手写一款符合Promise/A+规范的Promise
长篇预警！有点长，可以选择性观看。如果对Promise源码不是很清楚，还是推荐从头看，相信你认真从头看到尾，并且去实际操作了，肯定会有收获的。主要是代码部分有点多，不过好多都是重复的，不必担心
Promise的一些用法在此不多赘述，本篇主要带领你手写一个Promise源码，学完你就会发现：Promise没有你想象中的那么难
本篇大概分为以下步骤
- 实现简单的同步Promise
- 增加异步功能
- 增加链式调用then
- 增加catch finally方法
- 增加all race 等方法
- 实现一个promise的延迟对象defer
- 最终测试
实现简单的同步Promise
先大概说一下基本概念：
Promise内部维护着三种状态，即pending,resolved和rejected。初始状态是pending，状态可以有pending--->relolved,或者pending--->rejected.不能从resolve转换为rejected 或者从rejected转换成resolved.
即 只要Promise由pending状态转换为其他状态后，状态就不可变更。
ok.知道了这些后，我们开始手撸代码：
注意观看序号 1 2 3 4 5 ...
function Promise(executor){
let that = this;
/** 2 定义初始的一些变量 */
that.status = 'pending';
that.value = null;
that.reason = null;
/** 3 定义初始的成功和失败函数 */
function resolve(value){
/** 4 判断状态是不是初始状态pending
* 是就转换状态 否则不转换
* 确保状态的变化后的不可变性 */
if(that.status === 'pending'){
that.status = 'resolved';
that.value = value;
}
}
function reject(reason){
/** 5 同上 */
if(that.status === 'pending'){
that.status = 'rejected';
that.reason = reason;
}
}
/**
* 1 Promise中首先传了一个executor，它是一个函数
* executor函数中又传了两个函数，分别是resolve和reject
* 很显然 resolve是成功回调，reject是失败的回调
*/
executor(resolve,reject);
}
/** 6 在Promise原型上面定义then方法
* then方法上面有两个回调 一个是成功后的方法 另一个是失败后的方法
* 根据成功或失败的状态去执行相关成功onFilfulled()或者失败onRejected()的回调方法
*/
Promise.prototype.then = function(onFilfulled,onRejected){
let that = this;
if(that.status === 'resolved'){
/** 7 如果状态已经变更为resolved
* 说明resolve方法已经被调用
* 那么此时就执行成功的回调函数onFilfulled
* 并传入参数 that.value
* */
onFilfulled(that.value);
}
if(that.status === 'rejected'){
/** 8 同上
* 传入参数 that.reason
*/
onRejected(that.reason);
}
}
module.exports = Promise;通过require()引入手撸的Promise
let Promise = require('./myPromise');
let p1 = ()=>{
return new Promise((resolve,reject)=>{
resolve('success.1');
});
}
p1().then((data)=>{
console.log(data); // 打印 success.1
},(err)=>{
console.log(err);
});ok.经调用发现 此代码可以实现部分Promise的功能，但仅仅是同步下才有效果。
那异步呢？ 别急这就来~：
增加异步功能
注意观看序号 1 2 3 4 5 ...
