关于Promise详解

异步回调

回调地狱

在需要多个操作的时候，会导致多个回调函数嵌套，导致代码不够直观，就是常说的回调地狱

并行结果

如果几个异步操作之间并没有前后顺序之分,但需要等多个异步操作都完成后才能执行后续的任务，无法实现并行节约时间

Promise

Promise本意是承诺，在程序中的意思就是承诺我过一段时间后会给你一个结果。 什么时候会用到过一段时间？答案是异步操作，异步是指可能比较长时间才有结果的才做，例如网络请求、读取本地文件等

Promise的三种状态

• Pending Promise对象实例创建时候的初始状态
• Fulfilled 可以理解为成功的状态
• Rejected 可以理解为失败的状态
  

  then 方法就是用来指定Promise 对象的状态改变时确定执行的操作，resolve 时执行第一个函数（onFulfilled），reject 时执行第二个函数（onRejected

构造一个Promise函数

使用promise

let promise = new Promise((resolve, reject) => {
setTimeout(() => {
if(Math.random()>0.5)
resolve('This is resolve!');
else
reject('This is reject!');
}, 1000);
});
promise.then(Fulfilled,Rejected)

• 构造一个Promise实例需要给Promise构造函数传入一个函数。
• 传入的函数需要有两个形参，两个形参都是function类型的参数。
  • 第一个形参运行后会让Promise实例处于resolve状态，所以我们一般给第一个形参命名为resolve,使 Promise 对象的状态改变成成功，同时传递一个参数用于后续成功后的操作
  • 第一个形参运行后会让Promise实例处于reject状态，所以我们一般给第一个形参命名为reject,将 Promise 对象的状态改变为失败，同时将错误的信息传递到后续错误处理的操作

es5模拟Promise

function Promise(fn) {
fn((data)=> {
this.success(data);
}, (error)=> {
this.error();
});
}

Promise.prototype.resolve = function (data) {
this.success(data);
}

Promise.prototype.reject = function (error) {
this.error(error);
}

Promise.prototype.then = function (success, error) {
this.success = success;
this.error = error;
}

es6模拟Promise

class Promise {
constructor(fn) {
fn((data)=> {
this.success(data);
}, (error)=> {
this.error();
});
}

resolve(data) {
this.success(data);
}

reject(error) {
this.error(error);
}

then(success, error) {
this.success = success;
this.error = error;
console.log(this);
}
}

Primose作为函数的返回值

function ajaxPromise (queryUrl) {
return new Promise((resolve, reject) => {
let xhr = new XMLHttpRequest();
xhr.open('GET', queryUrl, true);
xhr.send(null);
xhr.onreadystatechange = () => {
if (xhr.readyState === 4) {
if (xhr.status === 200) {
resolve(xhr.responseText);
} else {
reject(xhr.responseText);
}
}
}
});
}

ajaxPromise('http://www.baidu.com')
.then((value) => {
console.log(value);
})
.catch((err) => {
console.error(err);
});

Promise的链式调用

• 每次调用返回的都是一个新的Promise实例
• 链式调用的参数通过返回值传递
  

  then可以使用链式调用的写法原因在于，每一次执行该方法时总是会返回一个Promise对象

  readFile('1.txt').then(function (data) {
  console.log(data);
  return data;
  }).then(function (data) {
  console.log(data);
  return readFile(data);
  }).then(function (data) {
  console.log(data);
  }).catch(function(err){
  console.log(err);
  });

Promise API

Promise.all

• 参数：接受一个数组，数组内都是Promise实例
• 返回值：返回一个Promise实例，这个Promise实例的状态转移取决于参数的Promise实例的状态变化。当参数中所有的实例都处于resolve状态时，返回的Promise实例会变为resolve状态。如果参数中任意一个实例处于reject状态，返回的Promise实例变为reject状态。

  Promise.all([p1, p2]).then(function (result) {
  console.log(result); // [ '2.txt', '2' ]
  });

不管两个promise谁先完成，Promise.all 方法会按照数组里面的顺序将结果返回

Promise.race

• 参数：接受一个数组，数组内都是Promise实例
• 返回值：返回一个Promise实例，这个Promise实例的状态转移取决于参数的Promise实例的状态变化。当参数中任何一个实例处于resolve状态时，返回的Promise实例会变为resolve状态。如果参数中任意一个实例处于reject状态，返回的Promise实例变为reject状态。

  Promise.race([p1, p2]).then(function (result) {
  console.log(result); // [ '2.txt', '2' ]
  });

