an emitter of typed events

TypedEvent.ts

export interface TypedEventListener<T> {
  (event: T): void;
}

export interface TypedEventDisposable {
  dispose: () => void;
}

export class TypedEvent<T> {
  private readonly listeners: TypedEventListener<T>[] = [];

  private oneOffs: TypedEventListener<T>[] = [];

  on(listener: TypedEventListener<T>): TypedEventDisposable {
    this.listeners.push(listener);
    return {
      dispose: () => this.off(listener),
    };
  }

  once(listener: TypedEventListener<T>): void {
    this.oneOffs.push(listener);
  }

  off(listener: TypedEventListener<T>): void {
    this.spliceListeners(this.listeners, listener);
    this.spliceListeners(this.oneOffs, listener);
  }

  emit(event: T): void {
    this.listeners.forEach(listener => listener(event));
    if (this.oneOffs.length > 0) {
      const toCall = this.oneOffs;
      this.oneOffs = [];
      toCall.forEach(listener => listener(event));
    }
  }

  pipe(te: TypedEvent<T>): TypedEventDisposable {
    return this.on(e => te.emit(e));
  }

  clear() {
    this.listeners.splice(0, this.listeners.length);
    this.oneOffs.splice(0, this.oneOffs.length);
  }

  // eslint-disable-next-line class-methods-use-this
  private spliceListeners(listeners: TypedEventListener<T>[], listener: TypedEventListener<T>) {
    const callbackIndex = listeners.indexOf(listener);
    if (callbackIndex > -1) {
      listeners.splice(callbackIndex, 1);
    }
  }
}