function Promise(executor){
let that = this;
that.status = 'pending';
that.value = null;
that.reason = null;
/** 1 因为异步不是立即执行 状态不会变更 成功或失败的回调函数也不会执行
* 所以先定义好存放成功或失败回调函数的数组
* 以便将成功或失败的回调函数先保存起来
* */
that.onFilFulledCallbacks = [];
that.onRejectedCallbacks = [];
function resolve(value){
if(that.status === 'pending'){
that.status = 'resolved';
that.value = value;
/** 3 发布
* 等待状态发生变更
* 状态变更后 立即执行之前存放在相应数组中所有的成功或失败的回调函数
* 即 发布
*/
that.onFilFulledCallbacks.forEach((fn)=>{
fn();
});
}
}
function reject(reason){
if(that.status === 'pending'){
that.status = 'rejected';
that.reason = reason;
/** 4 同上 */
that.onRejectedCallbacks.forEach((fn)=>{
fn();
});
}
}
executor(resolve,reject);
}
Promise.prototype.then = function(onFilfulled,onRejected){
let that = this;
if(that.status === 'resolved'){
onFilfulled(that.value);
}
if(that.status === 'rejected'){
onRejected(that.reason);
}
/** 2 订阅
* 因为是异步 状态当时并没有立即变更 所以状态还是pending
* 此时需要把成功或者失败的回调函数存放到对应的数组中
* 等待状态变更时 再从数组中拿出来去执行
* 即 订阅
* *存放数组时 为了执行时方便 需要把回调函数的外层包裹一层空函数
*/
if(that.status === 'pending'){
that.onFilFulledCallbacks.push(function(){
onFilfulled(that.value);
});
}
if(that.status === 'pending'){
that.onRejectedCallbacks.push(function(){
onRejected(that.reason);
});
}
}
module.exports = Promise;代码测试：
let Promise = require('./myPromise');
let p1 = ()=>{
return new Promise((resolve,reject)=>{
setTimeout(function(){
resolve('success.1');
// reject('fail.');
},1500);
});
}
p1().then((data)=>{
console.log(data); // success.1
},(err)=>{
console.log(err);
});可以看到 1.5s后 执行了resolve() 并打印了success.1，至此，我们实现了异步的Promise.其实这里的实现异步的思想就是发布订阅.
en～ok.高能预警?.接下来就稍稍复杂了 因为我们要实现链式调用then。 要实现这个功能那我们就要重写then方法，并在then方法中重新返回一个Promise,只有这样，才可以实现多次调用then.而且要新增一个解析返回值是否为promise的函数.
稍微捋下逻辑：
- 如果一个then方法返回一个普通值的话，这个值会传递给下一个then中作为resolve成功的结果
- 如果一个then方法返回一个promise的话，会根据返回的promise是成功还是失败，决定下一个then是成功还是失败
? 上代码：
增加链式调用then
注意观看序号 1 2 3 4 5 ...
function Promise(executor){
let that = this;
that.status = 'pending';
that.value = null;
that.reason = null;
that.onFilFulledCallbacks = [];
that.onRejectedCallbacks = [];
function resolve(value){
if(that.status === 'pending'){
that.status = 'resolved';
that.value = value;
that.onFilFulledCallbacks.forEach((fn)=>{
fn();
});
}
}
function reject(reason){
if(that.status === 'pending'){
that.status = 'rejected';
that.reason = reason;
that.onRejectedCallbacks.forEach((fn)=>{
fn();
});
}
}
executor(resolve,reject);
}
Promise.prototype.then = function(onFilfulled,onRejected){
let that = this;
/** 1 让promise2等于一个新的Promise 并将promise2返回 */
let promise2 = new Promise((resolve,reject)=>{
if(that.status === 'resolved'){
/** 2 因为返回了promise2
* 并且第3步resolvePromiseRelation函数中传递了promise2
* 而目前promise2并没有拿到
* 所以加一个定时器 异步执行 等到promise2拿到后
* 再去执行 resolvePromiseRelation()方法 并将promise2传递进去*/
setTimeout(()=>{
try{
let promise3 = onFilfulled(that.value);
/** 3 判断新返回值是什么类型的函数
* 并将当前的promise：promise2 新的返回值：promise3
* 和 成功时回调：esolve 失败时回调：reject 作为参数传进去 */
resolvePromiseRelation(promise2,promise3,resolve,reject);
