Event Aggregator a Java Implementation

The Event Aggregator consist of three distinct parts:

• The publishers, which publish different kind of Messages.
• The Subscribers, which are interested for different kind of Messages.
• And the Event Aggregator, which aggregates all the Messages that are emitted by the Publishers and transmit them in every interested Subscriber.

How It Works

The Publishers publish different type of Messages via the Event Aggregator. The Event Aggregator from its side propagate these Messages to all interested Subscribers.

Each published Message has a distinct IEventType and a specific Content.

Every Subscriber is registered to the Event Aggregator in order to get notifications for specific IEventType of Messages.

Also, every subscriber assign, for every IEvenType which is interested, a specific Action. So, when it gets a notification relevant for a specific IEventType, then the corresponding Action is executed.

Every Subscriber can watch more than one IEventType of Messages and also every Publisher can publish more than one different IEventType of Messages.

Event Aggregator UML Diagram

Code Example

public enum Event implements IEventType {
    ITEM_CREATED, ITEM_SAVED, ITEM_SELECTED,
}

public class Item {

    private final int itemNumber;

    public Item(int itemNumber) {
        this.itemNumber = itemNumber;
    }

    public int getItemNumber() {
        return itemNumber;
    }
}

public class App {

    public static void main(String[] args) {

        final EventAggregator eventAggregator = new EventAggregator();

        final ISubscriber itemView = new Subscriber(eventAggregator);
        itemView.subscribe(Event.ITEM_CREATED, message ->
                System.out.println("Item created " + ((Item)message.getContent()).getItemNumber())
        );

        itemView.subscribe(Event.ITEM_SAVED, message ->
                System.out.println("Item saved " + ((Item)message.getContent()).getItemNumber())
        );

        itemView.subscribe(Event.ITEM_SELECTED, message ->
                System.out.println("Item selected " + ((Item)message.getContent()).getItemNumber())
        );

        final IPublisher publisher = new Publisher(eventAggregator);

        final Item item1 = new Item(1);
        final Item item2 = new Item(2);
        final Item item3 = new Item(3);

        System.out.println("\n******************** ITEM_CREATED ********************");
        publisher.publish(Event.ITEM_CREATED, item1);
        publisher.publish(Event.ITEM_CREATED, item2);
        publisher.publish(Event.ITEM_CREATED, item3);
        System.out.println("******************************************************");

        System.out.println("\n********************* ITEM_SAVED *********************");
        publisher.publish(Event.ITEM_SAVED, item1);
        publisher.publish(Event.ITEM_SAVED, item3);
        System.out.println("******************************************************");

        System.out.println("\n******************* ITEM_SELECTED ********************");
        publisher.publish(Event.ITEM_SELECTED, item2);
        publisher.publish(Event.ITEM_SELECTED, item3);
        System.out.println("******************************************************");
    }
}