1.Promise产生背景及规范
众所周知，Promise是ES6引入的新特性，旨在解决回调地狱。下面是一个简单的例子：控制接口调用顺序：
apiA-->apiB-->apiC。复杂的业务，开发人员会裂开。后生在此向老前辈致敬。
// 回调地狱
apiA({
handleSuccess(resA){
apiB({
handleSuccess(resB){
apiC({
handleSuccess(resC){
}
})
}
})
}
})因此Promise/A+规范应运而生，ES6的Promise就是遵循规范开发出来的。
2. 同步Promise
阅读规范可得下面几点基本要求:
- Promise存在三个状态：pending（等待态）、fulfilled（成功态）、rejected（失败态）
- pending为初始态，并可以转化为fulfilled和rejected
- 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(executor=(resolve,reject)=>{resolve(value)})，resolve(value)将状态置为 fulfilled
- new Promise(executor=(resolve,reject)=>{reject(reson)})，reject(reson)将状态置为 rejected
- 若是executor运行异常执行reject()
thenable：then(onFulfilled, onRejected)
- onFulfilled：status为fulfilled，执行onFulfilled，传入value
- onRejected：status为rejected，执行onRejected，传入reason
// 1.Promise存在三个状态：pending（等待态）、fulfilled（成功态）、rejected（失败态）
const STATUS_PENDING = 'pending'
const STATUS_FULFILLED = 'fulfilled'
const STATUS_REJECTED = 'rejected'
class myPromise {
constructor(executor) {
// pending为初始态，并可以转化为fulfilled和rejected
this.status = STATUS_PENDING
this.value = '' // 3
this.reason = '' // 4
let resolve = value => {
// 5.
if (this.status === STATUS_PENDING) {
this.status = STATUS_FULFILLED
this.value = value
}
}
let reject = reason => {
//6.
if (this.status === STATUS_PENDING) {
this.status = STATUS_REJECTED
this.reason = reason
}
}
// 7.
try {
executor(resolve, reject);
} catch (err) {
reject(err);
}
}
// 8.
then(onFulfilled = () => {}, onRejected = () => {}) {
// 8.1
if (this.status === STATUS_FULFILLED) {
onFulfilled(this.value)
}
// 8.2
if (this.status === STATUS_REJECTED) {
onRejected(this.reason)
}
}
}
new myPromise(resolve => {
console.log('before resolve')
resolve(1)
}).then(res => {
console.log(res)
})
new myPromise((resolve, reject) => {
console.log('before reject')
reject('reject error')
}).then(res => {
console.log(res)
}, error => {
console.log(error)
})3. 异步Promise
new myPromise(resolve => {
console.log('before resolve')
setTimeout(()=>{
resolve(1)
},1000)
}).then(res => {
console.log(res)
})promise的状态只能在resolve或者reject的时候改变，同步代码执行到then回调的时候promise的状态还是pending，明细不符合我们的期望。
如果换做是你，你会怎么解决这个问题？举个栗子，你正在处理一堆事情（pending状态），然后（then），老板曰: 一秒内做完手上的事来一下我办公室，做不完滚蛋。你怕忘记，一般会用清单记录onResolvedCallbacks = ['做完了手上的事,去老板办公室']，onRejectedCallbacks = ['做不完,滚蛋']，1秒后看完成结果，再依据选择下一步。
完善后的代码如下：参考 前端进阶面试题详细解答
const STATUS_PENDING = 'pending'
const STATUS_FULFILLED = 'fulfilled'
const STATUS_REJECTED = 'rejected'
class myPromise {
constructor(executor) {
this.status = STATUS_PENDING
this.value = ''
this.reason = ''
// 成功存放的数组
this.onResolvedCallbacks = [];
// 失败存放法数组
this.onRejectedCallbacks = [];
let resolve = value => {
if (this.status === STATUS_PENDING) {
this.status = STATUS_FULFILLED
this.value = value
// pending->fulfilled 按照成功清单执行
this.onResolvedCallbacks.forEach(fn => fn())
}
}
let reject = reason => {
if (this.status === STATUS_PENDING) {
this.status = STATUS_REJECTED
this.reason = reason
// pending->rejected 按照异常清单执行
