observer.ts

export interface Observer<T> {
  update(value: T): void;
}

export interface Observable<T> {
  observe(observer: Observer<T>): Abortable;
}

export interface Abortable {
  abort(): void;
}

export interface Relay<T> extends Observable<T>, Observer<T> {}

// implementation

export function noop(_?: unknown) {}

export function update<T>(value: T, raise = console.error) {
  return function update(observer: Observer<T>) {
    try {
      observer.update(value);
    } catch (notice) {
      raise(notice);
    }
  };
}

export class Subject<T> implements Relay<T> {
  private _observers = new Set<Observer<T>>();
  constructor(private _state: T) {}
  get state(): T {
    return this._state;
  }
  observe(observer: Observer<T>) {
    this._observers.add(observer);
    return { abort: () => this._observers.delete(observer) };
  }
  update(value: T) {
    this._observers.forEach(update(this._state = value));
  }
}