}catch(e){
reject(e);
}
},0);
}
if(that.status === 'rejected'){
/** 同2 */
setTimeout(()=>{
try{
let promise3 = onRejected(that.reason);
/** 同3*/
resolvePromiseRelation(promise2,promise3,resolve,reject);
}catch(e){
reject(e);
}
},0);
}
if(that.status === 'pending'){
that.onFilFulledCallbacks.push(function(){
/** 同2 */
setTimeout(()=>{
try{
let promise3 = onFilfulled(that.value);
/** 同3*/
resolvePromiseRelation(promise2,promise3,resolve,reject);
}catch(e){
reject(e);
}
},0);
});
}
if(that.status === 'pending'){
that.onRejectedCallbacks.push(function(){
/** 同2 */
setTimeout(()=>{
try{
let promise3 = onRejected(that.reason);
/** 同3*/
resolvePromiseRelation(promise2,promise3,resolve,reject);
}catch(e){
reject(e);
}
},0);
});
}
});
/** 同1 */
return promise2;
}
function resolvePromiseRelation(promise2,promise3,resolve,reject){
/** 4 防止自己等待自己 一直循环等待 */
if(promise2 === promise3){
return reject(new TypeError('循环引用了!'));
}
/** 8 一个标示 表示当前没有被调用过
* 确保resolve或者reject后的状态不会再次发生变更
*/
let called;
/** 5 保证promise3是一个引用类型
* 判断新返回值promise3的类型
* 如果是普通值常量 就直接resolve导出 */
if(promise3!==null&&(typeof promise3 === 'object'||typeof promise3 === 'function')){
try{
let then = promise3.then;
/** 6 确保promise3是一个Promise
* 判断promise3的then方法
* 如果存在 并且是一个function类型
* 就表示promise3是一个Promise */
if(typeof then === 'function'){
/** 9 执行promise3的then方法
* 因为promise3也是一个Promise
* 需要再次解析promise3的then方法
* 直到解析到最后的返回值不是一个Promise类型为止
*/
then(promise3, (promise4)=>{
/** 同8 */
if(called) return;
called = true;
/** 10 递归解析新的返回值的类型
* 解析到返回值不是一个Promise类型为止
*/
resolvePromiseRelation(promise3,promise4,resolve,reject);
},(r)=>{
/** 同8 */
if(called) return;
called = true;
reject(r);
});
}else{
/** 7 此时promise3是一个普通对象 直接resolve() */
resolve(promise3);
}
}catch(e){
/** 同8 */
if(called) return;
called = true;
reject(e);
};
}else{
/** 同5 普通值直接resolve()*/
resolve(promise3);
}
}
module.exports = Promise;ok. 至此 我们已经实现了Promsie的异步和链式调用. Promise中比较复杂的部分我们已经搞定了 接下来就是添加一些方法，其实这部分反而没那么复杂了.
catch : catch方法本质上就是一个then方法的变形，只有失败时的回调 没有成功时的回调
finally : finally方法的作用是不管 Promise 对象最后状态如何，都会执行操作.其实说白了就是在then方法的成功和失败的回调函数中都执行该方法就行了.
ok.老规矩 上代码～
增加catch finally方法
function Promise(executor){
let that = this;
that.status = 'pending';
that.value = null;
that.reason = null;
that.onFilFulledCallbacks = [];
that.onRejectedCallbacks = [];
function resolve(value){
if(that.status === 'pending'){
that.status = 'resolved';
that.value = value;
that.onFilFulledCallbacks.forEach((fn)=>{
fn();
});
}
}
function reject(reason){
if(that.status === 'pending'){
that.status = 'rejected';
that.reason = reason;
that.onRejectedCallbacks.forEach((fn)=>{
fn();
});
}
}
executor(resolve,reject);
}
Promise.prototype.then = function(onFilfulled,onRejected){
/** 2 此处有个坑 如果只写1 不写2的话
* 会报一个TypeError ：onRejected is not a function
* 在此处给它一个默认的成功和失败的回调函数就好 */
onFilfulled = typeof onFilfulled === 'function'?onFilfulled:value=>value;
onRejected = typeof onRejected === 'function'?onRejected:err=>{throw err};
let that = this;
