module.exports = class Observable {
    constructor() {
        this._observers = new Set();
    }
    
    subscribe(observer) {
        this._observers.add(observer);
    }

    unsubscribe(observer) {
        this._observers = [...this._observers].filter(subscriber => subscriber !== observer);
    }

    notify(data) {
        this._observers.forEach(observer => observer(data));
    }
};