Promise的声明
首先，promise肯定是一个类，我们就用class来声明。
- 由于
new Promise((resolve, reject)=>{})，所以传入一个参数（函数），promisesA+规范里叫他executor，传入就执行。 -
executor里面有两个参数，一个叫resolve（成功），一个叫reject（失败）。 - 由于
resolve和reject可执行，所以都是函数，我们用let声明。
class Promise {
// 构造器
constructor(executor) {
// 成功
let resolve = () => { };
// 失败
let reject = () => { };
// 立即执行
executor(resolve, reject);
}
}
解决基本状态
promisesA+规范对Promise有规定：
-
Promise存在三个状态（state）：pending、fulfilled、rejected -
pending（等待态）为初始态，并可以转化为fulfilled（成功态）和rejected（失败态） - 成功时，不可转为其他状态，且必须有一个不可改变的值（
value） - 失败时，不可转为其他状态，且必须有一个不可改变的原因（
reason） -
new Promise((resolve, reject)=>{resolve(value)})，resolve为成功，接收参数value，状态改变为fulfilled，不可再次改变 -
new Promise((resolve, reject)=>{reject(reason)})，reject为失败，接收参数reason，状态改变为rejected，不可再次改变 - 若是
executor函数报错，直接执行reject()
于是乎，我们获得以下代码：
class Promise {
constructor(executor) {
// 初始化state为等待态
this.state = 'pending';
// 成功的值
this.value = undefined;
// 失败的原因
this.reason = undefined;
let resolve = value => {
// state改变,resolve调用就会失败
if (this.state === 'pending') {
// resolve调用后，state转化为成功态
this.state = 'fulfilled';
// 储存成功的值
this.value = value;
}
};
let reject = reason => {
// state改变,reject调用就会失败
if (this.state === 'pending') {
// reject调用后，state转化为失败态
this.state = 'rejected';
// 储存失败的原因
this.reason = reason;
}
};
// 如果executor执行报错，直接执行reject
try{
executor(resolve, reject);
} catch (err) {
reject(err);
}
}
}
then方法
promisesA+规范规定：Promise有一个叫做then的方法，里面有两个参数：onFulfilled, onRejected，成功有成功的值，失败有失败的原因。
- 当状态
state为fulfilled，则执行onFulfilled，传入this.value。当状态state为rejected，则执行onRejected，传入this.reason -
onFulfilled, onRejected如果他们是函数，则必须分别在fulfilled，rejected后被调用，value或reason依次作为他们的第一个参数
class Promise {
constructor(executor) {...}
// then 方法 有两个参数onFulfilled onRejected
then(onFulfilled, onRejected) {
// 状态为fulfilled，执行onFulfilled，传入成功的值
if (this.state === 'fulfilled') {
onFulfilled(this.value);
};
// 状态为rejected，执行onRejected，传入失败的原因
if (this.state === 'rejected') {
onRejected(this.reason);
};
}
}
解决异步实现
现在基本可以实现简单的同步代码，但是当resolve在setTomeout内执行，then时state还是pending等待状态 我们就需要在then调用的时候，将成功和失败存到各自的数组，一旦reject或者resolve，就调用它们。
类似于发布订阅，先将then里面的两个函数储存起来，由于一个promise可以有多个then，所以存在同一个数组内。
// 多个then的情况
let p = new Promise();
p.then();
p.then();
成功或者失败时，forEach调用它们。
class Promise{
constructor(executor){
this.state = 'pending';
this.value = undefined;
this.reason = undefined;
// 成功存放的数组
this.onResolvedCallbacks = [];
// 失败存放法数组
this.onRejectedCallbacks = [];
let resolve = value => {
if (this.state === 'pending') {
this.state = 'fulfilled';
this.value = value;
// 一旦resolve执行，调用成功数组的函数
this.onResolvedCallbacks.forEach(fn=>fn());
}
};
let reject = reason => {
if (this.state === 'pending') {
this.state = 'rejected';
this.reason = reason;
// 一旦reject执行，调用失败数组的函数
this.onRejectedCallbacks.forEach(fn=>fn());
}
};
try{
executor(resolve, reject);
} catch (err) {
reject(err);
}
}
then(onFulfilled,onRejected) {
if (this.state === 'fulfilled') {
onFulfilled(this.value);
};
if (this.state === 'rejected') {
onRejected(this.reason);
};
// 当状态state为pending时
if (this.state === 'pending') {
// onFulfilled传入到成功数组
this.onResolvedCallbacks.push(()=>{
onFulfilled(this.value);
})
// onRejected传入到失败数组
this.onRejectedCallbacks.push(()=>{
onRejected(this.reason);
})
}
}
}
解决链式调用
我门常常用到new Promise().then().then(),这就是链式调用，用来解决回调地狱。
为了达成链式，我们默认在第一个then里返回一个promise。promisesA+规范规定了一种方法，就是在then里面返回一个新的promise，称为promise2：promise2 = new Promise((resolve, reject)=>{})
- 将这个
promise2返回的值传递到下一个then中 - 如果返回一个普通的值，则将普通的值传递给下一个
then中
当我们在第一个then中return了一个参数（参数未知，需判断）。这个return出来的新的promise就是onFulfilled()或onRejected()的值。
