Event Bus and Event System

What the EventSystem is used for

Many times there is a need to provide an object on many locations simultaneously. This design pattern is quite similar to Java’s Observer, but it is much simpler and readable while having the same functional sense.

Main principle

EventBus represents a communication line between multiple components. Objects can be passed through the bus and reach the listening method of another object which is registered on that EventBus instance. Hence, the passed object is available as a parameter in the listening method.

Register to the EventBus

Any listening method has to be annotated with @Subscribe keyword and must have only one accepting parameter. Furthermore, the object which contains such listening method(s) has to be registered using the register(Object) method provided by EventBus. The listening methods can be overloaded by using different parameter types.

Posting an object

post(object) posts an object through the EventBus which has been used to register the listening/subscribing methods.

Short example

/* Listener.java */

import com.google.common.eventbus.Subscribe;

public class Listener {

   private int value = 0;

   @Subscribe
   public void listen(int value) {
      this.value = value;
   }

   public int getValue() {
      return this.value;
   }
}

/* Main.java */

import com.google.common.eventbus.EventBus;

public class Main {
   private static EventBus eventBus = new EventBus();

   public static void main(String[] args) {
      Main main = new Main();
      Listener listener = new Listener();
      eventBus.register(listener);
      eventBus.post(1); // 1 represents the passed event

      // Output should be 1
      System.out.println(listener.getValue());
   }
}

Event handling in JabRef

The event package contains some specific events which occur in JabRef.

For example: Every time an entry was added to the database a new EntryAddedEvent is sent through the eventBus which is located in BibDatabase.

If you want to catch the event you’ll have to register your listener class with the registerListener(Object listener) method in BibDatabase. EntryAddedEvent provides also methods to get the inserted BibEntry.