准备
完善 resolve/reject
then
异步处理
链式调用
边界处理
catch
优化后完整代码
准备首先，promise 有三种状态：pending fulfilled rejected;
promise在实例化操作中， 有两个改变状态的方法，分别为resolve,reject;
promise有很多方法，详情请见 mdn, 本篇文章先实现 promise的核心api: then和catch;
我们使用 es6 提供的 class 来实现
class MyPromise {
// 准备三个状态
static PENDING = 'pending';
static FULFILLED = 'fulfilled';
static REJECTED = 'rejected';
constructor(executor) {
this.status = MyPromise.PENDING; // 表示promise的状态
this.value = null; // 表示promise的值
try {
executor(this.resolve.bind(this), this.reject.bind(this))
} catch (error) {
this.reject(error)
}
}
resolve() {
}
reject() {
}
}
在这里 executor 就是传递过来的函数，可以接收 resolve和reject,这里将内部的两个方法给传入，如果在调用的过程中报错了会调用reject方法
他们做的工作分为以下几部
将状态改为pending为fulfilled 或 rejected
可以接受一个值为当前的promise的value
resolve(value) {
if (this.status === MyPromise.PENDING) {
this.status = MyPromise.FULFILLED;
this.value = value
}
}
reject(value) {
if (this.status === MyPromise.PENDING) {
this.status = MyPromise.REJECTED;
this.value = value
}
}
then
then 函数可以接受两个参数，分别为成功的回调函数和失败的回调函数,并且回调函数的默认为一个函数
状态为 fulfilled 执行第一个回调，rejected 执行第二个回调
回调函数中给传入当前的 value
then的执行为异步的
then(onFulfilled, onRejected) {
if (typeof onFulfilled !== 'function') {
onFulfilled = value => value
}
if (typeof onFulfilled !== 'function') {
onRejected = value => value
}
if (this.status === MyPromise.FULFILLED) {
setTimeout(() => {
onFulfilled(this.value)
})
}
if (this.status === MyPromise.REJECTED) {
setTimeout(() => {
onRejected(this.value)
})
}
}
验证一下：
console.log(1)
new MyPromise((resolve, reject) => {
console.log(2)
resolve('成功')
}).then(res => console.log(res))
console.log(3)
// 打印 1 2 3 成功
当promise里面有异步代码的时候，这个时候运行到.then方法 状态为pending，下来增加一下异步任务的处理
当状态为pending的时候，表示执行的是异步任务，这个时候我们可以增加一个callback，把异步执行的内容添加到这个callback中，当执行完异步代码的时候，会执行异步函数的callback的任务
constructor(executor) {
// ...
this.callbacks = []; // 用来存储回调函数的容器
// ...
}
resolve(value) {
// ...
this.callbacks.forEach(({ onFulfilled }) => onFulfilled(value))
// 当执行到这里的时候 如果有onFulfilled 就说明已经执行完then方法给容器添加内容了。把resolve的值传递给onFulfilled
}
reject(value) {
// ...
this.callbacks.forEach(({ onRejected }) => onRejected(value))
// 当执行到这里的时候 如果有onRejected 就说明已经执行完then方法给容器添加内容了。把reject的值传递给onFulfilled
}
then(onFulfilled, onRejected) {
// ...
if (this.status === MyPromise.PENDING) {
this.callbacks.push({
onFulfilled: value => {
setTimeout(() => {
onFulfilled(value)
})
},
onRejected: value => {
setTimeout(() => {
onRejected(value)
})
}
})
}
}
验证一下：
new MyPromise((resolve, reject) => {
setTimeout(() => {
resolve('成功')
})
}).then(res => console.log(res))
// 打印 成功
then 函数可以链式调用，接下来我们完善一下
链式调用的核心就是返回一个新的 promise,当成功调用的时候调用新的promise的resolve，失败reject，并且链式调用会把前一个的返回值当作下一个的 resolve 的状态
then(onFulfilled, onRejected) {
if (typeof onFulfilled !== 'function') {
onFulfilled = value => value
}
if (typeof onFulfilled !== 'function') {
onRejected = value => value
}
return new MyPromise((resolve, reject) => {
if (this.status === MyPromise.FULFILLED) {
setTimeout(() => {
const result = onFulfilled(this.value)
resolve(result)
})
}
if (this.status === MyPromise.REJECTED) {
setTimeout(() => {
const result = onRejected(this.value)
resolve(result)
})
}
if (this.status === MyPromise.PENDING) {
this.callbacks.push({
onFulfilled: value => {
setTimeout(() => {
const result = onFulfilled(value)
resolve(result)
})
},
onRejected: value => {
setTimeout(() => {
const result = onRejected(value)
resolve(result)
})
}
})
}
})
}
验证一下：
new MyPromise((resolve, reject) => {
setTimeout(() => {
reject('失败')
})
}).then(res => res, err => err).then(res => console.log(res))
// 打印 失败
如果.then的回调函数返回的是promise的情况也要做个处理
实现前：
new MyPromise((resolve, reject) => {
setTimeout(() => {
resolve('成功')
})
}).then(
res => new MyPromise((resolve, reject) => {
resolve(res)
}),
err => err
).then(res => console.log(res))