promisesA+规范则规定onFulfilled()或onRejected()的值，即第一个then返回的值，叫做x，判断x的函数叫做resolvePromise。
- 首先，要看
x是不是promise - 如果是
promise，则取它的结果，作为新的promise2成功的结果 - 如果是普通值，直接作为
promise2成功的结果 - 所以要比较
x和promise2 -
resolvePromise的参数有promise2（默认返回的promise）、x（我们自己return的对象）、resolve、reject -
resolve和reject是promise2的
class Promise {
constructor(executor) { ... }
then(onFulfilled, onRejected) {
// 声明返回的promise2
let promise2 = new Promise((resolve, reject)=>{
if (this.state === 'fulfilled') {
let x = onFulfilled(this.value);
// resolvePromise函数，处理自己return的promise和默认的promise2的关系
resolvePromise(promise2, x, resolve, reject);
};
if (this.state === 'rejected') {
let x = onRejected(this.reason);
resolvePromise(promise2, x, resolve, reject);
};
if (this.state === 'pending') {
this.onResolvedCallbacks.push(()=>{
let x = onFulfilled(this.value);
resolvePromise(promise2, x, resolve, reject);
})
this.onRejectedCallbacks.push(()=>{
let x = onRejected(this.reason);
resolvePromise(promise2, x, resolve, reject);
})
}
});
// 返回promise，完成链式
return promise2;
}
}
完成resolvePromise函数
promisesA+规范规定了一段代码，让不同的promise代码互相套用，叫做resolvePromise。
如果x === promise2，则是会造成循环引用，自己等待自己完成，则报“循环引用”错误。
let p = new Promise(resolve => {
resolve(0);
});
var p2 = p.then(data => {
// 循环引用，自己等待自己完成，一辈子完不成
return p2;
})
- 判断
x：
-
x不能是null -
x是普通值 直接resolve(x) -
x是对象或者函数（包括promise），let then = x.then
- 当
x是对象或者函数（默认promise）
- 声明了
then - 如果取
then报错，则走reject() - 如果
then是个函数，则用call执行then，第一个参数是this，后面是成功的回调和失败的回调 - 如果成功的回调还是
pormise，就递归继续解析
- 成功和失败只能调用一个，所以设定一个
called来防止多次调用
function resolvePromise(promise2, x, resolve, reject){
// 循环引用报错
if(x === promise2){
// reject报错
return reject(new TypeError('Chaining cycle detected for promise'));
}
// 防止多次调用
let called;
// x不是null 且x是对象或者函数
if (x != null && (typeof x === 'object' || typeof x === 'function')) {
try {
// A+规定，声明then = x的then方法
let then = x.then;
// 如果then是函数，就默认是promise了
if (typeof then === 'function') {
// 就让then执行 第一个参数是this 后面是成功的回调 和 失败的回调
then.call(x, y => {
// 成功和失败只能调用一个
if (called) return;
called = true;
// resolve的结果依旧是promise 那就继续解析
resolvePromise(promise2, y, resolve, reject);
}, err => {
// 成功和失败只能调用一个
if (called) return;
called = true;
reject(err);// 失败了就失败了
})
} else {
resolve(x); // 直接成功即可
}
} catch (e) {
// 也属于失败
if (called) return;
called = true;
// 取then出错了那就不要在继续执行了
reject(e);
}
} else {
resolve(x);
}
}
解决其他问题
-
promisesA+规范规定onFulfilled, onRejected都是可选参数，如果他们不是函数，必须被忽略
-
onFulfilled返回一个普通的值，成功时直接等于value => value -
onRejected返回一个普通的值，失败时如果直接等于value => value，则会跑到下一个then中的onFulfilled中，所以直接扔出一个错误reason => throw err
-
promisesA+规范规定onFulfilled或onRejected不能同步被调用，必须异步调用。我们就用setTimeout解决异步问题
- 如果
onFulfilled或onRejected报错，则直接返回reject()
class Promise{
constructor(executor){
this.state = 'pending';
this.value = undefined;
this.reason = undefined;
this.onResolvedCallbacks = [];
this.onRejectedCallbacks = [];
let resolve = value => {
if (this.state === 'pending') {
this.state = 'fulfilled';
this.value = value;
this.onResolvedCallbacks.forEach(fn=>fn());
}
};
let reject = reason => {
if (this.state === 'pending') {
this.state = 'rejected';
this.reason = reason;
this.onRejectedCallbacks.forEach(fn=>fn());
}
};
try{
executor(resolve, reject);
} catch (err) {
reject(err);
}
}
then(onFulfilled,onRejected) {
// onFulfilled如果不是函数，就忽略onFulfilled，直接返回value
onFulfilled = typeof onFulfilled === 'function' ? onFulfilled : value => value;
// onRejected如果不是函数，就忽略onRejected，直接扔出错误
onRejected = typeof onRejected === 'function' ? onRejected : err => { throw err };
let promise2 = new Promise((resolve, reject) => {
if (this.state === 'fulfilled') {
// 异步
setTimeout(() => {
try {
let x = onFulfilled(this.value);
