Tom Boullay
75 lines
1.9 KiB
TypeScript
75 lines
1.9 KiB
TypeScript
type Listener<TPayload> = (payload: TPayload) => void;
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// TypeScript cannot narrow mapped-type indexed access by a generic key TKey
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// (microsoft/TypeScript#30581). The helper below encapsulates the one necessary
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// cast so the rest of the class stays cast-free.
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type ListenerMap<TEvents extends Record<string, unknown>> = {
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[TKey in keyof TEvents]?: Set<Listener<TEvents[TKey]>>;
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};
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function getListeners<
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TEvents extends Record<string, unknown>,
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TKey extends keyof TEvents,
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>(
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map: ListenerMap<TEvents>,
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key: TKey,
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): Set<Listener<TEvents[TKey]>> | undefined {
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return map[key] as Set<Listener<TEvents[TKey]>> | undefined;
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}
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export class EventEmitter<TEvents extends Record<string, unknown>> {
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private readonly listeners: ListenerMap<TEvents> = {};
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on<TKey extends keyof TEvents>(
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event: TKey,
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listener: Listener<TEvents[TKey]>,
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): () => void {
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const existing = getListeners(this.listeners, event);
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if (existing) {
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existing.add(listener);
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} else {
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this.listeners[event] = new Set([listener]) as ListenerMap<TEvents>[TKey];
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}
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return () => {
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this.off(event, listener);
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};
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}
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off<TKey extends keyof TEvents>(
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event: TKey,
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listener: Listener<TEvents[TKey]>,
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): void {
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const currentListeners = getListeners(this.listeners, event);
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if (!currentListeners) {
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return;
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}
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currentListeners.delete(listener);
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if (currentListeners.size === 0) {
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delete this.listeners[event];
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}
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}
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emit<TKey extends keyof TEvents>(event: TKey, payload: TEvents[TKey]): void {
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const currentListeners = getListeners(this.listeners, event);
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if (!currentListeners) {
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return;
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}
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currentListeners.forEach((listener) => {
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listener(payload);
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});
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}
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clear(): void {
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for (const key of Object.keys(this.listeners) as (keyof TEvents)[]) {
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delete this.listeners[key];
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}
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}
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}
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