class Event {
    constructor(){
        this.events = {}
    }
    on(type, handler) {
        if(!this.events.hasOwnProperty(type)){
            this.events[type] = [];
        }
        this.events[type].push(handler)
                return this;
    }
    emit(type, ...args) {
        for(let i = 0; i < this.events[type].length; i++) {
           this.events[type][i].apply(this, args)
        }
            return this;
    }
    off(type, handler){
        if(!this.events.hasOwnProperty(type)){
            throw new Error(`不存在${type}`)
        }
        const len = this.events[type].length
        const eventHandler = this.events[type]
        for(let i = 0; i < len; i++) {
            if(eventHandler[i] == handler){
                eventHandler.splice(i, 1)
            }
        }
        return this;
    }
    once(type, handler){
        const on = (...args)  => {
          handler.apply(this, args)
          this.off(type, on)
        }
        this.on(type, on)
            return this;
    }
}

const event = new Event()
const a1 = function(data){
    console.log(1 + data);
}
// event.on('a', a1);
// event.emit('a', '我是第1次调用的参数');
// event.emit('a', '我是第2次调用的参数');　　
event.once('b', a1);
event.emit('b', '一次');
event.emit('b', '两次');



// vue中数据劫持结合发布/订阅模式
class Observer {
    constructor (value) {
        this.value = value
        this.dep = new Dep()
        this.walk(value)
    }
    walk (obj) {
        const keys = Object.keys(obj)
        for (let i = 0; i < keys.length; i++) {
          defineReactive(obj, keys[i],obj[keys[i]])
        }
    }
}


function defineReactive (obj, key, value){
    const dep = new Dep()

    Object.defineProperty(obj, key, {
    enumerable: true,
    configurable: true,
    get: function reactiveGetter () {
        console.log('get')
        dep.addSub({
            update:()=>{
                console.log('执行update')
            }
        });
        return value
    },
    set: function reactiveSetter (newVal) {
      if (newVal === value || (newVal !== newVal && value !== value)) {
        return
      }
      console.log('set')
      value = newVal;
      dep.notify();
    }
  })

}


class Dep {
    constructor (valuey) {
        this.subs = [];
    }
    addSub(sub){
         this.subs.push(sub);
    }
    notify(){
         this.subs.forEach(function(sub){
             sub.update();
         });
    }
}
let obj ={a:1,b:2,c:3}

new Observer(obj)
obj.a
obj.a = 2

对发布订阅者的通俗理解

订阅可以理解为寄存，发布理解为寄存的拿出调用，可以寄存很多个，拿出很多个

观察者模式主要是监听