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 方法
Promise.prototype.then / Promise.prototype.catch
如何生成一个promise对象
function A(){
return new Promise (function(x,y){
setTimeout(()=>{
x("hello")
}, 3000)
})
}
A().then((e)=>{
console.log(e) //"hello"
})
链式调用
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
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)
})
作用:解决回调地狱,也就是多层嵌套回调函数问题
范例:
- 实现: 1秒钟之后输出 fn1, 再过疫苗输出 fn2, 再过1秒输出 fn3
//回调函数方法
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()
})
})
//Promise
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)
把如下 ajax 改造成一个返回 Promise对象的方法
改造前:jQuery 的 success / error 形式回调
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:Promise回调
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') {
var ret = ( JSON.parse(xhr.responseText))
resolve(ret)
} else {
var ret = ( xhr.responseText)
resolve(ret)
}
} else {
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')
})
简化版
function ajax(){
return new Promise((resolve, reject)=>{
做事
如果成功就调用 resolve
如果失败就调用 reject
})
}
var promise = ajax()
promise.then(successFn, errorFn)
在 Node.js中的应用
回调函数的第一个参数,必须是错误对象err(如果没有错误,该参数就是null);原因是执行分成两段,在这两段之间抛出的错误,程序无法捕捉,只能当作参数,传入第二段
fs.readFile(fileA, 'utf-8', function (err, data) {
fs.readFile(fileB, 'utf-8', function (err, data) {
if (err) throw err;
console.log(data);
});
});
var readFile = require('fs-readfile-promise');
readFile(fileA)
.then(function (data) {
console.log(data.toString());
})
.then(function () {
return readFile(fileB);
})
.then(function (data) {
console.log(data.toString());
})
.catch(function (err) {
console.log(err);
});