Chore: Migrate EventDispatcher to TypeScript, add tests (#6673)

packages/lib/EventDispatcher.ts

@@ -0,0 +1,49 @@
+type Listener<Value> = (data: Value)=> void;
+type CallbackHandler<EventType> = (data: EventType)=> void;
+
+// EventKeyType is used to distinguish events (e.g. a 'ClickEvent' vs a 'TouchEvent')
+// while EventMessageType is the type of the data sent with an event (can be `void`)
+export default class EventDispatcher<EventKeyType extends string|symbol|number, EventMessageType> {
+	// Partial marks all fields as optional. To initialize with an empty object, this is required.
+	// See https://stackoverflow.com/a/64526384
+	private listeners: Partial<Record<EventKeyType, Array<Listener<EventMessageType>>>>;
+	public constructor() {
+		this.listeners = {};
+	}
+
+	public dispatch(eventName: EventKeyType, event: EventMessageType = null) {
+		if (!this.listeners[eventName]) return;
+
+		const ls = this.listeners[eventName];
+		for (let i = 0; i < ls.length; i++) {
+			ls[i](event);
+		}
+	}
+
+	public on(eventName: EventKeyType, callback: CallbackHandler<EventMessageType>) {
+		if (!this.listeners[eventName]) this.listeners[eventName] = [];
+		this.listeners[eventName].push(callback);
+
+		return {
+			// Retuns false if the listener has already been removed, true otherwise.
+			remove: (): boolean => {
+				const originalListeners = this.listeners[eventName];
+				this.off(eventName, callback);
+
+				return originalListeners.length !== this.listeners[eventName].length;
+			},
+		};
+	}
+
+	// Equivalent to calling .remove() on the object returned by .on
+	public off(eventName: EventKeyType, callback: CallbackHandler<EventMessageType>) {
+		if (!this.listeners[eventName]) return;
+
+		// Replace the current list of listeners with a new, shortened list.
+		// This allows any iterators over this.listeners to continue iterating
+		// without skipping elements.
+		this.listeners[eventName] = this.listeners[eventName].filter(
+			otherCallback => otherCallback !== callback
+		);
+	}
+}