Promise.resolve

返回一个Promise实例，这个实例处于resolve状态。
根据传入的参数不同有不同的功能：

• 值(对象、数组、字符串等)：作为resolve传递出去的值
• Promise实例：原封不动返回

Promise.reject

返回一个Promise实例，这个实例处于reject状态。

• 参数一般就是抛出的错误信息。

Q

Q是一个在Javascript中实现promise的模块

q的基本用法

var Q = require('q');
var fs = require('fs');
function read(filename) {
var deferred = Q.defer();
fs.readFile(filename,'utf8', function (err, data) {
if(err){
deferred.reject(err);
}else{
deferred.resolve(data);
}
});
return deferred.promise;
}

read('1.txt1').then(function(data){
console.log(data);
},function(error){
console.error(error);
});

q的简单实现

module.exports = {
defer(){
var _success,_error;
return {
resolve(data){
_success(data);
},
reject(err){
_error(err);
},
promise:{
then(success,error){
_success = success;
_error = error;
}
}
}
}
}

q的实现

var defer = function () {
var pending = [], value;
return {
resolve: function (_value) {
if (pending) {
value = _value;
for (var i = 0, ii = pending.length; i < ii; i++) {
var callback = pending[i];
callback(value);
}
pending = undefined;
}
},
promise: {
then: function (callback) {
if (pending) {
pending.push(callback);
} else {
callback(value);
}
}
}
};
};

var defer = function () {
var pending = [], value;
return {
resolve: function (_value) {
if (pending) {
value = _value;
for (var i = 0, ii = pending.length; i < ii; i++) {
var callback = pending[i];
callback(value);
}
pending = undefined;
}
},
promise: {
then: function (callback) {
if (pending) {
pending.push(callback);
} else {
callback(value);
}
}
}
};
};

bluebird

实现 promise 标准的库是功能最全，速度最快的一个库

bluebird经典使用

var Promise = require('./bluebird');

var readFile = Promise.promisify(require("fs").readFile);
readFile("1.txt", "utf8").then(function(contents) {
console.log(contents);
})

var fs = Promise.promisifyAll(require("fs"));

fs.readFileAsync("1.txt", "utf8").then(function (contents) {
console.log(contents);
})

bluebird简单实现

module.exports = {
promisify(fn){
return function () {
var args = Array.from(arguments);
return new Promise(function (resolve, reject) {
fn.apply(null, args.concat(function (err) {
if (err) {
reject(err);
} else {
resolve(arguments[1])
}
}));
})
}
},
promisifyAll(obj){
for(var attr in obj){
if(obj.hasOwnProperty(attr) && typeof obj[attr] =='function'){
obj[attr+'Async'] = this.promisify(obj[attr]);
}
}
return obj;
}
}

动画

<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>move</title>
<style>
.square{
width:40px;
height:40px;
border-radius: 50%;
}
.square1{
background-color: red;
}
.square2{
background-color: yellow;
}
.square3{
background-color: blue;
}
</style>
</head>
<body>
<div class="square square1" style="margin-left: 0"></div>
<div class="square square2" style="margin-left: 0"></div>
<div class="square square3" style="margin-left: 0"></div>
</body>
<script>
var square1 = document.querySelector('.square1');
var square2 = document.querySelector('.square2');
var square3 = document.querySelector('.square3');

/*function move(element,target,resolve){
let timer = setInterval(function(){
var marginLeft = parseInt(element.style.marginLeft, 10);
if(marginLeft == target){
resolve();
}else{
element.style.marginLeft = ++marginLeft+'px';
}
},13);
}*/
function move(element,target,resolve){
let current = 0;
let timer = setInterval(function(){
element.style.transform=`translateX(${++current}px)`;
if(current>target){
clearInterval(timer);
resolve();
};
},13);
}
function animate(element,target){
return new Promise(function(resolve,reject){
move(element,target,resolve);
});
}
animate(square1,100)
.then(function(){
return animate(square2,100);
})
.then(function(){
return animate(square3,100);
});
</script>
</html>

co

co初体验

let fs = require('fs');
function getNumber(){
return new Promise(function (resolve,reject) {
setTimeout(function(){
let number = Math.random();
if(number >.5){
resolve(number);
}else{
reject('数字太小');
}
},1000);
});
}
function *read(){
let a = yield getNumber();
console.log(a);
let b = yield 'b';
console.log(b);
let c = yield getNumber();
console.log(c);
}

