publisher
Event
Bus
Subscriber
Event
post
(MyEvent)
Event
onEvent(MyEvent)
Subscriber
onEvent(OtherEvent)

12. Code: Sender
MyEvent myEvent = new MyEvent(anyData);
EventBus.getDefault().post(myEvent);

13. Code: Receive
MyActivity extends Activity
// onCreate or onResume
EventBus.getDefault().register(this);
// onDestroy or onPause
EventBus.getDefault().unregister(this);
public onEvent(MyEvent event) { … }

14. Example:
Fragment to Fragment
ListFragment DetailFragment
Activity

15. DetailFragment
// Here‘s the action. How is it called?
public void loadDetails(Item item) {
…
}

16. ListFragment
interface Contract {
void onItemSelected(Item item);
}
// Propagate to Activity
((Contract)getActivity()).onItemSelected(item);

17. Activity
public class MyActivity implements ListFragment.Contract {
public void onItemSelected(Item item) {
DetailFragment detailFragment = (DetailFragment)
getFragmentManager().findFragmentBy…;
detailFragment.loadDetails(item);
}
}

18. EventBus Example

19. Event class
public class ItemSelectedEvent {
public final Item item;
public ItemSelectedEvent(Item item) {
this.item = item;
}
}

20. Post / Receive
// In ListFragment
EventBus.getDefault().post(new ItemSelectedEvent(item));
// In DetailFragment (extends EventBusFragment)
public void onEvent(ItemSelectedEvent event) {
…
}

21. Thread Modes
• PostThread: Direct call in same thread
• MainThread: UI updates etc.
• BackgroundThread: „not the main thread“
• Async: always asynchronous to posting
• Used in the method name: onEventXXX
• No additional code required

22. Tips

23. Sticky Event
• Events can be posted using sticky mode
postSticky(event); // Instead of post(…)
• The last sticky event is kept in memory
• Registration with getting sticky events
registerSticky(subscriber);
• Query for sticky event
getStickyEvent(Class<?> eventType)
• Remove sticky event (Class or Object)
removeStickyEvent(Class<?> eventType)

24. When to use EventBus
• Medium/high distance of components:
• Potentially multiple dependencies
• Asynchronous logic
• When not to use EventBus
• Strictly internal matters:
• Use methods and interfaces instead
• Inter-process communication