let promise2 = new Promise((resolve,reject)=>{
if(that.status === 'resolved'){
setTimeout(()=>{
try{
let promise3 = onFilfulled(that.value);
resolvePromiseRelation(promise2,promise3,resolve,reject);
}catch(e){
reject(e);
}
},0);
}
if(that.status === 'rejected'){
setTimeout(()=>{
try{
let promise3 = onRejected(that.reason);
resolvePromiseRelation(promise2,promise3,resolve,reject);
}catch(e){
reject(e);
}
},0);
}
if(that.status === 'pending'){
that.onFilFulledCallbacks.push(function(){
setTimeout(()=>{
try{
let promise3 = onFilfulled(that.value);
resolvePromiseRelation(promise2,promise3,resolve,reject);
}catch(e){
reject(e);
}
},0);
});
}
if(that.status === 'pending'){
that.onRejectedCallbacks.push(function(){
setTimeout(()=>{
try{
let promise3 = onRejected(that.reason);
resolvePromiseRelation(promise2,promise3,resolve,reject);
}catch(e){
reject(e);
}
},0);
});
}
});
return promise2;
}
function resolvePromiseRelation(promise2,promise3,resolve,reject){
if(promise2 === promise3){
return reject(new TypeError('循环引用了!'));
}
let called;
if(promise3!==null&&(typeof promise3 === 'object'||typeof promise3 === 'function')){
try{
let then = promise3.then;
if(typeof then === 'function'){
then(promise3, (promise4)=>{
if(called) return;
called = true;
resolvePromiseRelation(promise3,promise4,resolve,reject);
},(r)=>{
if(called) return;
called = true;
reject(r);
});
}else{
resolve(promise3);
}
}catch(e){
if(called) return;
called = true;
reject(e);
};
}else{
resolve(promise3);
}
}
/** 1 直接返回this的then方法
* 因为catch只捕获错误 所以resolve直接为null
* 返回reject就好*/
Promise.prototype.catch = function(errFn){
return this.then(null,errFn);
}
/** 3 finally实现起来也很简单
* 分别在resolve和reject中执行fn就好
* 最后再把this返回出去就好
*/
Promise.prototype.finally = function(fn){
this.then(()=>{
fn();
},()=>{
fn();
});
return this;
}
module.exports = Promise;增加all race resolve reject等方法
function Promise(executor){
let that = this;
that.status = 'pending';
that.value = null;
that.reason = null;
that.onFilFulledCallbacks = [];
that.onRejectedCallbacks = [];
function resolve(value){
if(that.status === 'pending'){
that.status = 'resolved';
that.value = value;
that.onFilFulledCallbacks.forEach((fn)=>{
fn();
});
}
}
function reject(reason){
if(that.status === 'pending'){
that.status = 'rejected';
that.reason = reason;
that.onRejectedCallbacks.forEach((fn)=>{
fn();
});
}
}
executor(resolve,reject);
}
Promise.prototype.then = function(onFilfulled,onRejected){
onFilfulled = typeof onFilfulled === 'function'?onFilfulled:value=>value;
onRejected = typeof onRejected === 'function'?onRejected:err=>{throw err};
let that = this;
let promise2 = new Promise((resolve,reject)=>{
if(that.status === 'resolved'){
setTimeout(()=>{
try{
let promise3 = onFilfulled(that.value);
resolvePromiseRelation(promise2,promise3,resolve,reject);
}catch(e){
reject(e);
}
},0);
}
if(that.status === 'rejected'){
setTimeout(()=>{
try{
let promise3 = onRejected(that.reason);
resolvePromiseRelation(promise2,promise3,resolve,reject);
