1.3 发布订阅模式

通过发布订阅模式实现异步接口的顺序调用

publish.js

class Publish{
  constructor() {
    this.events = {}
  }

  sub(event,callback){
    if(this.events[event]){
      this.events[event].push(callback)
    }
    else
      this.events[event]=[callback]
  }
  publish(event,...agrs){
    if(this.events[event].length){
      this.events[event].forEach(callback=>{
        callback.call(this,...agrs)
      })
    }
  }
}


module.exports = Publish

test.js

const axios = require('axios')
const Pub = require('./publish')

const pub = new Pub();

/**
axios.get是同步方法，所以按顺序先执行axios.get，然后是pub.sub
pub.sub的回调函数是异步，只有pub.publish('firstRequest')发布时，
才会执行
综上：同步方法先注册事件，然后依据执行顺序进行发布，即可完成按顺序执行
*/
axios.get('https://www.baidu.com').then(res =>{
  if(res.status === 200){
    console.log('first')
    pub.publish('firstRequest')
  }
})

pub.sub('firstRequest',()=>{
  axios.get('https://www.baidu.com').then(res=>{
    if(res.status === 200){
      console.log('second')

      pub.publish('secondRequest')
    }
    }
  )
})
pub.sub('secondRequest',()=>{
  axios.get('https://www.baidu.com').then(res=>{
      if(res.status === 200){
        console.log('third')
      }
    }
  )
})