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event-aggr
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all-contri
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@ -9,10 +9,6 @@ tags:
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- Reactive
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- Reactive
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---
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---
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## Name
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Event Aggregator
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## Intent
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## Intent
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A system with lots of objects can lead to complexities when a
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A system with lots of objects can lead to complexities when a
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client wants to subscribe to events. The client has to find and register for
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client wants to subscribe to events. The client has to find and register for
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@ -21,136 +17,6 @@ requires a separate subscription. An Event Aggregator acts as a single source
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of events for many objects. It registers for all the events of the many objects
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of events for many objects. It registers for all the events of the many objects
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allowing clients to register with just the aggregator.
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allowing clients to register with just the aggregator.
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## Explanation
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Real-world example
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> King Joffrey sits on the iron throne and rules the seven kingdoms of Westeros. He receives most
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> of his critical information from King's Hand, the second in command. King's hand has many
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> close advisors himself, feeding him with relevant information about events occurring in the
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> kingdom.
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In Plain Words
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> Event Aggregator is an event mediator that collects events from multiple sources and delivers
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> them to registered observers.
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**Programmatic Example**
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In our programmatic example, we demonstrate the implementation of an event aggregator pattern. Some of
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the objects are event listeners, some are event emitters, and the event aggregator does both.
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```java
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public interface EventObserver {
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void onEvent(Event e);
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}
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public abstract class EventEmitter {
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private final Map<Event, List<EventObserver>> observerLists;
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public EventEmitter() {
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observerLists = new HashMap<>();
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}
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public final void registerObserver(EventObserver obs, Event e) {
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...
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}
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protected void notifyObservers(Event e) {
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...
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}
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}
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```
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`KingJoffrey` is listening to events from `KingsHand`.
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```java
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@Slf4j
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public class KingJoffrey implements EventObserver {
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@Override
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public void onEvent(Event e) {
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LOGGER.info("Received event from the King's Hand: {}", e.toString());
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}
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}
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```
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`KingsHand` is listening to events from his subordinates `LordBaelish`, `LordVarys`, and `Scout`.
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Whatever he hears from them, he delivers to `KingJoffrey`.
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```java
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public class KingsHand extends EventEmitter implements EventObserver {
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public KingsHand() {
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}
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public KingsHand(EventObserver obs, Event e) {
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super(obs, e);
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}
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@Override
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public void onEvent(Event e) {
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notifyObservers(e);
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}
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}
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```
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For example, `LordVarys` finds a traitor every Sunday and notifies the `KingsHand`.
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```java
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@Slf4j
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public class LordVarys extends EventEmitter implements EventObserver {
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@Override
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public void timePasses(Weekday day) {
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if (day == Weekday.SATURDAY) {
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notifyObservers(Event.TRAITOR_DETECTED);
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}
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}
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}
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```
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The following snippet demonstrates how the objects are constructed and wired together.
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```java
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var kingJoffrey = new KingJoffrey();
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var kingsHand = new KingsHand();
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kingsHand.registerObserver(kingJoffrey, Event.TRAITOR_DETECTED);
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kingsHand.registerObserver(kingJoffrey, Event.STARK_SIGHTED);
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kingsHand.registerObserver(kingJoffrey, Event.WARSHIPS_APPROACHING);
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kingsHand.registerObserver(kingJoffrey, Event.WHITE_WALKERS_SIGHTED);
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var varys = new LordVarys();
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varys.registerObserver(kingsHand, Event.TRAITOR_DETECTED);
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varys.registerObserver(kingsHand, Event.WHITE_WALKERS_SIGHTED);
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var scout = new Scout();
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scout.registerObserver(kingsHand, Event.WARSHIPS_APPROACHING);
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scout.registerObserver(varys, Event.WHITE_WALKERS_SIGHTED);
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var baelish = new LordBaelish(kingsHand, Event.STARK_SIGHTED);
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var emitters = List.of(
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kingsHand,
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baelish,
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varys,
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scout
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);
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Arrays.stream(Weekday.values())
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.<Consumer<? super EventEmitter>>map(day -> emitter -> emitter.timePasses(day))
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.forEachOrdered(emitters::forEach);
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```
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The console output after running the example.
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```
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18:21:52.955 [main] INFO com.iluwatar.event.aggregator.KingJoffrey - Received event from the King's Hand: Warships approaching
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18:21:52.960 [main] INFO com.iluwatar.event.aggregator.KingJoffrey - Received event from the King's Hand: White walkers sighted
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18:21:52.960 [main] INFO com.iluwatar.event.aggregator.KingJoffrey - Received event from the King's Hand: Stark sighted
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18:21:52.960 [main] INFO com.iluwatar.event.aggregator.KingJoffrey - Received event from the King's Hand: Traitor detected
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```
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## Class diagram
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## Class diagram
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@ -160,13 +26,9 @@ Use the Event Aggregator pattern when
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* Event Aggregator is a good choice when you have lots of objects that are
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* Event Aggregator is a good choice when you have lots of objects that are
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potential event sources. Rather than have the observer deal with registering
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potential event sources. Rather than have the observer deal with registering
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with them all, you can centralize the registration logic to the Event
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with them all, you can centralize the registration logic to the Event
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Aggregator. As well as simplifying registration, an Event Aggregator also
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Aggregator. As well as simplifying registration, a Event Aggregator also
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simplifies the memory management issues in using observers.
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simplifies the memory management issues in using observers.
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## Related patterns
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* [Observer](https://java-design-patterns.com/patterns/observer/)
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## Credits
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## Credits
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* [Martin Fowler - Event Aggregator](http://martinfowler.com/eaaDev/EventAggregator.html)
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* [Martin Fowler - Event Aggregator](http://martinfowler.com/eaaDev/EventAggregator.html)
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