}catch(e){
reject(e);
}
},0);
}
if(that.status === 'pending'){
that.onFilFulledCallbacks.push(function(){
setTimeout(()=>{
try{
let promise3 = onFilfulled(that.value);
resolvePromiseRelation(promise2,promise3,resolve,reject);
}catch(e){
reject(e);
}
},0);
});
}
if(that.status === 'pending'){
that.onRejectedCallbacks.push(function(){
setTimeout(()=>{
try{
let promise3 = onRejected(that.reason);
resolvePromiseRelation(promise2,promise3,resolve,reject);
}catch(e){
reject(e);
}
},0);
});
}
});
return promise2;
}
function resolvePromiseRelation(promise2,promise3,resolve,reject){
if(promise2 === promise3){
return reject(new TypeError('循环引用了!'));
}
let called;
if(promise3!==null&&(typeof promise3 === 'object'||typeof promise3 === 'function')){
try{
let then = promise3.then;
if(typeof then === 'function'){
then(promise3, (promise4)=>{
if(called) return;
called = true;
resolvePromiseRelation(promise3,promise4,resolve,reject);
},(r)=>{
if(called) return;
called = true;
reject(r);
});
}else{
resolve(promise3);
}
}catch(e){
if(called) return;
called = true;
reject(e);
};
}else{
resolve(promise3);
}
}
Promise.prototype.catch = function(errFn){
return this.then(null,errFn);
}
Promise.prototype.finally = function(fn){
this.then(()=>{
fn();
},()=>{
fn();
});
return this;
}
/** 1 直接在构造函数上增加all方法
* 它返回的也是一个Promise
* 等待参数数组中所有的promise都执行完毕后
* 再返回结果
*/
Promise.all = function(values){
return new Promise((resolve,reject)=>{
/** 2 定义一个存放最终结果的数组result和一个index */
let results = [];
let index = 0;
/** 3 定义一个方法addToArr()
* 让index每次执行增加results数组元素的函数的时候都+1
* 当index === values的长度的时候 说明此时所有promsie都执行完毕并放到的数组中
* 然后直接resolve(results)就行了
*/
function addToArr(key,value){
index++;
results[key] = value;
/** 6 当满足条件时 说明所有的promise都执行完毕 直接resolve(results) */
if(index === values.length){
resolve(results);
}
}
/** 4 循环values中的每一项promsie */
for(let i = 0; i < values.length; i++){
let current = values[i];
/** 5 判断每一项promise的返回值是不是一个Promsie
* 是的话就执行该Promise的then方法 拿到返回值 并放到数组results中
* 是一个普通值的话就直接将该值放到数组results中
*/
if(current && current.then && typeof current.then === 'function'){
current.then((value)=>{
/** 同5 把返回值放到数组results中*/
addToArr(i,value);
},reject);
}else{
/** 同5 把返回值放到数组results中*/
addToArr(i,current);
}
}
});
}
/** race方法相比较于all方法简单很多
* 因为race中的promsie成功resolve一个
* 整个race就resolve */
Promise.race = function(values){
return new Promise((resolve,reject)=>{
/** 同4 */
for(let i = 0; i < values.length; i++){
let current = values[i];
/** 同5 */
if(current&¤t.then&&typeof current.then === 'function'){
/** 7 直接执行then就好 */
current.then(resolve,reject);
}else{
/** 8 普通值直接resolve */
resolve(current);
}
}
});
}
// resolve方法
Promise.resolve = function(value){
return new Promise((resolve,reject)=>{
resolve(value);
});
}
// reject方法
Promise.reject = function(reason){
return new Promise((resolve,reject)=>{
reject(reason);
});
}
module.exports = Promise;实现一个promise的延迟对象defer
此步是为了测试我们手写的Promsie符不符合Promsie/A+规范，如果没有defer的话，我们在测试过程中就会报一个TypeError: adapter.deferred is not a function.