resolvePromise(promise2, x, resolve, reject);
} catch (e) {
reject(e);
}
}, 0);
};
if (this.state === 'rejected') {
// 异步
setTimeout(() => {
// 如果报错
try {
let x = onRejected(this.reason);
resolvePromise(promise2, x, resolve, reject);
} catch (e) {
reject(e);
}
}, 0);
};
if (this.state === 'pending') {
this.onResolvedCallbacks.push(() => {
// 异步
setTimeout(() => {
try {
let x = onFulfilled(this.value);
resolvePromise(promise2, x, resolve, reject);
} catch (e) {
reject(e);
}
}, 0);
});
this.onRejectedCallbacks.push(() => {
// 异步
setTimeout(() => {
try {
let x = onRejected(this.reason);
resolvePromise(promise2, x, resolve, reject);
} catch (e) {
reject(e);
}
}, 0)
});
};
});
// 返回promise，完成链式
return promise2;
}
}
catchresolve、reject、race和all方法
class Promise {
constructor(executor) {
this.state = 'pending';
this.value = undefined;
this.reason = undefined;
this.onResolvedCallbacks = [];
this.onRejectedCallbacks = [];
let resolve = value => {
if (this.state === 'pending') {
this.state = 'fulfilled';
this.value = value;
this.onResolvedCallbacks.forEach(fn=>fn());
}
};
let reject = reason => {
if (this.state === 'pending') {
this.state = 'rejected';
this.reason = reason;
this.onRejectedCallbacks.forEach(fn=>fn());
}
};
try {
executor(resolve, reject);
} catch (err) {
reject(err);
}
}
then(onFulfilled,onRejected) {
onFulfilled = typeof onFulfilled === 'function' ? onFulfilled : value => value;
onRejected = typeof onRejected === 'function' ? onRejected : err => { throw err };
let promise2 = new Promise((resolve, reject) => {
if (this.state === 'fulfilled') {
setTimeout(() => {
try {
let x = onFulfilled(this.value);
resolvePromise(promise2, x, resolve, reject);
} catch (e) {
reject(e);
}
}, 0);
};
if (this.state === 'rejected') {
setTimeout(() => {
try {
let x = onRejected(this.reason);
resolvePromise(promise2, x, resolve, reject);
} catch (e) {
reject(e);
}
}, 0);
};
if (this.state === 'pending') {
this.onResolvedCallbacks.push(() => {
setTimeout(() => {
try {
let x = onFulfilled(this.value);
resolvePromise(promise2, x, resolve, reject);
} catch (e) {
reject(e);
}
}, 0);
});
this.onRejectedCallbacks.push(() => {
setTimeout(() => {
try {
let x = onRejected(this.reason);
resolvePromise(promise2, x, resolve, reject);
} catch (e) {
reject(e);
}
}, 0)
});
};
});
return promise2;
}
catch(fn){
return this.then(null,fn);
}
}
function resolvePromise(promise2, x, resolve, reject){
if(x === promise2){
return reject(new TypeError('Chaining cycle detected for promise'));
}
let called;
if (x != null && (typeof x === 'object' || typeof x === 'function')) {
try {
let then = x.then;
if (typeof then === 'function') {
then.call(x, y => {
if(called)return;
called = true;
resolvePromise(promise2, y, resolve, reject);
}, err => {
if(called)return;
called = true;
reject(err);
})
} else {
resolve(x);
}
} catch (e) {
if(called)return;
called = true;
reject(e);
}
} else {
resolve(x);
}
}
//resolve方法
Promise.resolve = function(val){
return new Promise((resolve,reject)=>{
resolve(val)
});
}
//reject方法
Promise.reject = function(val){
return new Promise((resolve,reject)=>{
reject(val)
});
}
//race方法
Promise.race = function(promises){
return new Promise((resolve,reject)=>{
for(let i=0;i<promises.length;i++){
promises[i].then(resolve,reject)
};
})
}
//all方法(获取所有的promise，都执行then，把结果放到数组，一起返回)
Promise.all = function(promises){
let arr = [];
let i = 0;
function processData(index,data){
arr[index] = data;
i++;
if(i == promises.length){
resolve(arr);
};
};
return new Promise((resolve,reject)=>{
for(let i=0;i<promises.length;i++){
promises[i].then(data=>{
processData(i,data);
},reject);
};
});
}
