观察者模式(Observer)

定义：属于行为型模式的一种，它定义了一种一对多的依赖关系，让多个观察者对象同时监听某一个主题对象。这个主题对象在状态变动时，会告诉所有的观察者对象，使他们可能自动更新本人。

实现

  <span id="num">1</span>  <button onclick="numAdd()">+</button>

    var num = document.getElementById('num')    function numAdd() {      notice.setState(Math.floor(Math.random() * 10))      num.innerText = notice.state + Math.floor(Math.random() * 10)    }    function Theme() {      this.observers = []      this.state = 0      this.setState = function (newVal) {        this.state = newVal        this.notifyAllObservers()      }      this.attach = function (observer) {        this.observers.push(observer)      }      this.notifyAllObservers = function () {        this.observers.forEach((item) => item.updated(this.state))      }    }    function Observer(name) {      this.name = name      this.updated = function (e) {        console.log(`${this.name}接管到了新音讯`, e)      }    }    var notice = new Theme()    var observer1 = new Observer('观察者1')    var observer2 = new Observer('观察者2')    notice.attach(observer1)    notice.attach(observer2)