其实写完defer后，我们就可以去进行测试我们手写的Promsie符不符合Promsie/A+规范了。
即：本篇手写一款符合Promise/A+规范的Promise的最终本为：
function Promise(executor){
let that = this;
that.status = 'pending';
that.value = null;
that.reason = null;
that.onFilFulledCallbacks = [];
that.onRejectedCallbacks = [];
function resolve(value){
if(that.status === 'pending'){
that.status = 'resolved';
that.value = value;
that.onFilFulledCallbacks.forEach((fn)=>{
fn();
});
}
}
function reject(reason){
if(that.status === 'pending'){
that.status = 'rejected';
that.reason = reason;
that.onRejectedCallbacks.forEach((fn)=>{
fn();
});
}
}
try{
executor(resolve,reject);
}catch(e){
reject(e);
}
}
Promise.prototype.then = function(onFilfulled,onRejected){
onFilfulled = typeof onFilfulled === 'function'?onFilfulled:value=>value;
onRejected = typeof onRejected === 'function'?onRejected:err=>{throw err};
let that = this;
let promise2 = new Promise((resolve,reject)=>{
if(that.status === 'resolved'){
setTimeout(()=>{
try{
let promise3 = onFilfulled(that.value);
resolvePromiseRelation(promise2,promise3,resolve,reject);
}catch(e){
reject(e);
}
},0);
}
if(that.status === 'rejected'){
setTimeout(()=>{
try{
let promise3 = onRejected(that.reason);
resolvePromiseRelation(promise2,promise3,resolve,reject);
}catch(e){
reject(e);
}
},0);
}
if(that.status === 'pending'){
that.onFilFulledCallbacks.push(function(){
setTimeout(()=>{
try{
let promise3 = onFilfulled(that.value);
resolvePromiseRelation(promise2,promise3,resolve,reject);
}catch(e){
reject(e);
}
},0);
});
that.onRejectedCallbacks.push(function(){
setTimeout(()=>{
try{
let promise3 = onRejected(that.reason);
resolvePromiseRelation(promise2,promise3,resolve,reject);
}catch(e){
reject(e);
}
},0);
});
}
});
return promise2;
}
function resolvePromiseRelation(promise2,promise3,resolve,reject){
if(promise2 == promise3){
return reject(new TypeError('循环引用了!'));
}
let called;
if(promise3!==null&&(typeof promise3 === 'object' || typeof promise3 === 'function')){
try{
let then = promise3.then;
if(typeof then === 'function'){
then.call(promise3, (promise4)=>{
if(called) return;
called = true;
resolvePromiseRelation(promise3,promise4,resolve,reject);
},(r)=>{
if(called) return;
called = true;
reject(r);
});
}else{
resolve(promise3);
}
}catch(e){
if(called) return;
called = true;
reject(e);
};
}else{
resolve(promise3);
}
}
Promise.prototype.catch = function(errFn){
return this.then(null,errFn);
}
Promise.prototype.finally = function(fn){
this.then(()=>{
fn();
},()=>{
fn();
});
return this;
}
Promise.all = function(values){
return new Promise((resolve,reject)=>{
let results = [];
let index = 0;
function addToArr(key,value){
index++;
results[key] = value;
if(index === values.length){
resolve(results);
}
}
for(let i = 0; i < values.length; i++){
let current = values[i];
if(current && current.then && typeof current.then === 'function'){
current.then((value)=>{
addToArr(i,value);
},reject);
}else{
addToArr(i,current);
}
}
});
}
Promise.race = function(values){
return new Promise((resolve,reject)=>{
for(let i = 0; i < values.length; i++){
let current = values[i];
if(current&¤t.then&&typeof current.then === 'function'){
current.then(resolve,reject);
}else{
resolve(current);
}
}
});
}
Promise.resolve = function(value){
return new Promise((resolve,reject)=>{
resolve(value);
});
}
Promise.reject = function(reason){
return new Promise((resolve,reject)=>{
reject(reason);
});
}
// 实现一个promise的延迟对象 defer
Promise.defer = Promise.deferred = function(){
let dfd = {};
dfd.promise = new Promise((resolve, reject)=>{
dfd.resolve = resolve;
dfd.reject = reject;
});
return dfd;
}
module.exports = Promise;最终测试
- 测试当前代码是否符合Promise/A+规范
- 全局安装 npm i -g promises-aplus-tests
- 文件所在目录运行以下命令 (例如你的文件名为:MyPrommise.js)
- promise-aplus-tests MyPrommise.js
- 等待
- ok.
源码在github上，写文章不易，欢迎star或fork，thx～
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