// 打印 { "status": "fulfilled", "value": "成功", "callbacks": [] }
当判断返回值为 MyPromise 的时候，需要手动调用 .then 的方法取他的值，并且吧当前的 promise 的改变状态的函数透出给 then 方法
then(onFulfilled, onRejected) {
if (typeof onFulfilled !== 'function') {
onFulfilled = value => value
}
if (typeof onFulfilled !== 'function') {
onRejected = value => value
}
return new MyPromise((resolve, reject) => {
if (this.status === MyPromise.FULFILLED) {
setTimeout(() => {
const result = onFulfilled(this.value)
if (result instanceof MyPromise) {
result.then(resolve, reject)
} else {
resolve(result)
}
})
}
if (this.status === MyPromise.REJECTED) {
setTimeout(() => {
const result = onRejected(this.value)
if (result instanceof MyPromise) {
result.then(resolve, reject)
} else {
resolve(result)
}
})
}
if (this.status === MyPromise.PENDING) {
this.callbacks.push({
onFulfilled: value => {
setTimeout(() => {
const result = onFulfilled(value)
if (result instanceof MyPromise) {
result.then(resolve, reject)
} else {
resolve(result)
}
})
},
onRejected: value => {
setTimeout(() => {
const result = onRejected(value)
if (result instanceof MyPromise) {
result.then(resolve, reject)
} else {
resolve(result)
}
})
}
})
}
})
}
验证：
new MyPromise((resolve, reject) => {
setTimeout(() => {
resolve('成功')
})
}).then(
res => new MyPromise((resolve, reject) => {
resolve(res)
}),
err => err
).then(res => console.log(res))
// 打印 成功
到这里 .then 方法就实现差不多了，接下来实现 catch 方法
catch 方法可以处理拒绝的状态和错误的状态：
catch(onFulfilled) {
if (typeof onFulfilled !== 'function') {
onFulfilled = value => value
}
return new MyPromise((resolve, reject) => {
if (this.status === MyPromise.REJECTED) {
setTimeout(() => {
const result = onFulfilled(this.value)
if (result instanceof MyPromise) {
result.then(resolve, reject)
} else {
resolve(result)
}
})
}
})
}
验证：
new MyPromise((resolve, reject) => {
reject('失败')
}).catch(res=> console.log(res))
// 打印 失败
道理其实和 then 是相同的，到这里主功能基本上就差不多了，但是有很多重复的地方，优化一下
class MyPromise {
// 准备三个状态
static PENDING = 'pending';
static FULFILLED = 'fulfilled';
static REJECTED = 'rejected';
constructor(executor) {
this.status = MyPromise.PENDING; // 表示promise的状态
this.value = null; // 表示promise的值
this.callbacks = [];
try {
executor(this.resolve.bind(this), this.reject.bind(this))
} catch (error) {
console.log(error)
this.reject(error)
}
}
resolve(value) {
if (this.status === MyPromise.PENDING) {
this.status = MyPromise.FULFILLED;
this.value = value
}
this.callbacks.forEach(({ onFulfilled }) => onFulfilled(value))
}
reject(value) {
if (this.status === MyPromise.PENDING) {
this.status = MyPromise.REJECTED;
this.value = value
}
this.callbacks.forEach(({ onRejected }) => onRejected(value))
}
parse({ callback, resolve, reject, value = this.value }) {
setTimeout(() => {
const result = callback(value)
if (result instanceof MyPromise) {
result.then(resolve, reject)
} else {
resolve(result)
}
})
}
then(onFulfilled, onRejected) {
if (typeof onFulfilled !== 'function') {
onFulfilled = value => value
}
if (typeof onFulfilled !== 'function') {
onRejected = value => value
}
return new MyPromise((resolve, reject) => {
if (this.status === MyPromise.FULFILLED) {
this.parse({ callback: onFulfilled, resolve, reject })
}
if (this.status === MyPromise.REJECTED) {
this.parse({ callback: onRejected, resolve, reject })
}
if (this.status === MyPromise.PENDING) {
this.callbacks.push({
onFulfilled: value => {
this.parse({ callback: onFulfilled, resolve, reject, value })
},
onRejected: value => {
this.parse({ callback: onRejected, resolve, reject, value })
}
})
}
})
}
catch(onFulfilled) {
if (typeof onFulfilled !== 'function') {
onFulfilled = value => value
}
return new MyPromise((resolve, reject) => {
if (this.status === MyPromise.REJECTED) {
this.parse({ callback: onFulfilled, resolve, reject })
}
})
}
}
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