我们不是年复一年的老去，而是日复一日的焕然一新。（送给自己，也送个你们【娇羞脸】）
我很早就有手动实现Promise的想法，拖延了很久，终于在周末处女座情节爆发，硬扛了好久，结果越写越多，自己写了一般结果发现想的还是不周全。然后查阅了一些资料,遂发现promise/aplus规范，也参考了一些实现，受益颇多。
说到Promise,我们能想到什么？
由其自身的三个状态出发（pending,fulfilled,reject），不管函数嵌套几层，函数的过程只需要维护这几个状态，从这个思路出发，因此可以设计成一个状态机。
Version1:
const PENDING = "pending";
const FULFILLED = "fulfilled";
const REJECTED = "rejected";
function Promise(executor) {
let self = this;
self.status = PENDING;
self.value = undefined;
function resolve(value) {
if(self.status === PENDING){
self.status = FULFILLED;
self.value = value;
}
}
function reject(reason) {
if(self.status === PENDING){
self.status = REJECTED;
self.value = reason
}
}
try{
executor(resolve ,reject)
}catch(e) {
reject(e)
}
}
Promise.prototype.then = function(onFulfilled, onRejected) {
let self = this;
if(self.status === FULFILLED){
onFulfilled(self.value);
}
if(self.status === REJECTED){
onRejected(self.value)
}
}
嗯哼，可以了么？貌似并没有那么简单，虽然同步能执行下去，但是当异步操作时，譬如 Version2(Promise主体部分优化)
new Promsie(function(resolve, reject){
resolve(2)
).then(function(data){
console.log(data)
},function(reason){
console.log(reason)
})
})
//输出2
new Promsie(function(resolve, reject){
setTimeout(function(){
resolve(2)
}).then(function(data){
console.log(data)
},function(reason){
console.log(reason)
})
})
//无输出
为什么呢？在异步操作还没有返回的时候，函数已经运行完then部分，而此刻返回resolve(1)，已经没有意义
怎么解决呢？观察者（发布订阅）模式模式应运而生。这边先声明两个数组onResolveCallBacks,onRejectCallBacks分别保存成功失败的方法，当状态机变化的时候，依次遍历执行数组中的方法
const PENDING = "pending";
const FULFILLED = "fulfilled";
const REJECTED = "rejected";
function Promise(executor) {
let self = this.;
self.status = PENDING;
self.value = undefined;
//添加的数组
self.onResolveCallBacks = [];
self.onRejectCallBacks = [];
function resolve(value) {
if(self.status === PENDING){
self.status = FULFILLED;
self.value = value;
self.onResolveCallBacks.forEach(cb=>cb(self.value));
}
}
function reject(reason) {
if(self.status === PENDING){
self.status = REJECTED;
self.value = reason；
self.onRejectCallBacks.forEach(cb=>cb(self.value));
}
}
try{
exectutor(resolve ,reject)
}catch(e) {
reject(e)
}
}
Promise.prototype.then = function(onFulfilled, onRejected) {
let self = this;
if(self.status === FULFILLED){
onFulfilled(self.value);
}
if(self.status === REJECTED){
onRejected(self.value)
}
}
现在我们异步操作也能准确了实现了，然后咧~Promise的精髓，链式调用貌似还是不能准确支持，要在then方法之后还能继续调用then，那恐怕解铃还需系铃人了，在then执行完之后再返回一个Promise不就行了么？ Version2(then部分优化)
Promise.prototype.then(onFulfilled, onRejected) {
let self = this;
if(self.status === FULFILLED){
return new Promise((resolve, reject)=>{
let x = onFulfilled(self.value);
resolve(x)
})
}
if(self.status === REJECTED){
return new Promise((resolve, reject)=>{
let x = onRejected(self.value);
reject(x)
})
}
if(self.status === PENDING){
return new Promise(function(resolve, reject){
self.onResolvedCallBacks.push(function(){
let x = onFulfilled(self.value);
resolve(x)
})
self.onRejectedCallBacks.push(function(){
let x = onFulfilled(self.value);
resolve(x)
})
})
}
}
看当前的代码貌似很完美了，但是如果then的成功函数onFulfilled要是返回一个Promise怎么办
return new Promise(function(resolve,reject){
self.onResolveCallBacks.push(function(){
let x = onFulfilled(self.value);
// 判断返回值是否是Promise，是的话调用thenable方法
if(x instanceof Promise){
x.then(resolve,reject)
}else{
resolve(x);
}
});
self.onRejectCallBacks.push(function(){
let x = onRejected(self.reason);
if(x instanceof Promise){
