[EventDispatcher] initial commit

src/Nuxed/Contract/EventDispatcher/Event/IEvent.hack

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+namespace Nuxed\EventDispatcher\Event;
+
+/**
+ * Marker interface indicating an event instance.
+ *
+ * Event instances may contain zero methods, or as many methods as they
+ * want. The interface MUST be implemented, however, to provide type-safety
+ * to both listeners as well as the dispatcher.
+ */
+interface IEvent {}

src/Nuxed/Contract/EventDispatcher/Event/IStoppableEvent.hack

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+namespace Nuxed\EventDispatcher\Event;
+
+/**
+ * An Event whose processing may be interrupted when the event has been handled.
+ *
+ * A Dispatcher implementation MUST check to determine an Event
+ * is marked as stopped after each listener is called. If it is then it should
+ * return immediately without calling any further Listeners.
+ */
+interface IStoppableEvent extends IEvent {
+  /**
+   * Is propagation stopped?
+   *
+   * This will typically only be used by the Dispatcher to determine if the
+   * previous listener halted propagation.
+   *
+   * @return bool
+   *   True if the Event is complete and no further listeners should be called.
+   *   False to continue calling listeners.
+   */
+  public function isPropagationStopped(): bool;
+}

src/Nuxed/Contract/EventDispatcher/EventListener/IEventListener.hack

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+namespace Nuxed\EventDispatcher\EventListener;
+
+use namespace Nuxed\EventDispatcher\Event;
+
+/**
+ * Defines a listener for an event.
+ */
+interface IEventListener<T as Event\IEvent> {
+  /**
+   * Process the given event.
+   */
+  public function process(T $event): Awaitable<void>;
+}

src/Nuxed/Contract/EventDispatcher/IEventDispatcher.hack

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+namespace Nuxed\EventDispatcher;
+
+/**
+ * Defines a dispatcher for events.
+ */
+interface IEventDispatcher {
+  /**
+   * Provide all relevant listeners with an event to process.
+   *
+   * @template T as IEvent
+   *
+   * @return T The Event that was passed, now modified by listeners.
+   */
+  public function dispatch<T as Event\IEvent>(T $event): Awaitable<T>;
+}

src/Nuxed/Contract/EventDispatcher/ListenerProvider/IListenerProvider.hack

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+namespace Nuxed\EventDispatcher\ListenerProvider;
+
+use namespace Nuxed\EventDispatcher\{Event, EventListener};
+
+/**
+ * Mapper from an event to the listeners that are applicable to that event.
+ */
+interface IListenerProvider {
+  /**
+   * Retrieve all listeners for the given event.
+   *
+   * @param T $event
+   *   An event for which to return the relevant listeners.
+   * @return AsyncIterator<EventListener\IEventListener<T>>
+   *   An async iterator (usually an async generator) of listeners. Each
+   *   listener MUST be type-compatible with $event.
+   */
+  public function getListeners<T as Event\IEvent>(
+    T $event,
+  ): AsyncIterator<EventListener\IEventListener<T>>;
+}