Promise的使用及方法

什么是Promise

---

Promise 是一个对象，对象里存储一个状态，这个状态是可以随着内部的执行转化的，为以下三种状态之一：等待态(Pending)、完成态(Fulfilled)、拒绝态(Rejected)。

一开始，我们先设置好等状态从 pending 变成 fulfilled 和 rejected 的预案（当成功后我们做什么，失败时我们做什么）。

Promise 启动之后，当满足成功的条件时我们让状态从 pending 变成 fullfilled （执行 resolve）；当满足失败的条件，我们让状态从 pending 变成 rejected（执行 reject）
Promise范例

---

范例1

Promise.prototype.then/Promise.prototype.catch

function getIp() {
  var promise = new Promise(function(resolve, reject){
    var xhr = new XMLHttpRequest()
    xhr.open('GET', 'https://easy-mock.com/mock/5ac2f80c3d211137b3f2843a/promise/getIp', true)
    xhr.onload = function(){
      var retJson = JSON.parse(xhr.responseText)  // {"ip":"58.100.211.137"}
      resolve(retJson.ip)
    }
    xhr.onerror = function(){
      reject('获取IP失败')
    }
    xhr.send()
  })
  return promise
}

function getCityFromIp(ip) {
  var promise = new Promise(function(resolve, reject){
    var xhr = new XMLHttpRequest()
    xhr.open('GET', 'https://easy-mock.com/mock/5ac2f80c3d211137b3f2843a/promise/getCityFromIp?ip='+ip, true)
    xhr.onload = function(){
      var retJson = JSON.parse(xhr.responseText)  // {"city": "hangzhou","ip": "23.45.12.34"}
      resolve(retJson.city)
    }
    xhr.onerror = function(){
      reject('获取city失败')
    }
    xhr.send()
  })
  return promise
}
function getWeatherFromCity(city) {
  var promise = new Promise(function(resolve, reject){
    var xhr = new XMLHttpRequest()
    xhr.open('GET', 'https://easy-mock.com/mock/5ac2f80c3d211137b3f2843a/promise/getWeatherFromCity?city='+city, true)
    xhr.onload = function(){
      var retJson = JSON.parse(xhr.responseText)   //{"weather": "晴天","city": "beijing"}
      resolve(retJson)
    }
    xhr.onerror = function(){
      reject('获取天气失败')
    }
    xhr.send()
  })
  return promise
}

getIp().then(function(ip){
  return getCityFromIp(ip)
}).then(function(city){
  return getWeatherFromCity(city)
}).then(function(data){
  console.log(data)
}).catch(function(e){
  console.log('出现了错误', e)
})

Promise.all

范例2

function getCityFromIp(ip) {
  var promise = new Promise(function(resolve, reject){
    var xhr = new XMLHttpRequest()
    xhr.open('GET', 'https://easy-mock.com/mock/5ac2f80c3d211137b3f2843a/promise/getCityFromIp?ip='+ip, true)
    xhr.onload = function(){
      var retJson = JSON.parse(xhr.responseText)  // {"city": "hangzhou","ip": "23.45.12.34"}
      resolve(retJson)
    }
    xhr.onerror = function(){
      reject('获取city失败')
    }
    xhr.send()
  })
  return promise
}

var p1 = getCityFromIp('10.10.10.1')
var p2 = getCityFromIp('10.10.10.2')
var p3 = getCityFromIp('10.10.10.3')

//Promise.all, 当所有的 Promise 对象都完成后再执行
Promise.all([p1, p2, p3]).then(data=>{
  console.log(data)
})

Promise.race

function getCityFromIp(ip) {
  var promise = new Promise(function(resolve, reject){
    var xhr = new XMLHttpRequest()
    xhr.open('GET', 'https://easy-mock.com/mock/5ac2f80c3d211137b3f2843a/promise/getCityFromIp?ip='+ip, true)
    xhr.onload = function(){
      var retJson = JSON.parse(xhr.responseText)  // {"city": "hangzhou","ip": "23.45.12.34"}
      resolve(retJson)
    }
    xhr.onerror = function(){
      reject('获取city失败')
    }
    setTimeout(()=>{
      xhr.send()
    }, Math.random()*1000)

  })
  return promise
}

var p1 = getCityFromIp('10.10.10.1')
var p2 = getCityFromIp('10.10.10.2')
var p3 = getCityFromIp('10.10.10.3')