x.then(resolve,reject);
}else{
reject(x);
}
});
})
关于Promise的实现确实包含了很多知识点，譬如Event Loop，递归等等，此处先做个铺垫，先上个基础版本的实现，后续待篇二分解 VERSION3（基础版本API终版）
const PENDING = 'pending';//初始态
const FULFILLED = 'fulfilled';//初始态
const REJECTED = 'rejected';//初始态
function Promise(executor){
let self = this;//先缓存当前promise实例
self.status = PENDING;//设置状态
//定义存放成功的回调的数组
self.onResolvedCallbacks = [];
//定义存放失败回调的数组
self.onRejectedCallbacks = [];
//当调用此方法的时候，如果promise状态为pending,的话可以转成成功态,如果已经是成功态或者失败态了，则什么都不做
//2.1
function resolve(value){ //2.1.1
if(value!=null &&value.then&&typeof value.then == 'function'){
return value.then(resolve,reject);
}
//如果是初始态，则转成成功态
//为什么要把它用setTimeout包起来
setTimeout(function(){
if(self.status == PENDING){
self.status = FULFILLED;
self.value = value;//成功后会得到一个值，这个值不能改
//调用所有成功的回调
self.onResolvedCallbacks.forEach(cb=>cb(self.value));
}
})
}
function reject(reason){ //2.1.2
setTimeout(function(){
//如果是初始态，则转成失败态
if(self.status == PENDING){
self.status = REJECTED;
self.value = reason;//失败的原因给了value
self.onRejectedCallbacks.forEach(cb=>cb(self.value));
}
});
}
try{
//因为此函数执行可能会异常，所以需要捕获，如果出错了，需要用错误 对象reject
executor(resolve,reject);
}catch(e){
//如果这函数执行失败了，则用失败的原因reject这个promise
reject(e);
};
}
function resolvePromise(promise2,x,resolve,reject){
if(promise2 === x){
return reject(new TypeError('循环引用'));
}
let called = false;//promise2是否已经resolve 或reject了
if(x instanceof Promise){
if(x.status == PENDING){
x.then(function(y){
resolvePromise(promise2,y,resolve,reject);
},reject);
}else{
x.then(resolve,reject);
}
//x是一个thenable对象或函数，只要有then方法的对象，
}else if(x!= null &&((typeof x=='object')||(typeof x == 'function'))){
//当我们的promise和别的promise进行交互，编写这段代码的时候尽量的考虑兼容性，允许别人瞎写
try{
let then = x.then;
if(typeof then == 'function'){
//有些promise会同时执行成功和失败的回调
then.call(x,function(y){
//如果promise2已经成功或失败了，则不会再处理了
if(called)return;
called = true;
resolvePromise(promise2,y,resolve,reject)
},function(err){
if(called)return;
called = true;
reject(err);
});
}else{
//到此的话x不是一个thenable对象，那直接把它当成值resolve promise2就可以了
resolve(x);
}
}catch(e){
if(called)return;
called = true;
reject(e);
}
}else{
//如果X是一个普通 的值，则用x的值去resolve promise2
resolve(x);
}
}
//onFulfilled 是用来接收promise成功的值或者失败的原因
Promise.prototype.then = function(onFulfilled,onRejected){
//如果成功和失败的回调没有传，则表示这个then没有任何逻辑，只会把值往后抛
//2.2.1
onFulfilled = typeof onFulfilled == 'function'?onFulfilled:function(value){return value};
onRejected = typeof onRejected == 'function'?onRejected:reason=>{throw reason};
//如果当前promise状态已经是成功态了，onFulfilled直接取值
let self = this;
let promise2;
if(self.status == FULFILLED){
return promise2 = new Promise(function(resolve,reject){
setTimeout(function(){
try{
let x =onFulfilled(self.value);
//如果获取到了返回值x,会走解析promise的过程
resolvePromise(promise2,x,resolve,reject);
}catch(e){
//如果执行成功的回调过程中出错了，用错误原因把promise2 reject
reject(e);
}
})
});
}
if(self.status == REJECTED){
return 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){
return promise2 = new Promise(function(resolve,reject){
self.onResolvedCallbacks.push(function(){
try{
let x =onFulfilled(self.value);
//如果获取到了返回值x,会走解析promise的过程
resolvePromise(promise2,x,resolve,reject);
}catch(e){
reject(e);
}
});
self.onRejectedCallbacks.push(function(){
try{
let x =onRejected(self.value);
resolvePromise(promise2,x,resolve,reject);
}catch(e){
reject(e);
}
});
});
}
}
module.exports = Promise;
