SOURCE

class Observer{
    constructor(fn){
        this.update = fn
    }
}
class Subject {
    constructor(){
        this.observers = [] //观察者列队
    }
    addObserver(observer){
        this.observers.push(observer) //向观察者列队添加观察者
    }
    notify(){ //通知所有的观察者，实际上是把观察者的update 都执行了一遍
        this.observers.forEach(observer=>{
            observer.update()
        })
    }

}

var subject = new Subject() //被观察者
const update = ()=>{
    console.log("被观察者发出通知")
}
var ob1 = new Observer(update);
var ob2 = new Observer(update);
subject.addObserver(ob1);
subject.addObserver(ob2);
subject.notify();