function co(gen){
return new Promise(function(resolve,reject){
let g = gen();
function next(lastValue){
let {done,value} = g.next(lastValue);
if(done){
resolve(lastValue);
}else{
if(value instanceof Promise){
value.then(next,function(val){
reject(val);
});
}else{
next(value);
}
}
}
next();
});
}
co(read).then(function(data){
console.log(data);
},function(reason){
console.log(reason);
});

co连续读文件

let fs = require('fs');
function readFile(filename){
return new Promise(function (resolve,reject) {
fs.readFile(filename,'utf8',function(err,data){
if(err)
reject(err);
else
resolve(data);
})
});
}
function *read(){
let a = yield readFile('./1.txt');
console.log(a);
let b = yield readFile('./2.txt');
console.log(b);
}

function co(gen){
let g = gen();
function next(val){
let {done,value} = g.next(val);
if(!done){
value.then(next);
}
}
next();
}

Promise/A+完整实现

function Promise(executor) {
let self = this;
self.status = "pending";
self.value = undefined;
self.onResolvedCallbacks = [];
self.onRejectedCallbacks = [];
function resolve(value) {
if (value instanceof Promise) {
return value.then(resolve, reject)
}
setTimeout(function () { // 异步执行所有的回调函数
if (self.status == 'pending') {
self.value = value;
self.status = 'resolved';
self.onResolvedCallbacks.forEach(item => item(value));
}
});

}

function reject(value) {
setTimeout(function () {
if (self.status == 'pending') {
self.value = value;
self.status = 'rejected';
self.onRejectedCallbacks.forEach(item => item(value));
}
});
}

try {
executor(resolve, reject);
} catch (e) {
reject(e);
}
}
function resolvePromise(promise2, x, resolve, reject) {
if (promise2 === x) {
return reject(new TypeError('循环引用'));
}
let then, called;

if (x != null && ((typeof x == 'object' || typeof x == 'function'))) {
try {
then = x.then;
if (typeof then == 'function') {
then.call(x, function (y) {
if (called)return;
called = true;
resolvePromise(promise2, y, resolve, reject);
}, function (r) {
if (called)return;
called = true;
reject(r);
});
} else {
resolve(x);
}
} catch (e) {
if (called)return;
called = true;
reject(e);
}
} else {
resolve(x);
}
}
Promise.prototype.then = function (onFulfilled, onRejected) {
let self = this;
onFulfilled = typeof onFulfilled == 'function' ? onFulfilled : function (value) {
return value
};
onRejected = typeof onRejected == 'function' ? onRejected : function (value) {
throw value
};
let promise2;
if (self.status == 'resolved') {
promise2 = new Promise(function (resolve, reject) {
setTimeout(function () {
try {
let x = onFulfilled(self.value);
resolvePromise(promise2, x, resolve, reject);
} catch (e) {
reject(e);
}
});

});
}
if (self.status == 'rejected') {
promise2 = new Promise(function (resolve, reject) {
setTimeout(function () {
try {
let x = onRejected(self.value);
resolvePromise(promise2, x, resolve, reject);
} catch (e) {
reject(e);
}
});
});
}
if (self.status == 'pending') {
promise2 = new Promise(function (resolve, reject) {
self.onResolvedCallbacks.push(function (value) {
try {
let x = onFulfilled(value);
resolvePromise(promise2, x, resolve, reject);
} catch (e) {
reject(e);
}
});
self.onRejectedCallbacks.push(function (value) {
try {
let x = onRejected(value);
resolvePromise(promise2, x, resolve, reject);
} catch (e) {
reject(e);
}
});
});
}
return promise2;
}
Promise.prototype.catch = function (onRejected) {
return this.then(null, onRejected);
}
Promise.all = function (promises) {
return new Promise(function (resolve, reject) {
let result = [];
let count = 0;
for (let i = 0; i < promises.length; i++) {
promises[i].then(function (data) {
result[i] = data;
if (++count == promises.length) {
resolve(result);
}
}, function (err) {
reject(err);
});
}
});
}

Promise.deferred = Promise.defer = function () {
var defer = {};
defer.promise = new Promise(function (resolve, reject) {
defer.resolve = resolve;
defer.reject = reject;
})
return defer;
}
/**
* npm i -g promises-aplus-tests
* promises-aplus-tests Promise.js
*/
try {
module.exports = Promise
} catch (e) {
}

资源

bluebirdAPI
javascript Promise迷你书
Promise/A+规范
Promise/A+