observer.ts

abstract class Subject {
    protected observers: Observer[] = [];

    public addObserver(observer: Observer) {
        this.observers.push(observer);
    }

    public removeObserver(observer: Observer) {
        let found = this.observers.indexOf(observer);
        if (found !== -1) {
            this.observers.splice(found, 1);
        }
    }

    public notify(eventName: string) {
        this.observers.forEach((observer) => {
            observer.onNotify(this, eventName);
        });
    }
}

abstract class Observer {
    public abstract onNotify(entity: Subject, eventName: string);
}


/**
 * Subject
 */
class Entity extends Subject {
    public init(achievements: Achievements, sound: Sound) {
        this.addObserver(achievements);
        this.addObserver(sound);
    }

    public update() {
        this.notify("PLAYER_GOT_HURT");
    }
}


/**
 * Observers
 */

class Sound extends Observer {
    public onNotify(entity: Entity, eventName: string) {
        switch (eventName) {
            case "PLAYER_GOT_HURT":
                this.play(eventName);
                break;
        }
    }

    private play(eventName: string) {
        console.log("play", eventName);
    }
}

class Achievements extends Observer {
    public onNotify(entity: Entity, eventName: string) {
        switch (eventName) {
            case "KILLED_10_NPCS":
                this.unlock(eventName);
                break;
            case "PLAYER_GOT_HURT":
                this.unlock(eventName);
                break;
        }
    }

    private unlock(achievementName: string) {
        console.log("unlock", achievementName);
    }
}


const a = new Achievements();
const s = new Sound();

const p = new Entity();
p.init(a, s);
p.update();