//Promise.all, 当所有的 Promise 对象都完成后再执行
Promise.race([p1, p2, p3]).then(data=>{
  console.log(data)
})

对于回调地狱的解决

function fn1() {
  setTimeout(()=>{
    console.log('fn1')
  }, 1000)
}

function fn2() {
  setTimeout(()=>{
    console.log('fn2')
  }, 1000)
}

function fn3() {
  setTimeout(()=>{
    console.log('fn3')
  }, 1000)
}

对于以上代码如何实现: 1秒钟之后输出 fn1, 再过疫苗输出 fn2, 再过1秒输出 fn3 ?

可如下改装：

function fn1(callback) {
  setTimeout(()=>{
    console.log('fn1')
    callback()
  }, 1000)
}

function fn2(callback) {
  setTimeout(()=>{
    console.log('fn2')
    callback()
  }, 1000)
}

function fn3() {
  setTimeout(()=>{
    console.log('fn3')
  }, 1000)
}
fn1(function(){
  fn2(function(){
    fn3()
  })
})
回调地狱
*我们可以利用Promise进行改写*
function fn1() {
  return new Promise((resolve, reject)=>{
    setTimeout(()=>{
      console.log('fn1...')
      resolve()
    }, 1000)    
  })
}

function fn2() {
  return new Promise((resolve, reject)=>{
    setTimeout(()=>{
      console.log('fn2...')
      resolve()
    }, 1000)    
  })
}

function fn3() {
  return new Promise((resolve, reject)=>{
    setTimeout(()=>{
      console.log('fn3...')
      resolve()
    }, 1000)    
  })
}

function onerror() {
  console.log('error')
}

fn1().then(fn2).then(fn3).catch(onerror)

利用Promise对ajax进行改造

---

改造前

function ajax(opts){
    var url = opts.url
    var type = opts.type || 'GET'
    var dataType = opts.dataType || 'json'
    var onsuccess = opts.onsuccess || function(){}
    var onerror = opts.onerror || function(){}
    var data = opts.data || {}

    var dataStr = []
    for(var key in data){
        dataStr.push(key + '=' + data[key])
    }
    dataStr = dataStr.join('&')

    if(type === 'GET'){
        url += '?' + dataStr
    }

    var xhr = new XMLHttpRequest()
    xhr.open(type, url, true)
    xhr.onload = function(){
        if((xhr.status >= 200 && xhr.status < 300) || xhr.status == 304){
            //成功了
            if(dataType === 'json'){
                onsuccess( JSON.parse(xhr.responseText))
            }else{
                onsuccess( xhr.responseText)
            }
        } else {
            onerror()
        }
    }
    xhr.onerror = onerror
    if(type === 'POST'){
        xhr.send(dataStr)
    }else{
        xhr.send()
    }
}

ajax({
    url: 'http://api.jirengu.com/weather.php',
    data: {
        city: '北京'
    },
    onsuccess: function(ret){
        console.log(ret)
    },
    onerror: function(){
        console.log('服务器异常')
    }
})

改造后

function ajax(opts){
    var url = opts.url
    var type = opts.type || 'GET'
    var dataType = opts.dataType || 'json'
    var data = opts.data || {}

    var dataStr = []
    for(var key in data){
        dataStr.push(key + '=' + data[key])
    }
    dataStr = dataStr.join('&')

    if(type === 'GET'){
        url += '?' + dataStr
    }
var promise = new Promise(function(resolve, reject){
  var xhr = new XMLHttpRequest()
    xhr.open(type, url, true)
    xhr.onload = function(){
        if((xhr.status >= 200 && xhr.status < 300) || xhr.status == 304){
            //成功了
            if(dataType === 'json'){
                resolve( JSON.parse(xhr.responseText))
            }else{
                resolve( xhr.responseText)
            }
        } else {
            reject()
        }
    }
    xhr.onerror = reject()
    if(type === 'POST'){
        xhr.send(dataStr)
    }else{
        xhr.send()
    }
})
  return promise
}

ajax({
    url: 'http://api.jirengu.com/weather.php',
    data: {
        city: '北京'
    }
}).then(function(ret){
    console.log(ret)
}).catch(function(){
    console.log('服务器异常')
})