this.onRejectedCallbacks.forEach(fn => fn());
}
}
try {
executor(resolve, reject);
} catch (err) {
reject(err);
}
}
then(onFulfilled = () => {}, onRejected = () => {}) {
if (this.status === STATUS_FULFILLED) {
onFulfilled(this.value)
}
if (this.status === STATUS_REJECTED) {
onRejected(this.reason)
}
// 忙碌状态,先记录老板吩咐的内容
if (this.status === STATUS_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))
}
}
}
// 异步
new myPromise((resolve, reject) => {
console.log('老板曰: 一秒做完手上的事来一下我办公室,做不完滚蛋')
setTimeout(() => {
if (false) { // 臣妾做不到啊
resolve('做完了手上的事,去老板办公室')
} else {
reject('做不完,滚蛋')
}
}, 1000)
}).then(res => {
console.log(`1s 后:${res}`)
}, error => {
console.log(`1s 后:${error}`)
})
// 老板曰: 一秒做完手上的事来一下我办公室,做不完滚蛋
// 1s 后:做不完,滚蛋4. new Promise().then().then()...
这个思路倒是挺简单，就是then函数返回值为另一个Promise实例。根据规范修改后如下：
const PENDING = 'pending'
const FULFILLED = 'fulfilled'
const REJECTED = 'rejected'
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);
}
}
class Promise {
constructor(executor) {
this.state = PENDING
this.value = ''
this.reason = ''
// 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
// pending->fulfilled 按照成功清单执行
this.onResolvedCallbacks.forEach(fn => fn())
}
}
let reject = reason => {
if (this.state === PENDING) {
this.state = REJECTED
this.reason = reason
// pending->rejected 按照异常清单执行
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，扔出错误
onRejected = typeof onRejected === 'function' ? onRejected : err => {
throw err
};
let promise2 = new Promise((resolve, reject) => {
if (this.state === FULFILLED) {
// 异步解决：
// 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，
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;
}
}
new Promise(resolve => {
console.log(0)
setTimeout(() => resolve(1), 3000)
})
.then(res => {
console.log(res)
return new Promise(resolve => {
console.log(2)
setTimeout(() => {
resolve(3)
}, 3000)
})
})
.then(res => {
console.log(res)
})5. catch、resolve、reject、race和all
1. catch(特殊的then方法)
catch(fn){
return this.then(null,fn)
}2. resolve(resolve一个值)
Promise.resolve = val => new Promise(resolve=> resolve(val))3. reject(reject一个值)
Promise.reject = val => new Promise((resolve,reject)=> reject(val))4. race
Promise.race([p1, p2, p3])里面哪个结果获得的快，就返回那个结果，不管结果本身是成功状态还是失败状态 。
Promise.race = promises =>
new Promise((resolve, reject) =>
promises.forEach(pro => pro.then(resolve, reject))
)5. all
Promise.all可以将多个Promise实例包装成一个新的Promise实例。同时，成功和失败的返回值是不同的，成功的时候返回的是一个结果数组，而失败的时候则返回最先被reject失败状态的值。
Promise.all = function (promises) {
return new Promise((resolve, reject) => {
let index = 0;
let result = [];
if (promises.length === 0) {
resolve(result);
} else {
function processValue(i, data) {
result[i] = data;
if (++index === promises.length) {
resolve(result);
}
}
for (let i = 0; i < promises.length; i++) {
//promises[i] 可能是普通值
Promise.resolve(promises[i]).then((data) => {
processValue(i, data);
}, (err) => {
reject(err);
return;
});
}
}
});
}本文旨在记录学习过程，通过手打代码加深印象