背景
promise 作为前端开发中常用的函数,解决了 js 处理异步时回调地狱的问题,大家应该也不陌生了,今天来学习一下 promise 的实现过程,这样可以加(面)深(试)理(要)解(考)。
需求
我们先来总结一下 promise 的特性:
使用:
const p1 = new Promise((resolve, reject) => {
console.log('1');
resolve('成功了');
})
console.log("2");
const p2 = p1.then(data => {
console.log('3')
throw new Error('失败了')
})
const p3 = p2.then(data => {
console.log('success', data)
}, err => {
console.log('4', err)
})
以上的示例可以看出 promise 的一些特点,也就是我们本次要做的事情:
- 在调用 Promise 时,会返回一个 Promise 对象,包含了一些熟悉和方法(all/resolve/reject/then…)
- 构建 Promise 对象时,需要传入一个 executor 函数,接受两个参数,分别是resolve和reject,Promise 的主要业务流程都在 executor 函数中执行。
- 如果运行在 excutor 函数中的业务执行成功了,会调用 resolve 函数;如果执行失败了,则调用 reject 函数。
- promise 有三个状态:pending,fulfilled,rejected,默认是 pending。只能从pending到rejected, 或者从pending到fulfilled,状态一旦确认,就不会再改变;
- promise 有一个then方法,接收两个参数,分别是成功的回调 onFulfilled, 和失败的回调 onRejected。
// 三个状态:PENDING、FULFILLED、REJECTED
const PENDING = "PENDING";
const FULFILLED = "FULFILLED";
const REJECTED = "REJECTED";
class Promise {
constructor(executor) {
// 默认状态为 PENDING
this.status = PENDING;
// 存放成功状态的值,默认为 undefined
this.value = undefined;
// 存放失败状态的值,默认为 undefined
this.reason = undefined;
// 调用此方法就是成功
let resolve = (value) => {
// 状态为 PENDING 时才可以更新状态,防止 executor 中调用了两次 resovle/reject 方法
if (this.status === PENDING) {
this.status = FULFILLED;
this.value = value;
}
};
// 调用此方法就是失败
let reject = (reason) => {
// 状态为 PENDING 时才可以更新状态,防止 executor 中调用了两次 resovle/reject 方法
if (this.status === PENDING) {
this.status = REJECTED;
this.reason = reason;
}
};
try {
// 立即执行,将 resolve 和 reject 函数传给使用者
executor(resolve, reject);
} catch (error) {
// 发生异常时执行失败逻辑
reject(error);
}
}
// 包含一个 then 方法,并接收两个参数 onFulfilled、onRejected
then(onFulfilled, onRejected) {
if (this.status === FULFILLED) {
onFulfilled(this.value);
}
if (this.status === REJECTED) {
onRejected(this.reason);
}
}
}
调用一下:
const promise = new Promise((resolve, reject) => {
resolve('成功');
}).then(
(data) => {
console.log('success', data)
},
(err) => {
console.log('faild', err)
}
)
这个时候我们很开心,但是别开心的太早,promise 是为了处理异步任务的,我们来试试异步任务好不好使:
const promise = new Promise((resolve, reject) => {
// 传入一个异步操作
setTimeout(() => {
resolve('成功');
},1000);
}).then(
(data) => {
console.log('success', data)
},
(err) => {
console.log('faild', err)
}
)
发现没有任何输出,我们来分析一下为什么:
new Promise 执行的时候,这时候异步任务开始了,接下来直接执行 .then 函数,then函数里的状态目前还是 padding,所以就什么也没执行。
那么我们想想应该怎么改呢?
我们想要做的就是,当异步函数执行完后,也就是触发 resolve 的时候,再去触发 .then 函数执行并把 resolve 的参数传给 then。
这就是典型的发布订阅的模式,可以看我这篇文章。
const PENDING = 'PENDING';
const FULFILLED = 'FULFILLED';
const REJECTED = 'REJECTED';
class Promise {
constructor(executor) {
this.status = PENDING;
this.value = undefined;
this.reason = undefined;
// 存放成功的回调
this.onResolvedCallbacks = [];
// 存放失败的回调
this.onRejectedCallbacks= [];
let resolve = (value) => {
if(this.status === PENDING) {
this.status = FULFILLED;
this.value = value;
// 依次将对应的函数执行
this.onResolvedCallbacks.forEach(fn=>fn());
}
}
let reject = (reason) => {
if(this.status === PENDING) {
this.status = REJECTED;
this.reason = reason;
// 依次将对应的函数执行
this.onRejectedCallbacks.forEach(fn=>fn());
}
}
try {
executor(resolve,reject)
} catch (error) {
reject(error)
}
}
then(onFulfilled, onRejected) {
if (this.status === FULFILLED) {
onFulfilled(this.value)
}
if (this.status === REJECTED) {
onRejected(this.reason)
}
if (this.status === PENDING) {
// 如果promise的状态是 pending,需要将 onFulfilled 和 onRejected 函数存放起来,等待状态确定后,再依次将对应的函数执行
this.onResolvedCallbacks.push(() => {
onFulfilled(this.value)
});
// 如果promise的状态是 pending,需要将 onFulfilled 和 onRejected 函数存放起来,等待状态确定后,再依次将对应的函数执行
this.onRejectedCallbacks.push(()=> {
onRejected(this.reason);
})
}
}
}
这下再进行测试:
const promise = new Promise((resolve, reject) => {
setTimeout(() => {
resolve('成功');
},1000);
}).then(
(data) => {
console.log('success', data)
},
(err) => {
console.log('faild', err)
}
)
1s 后输出了:
说明成功啦
then的链式调用
这里就不分析细节了,大体思路就是每次 .then 的时候重新创建一个 promise 对象并返回 promise,这样下一个 then 就能拿到前一个 then 返回的 promise 了。
const PENDING = 'PENDING';
const FULFILLED = 'FULFILLED';
const REJECTED = 'REJECTED';
const resolvePromise = (promise2, x, resolve, reject) => {
// 自己等待自己完成是错误的实现,用一个类型错误,结束掉 promise Promise/A+ 2.3.1
if (promise2 === x) {
return reject(new TypeError('Chaining cycle detected for promise #<Promise>'))
}
// Promise/A+ 2.3.3.3.3 只能调用一次
let called;
// 后续的条件要严格判断 保证代码能和别的库一起使用
if ((typeof x === 'object' && x != null) || typeof x === 'function') {
try {
// 为了判断 resolve 过的就不用再 reject 了(比如 reject 和 resolve 同时调用的时候) Promise/A+ 2.3.3.1
let then = x.then;
if (typeof then === 'function') {
// 不要写成 x.then,直接 then.call 就可以了 因为 x.then 会再次取值,Object.defineProperty Promise/A+ 2.3.3.3
then.call(x, y => { // 根据 promise 的状态决定是成功还是失败
if (called) return;
called = true;
// 递归解析的过程(因为可能 promise 中还有 promise) Promise/A+ 2.3.3.3.1
resolvePromise(promise2, y, resolve, reject);
}, r => {
// 只要失败就失败 Promise/A+ 2.3.3.3.2
if (called) return;
called = true;
reject(r);
});
} else {
// 如果 x.then 是个普通值就直接返回 resolve 作为结果 Promise/A+ 2.3.3.4
resolve(x);
}
} catch (e) {
// Promise/A+ 2.3.3.2
if (called) return;
called = true;
reject(e)
}
} else {
// 如果 x 是个普通值就直接返回 resolve 作为结果 Promise/A+ 2.3.4
resolve(x)
}
}
class Promise {
constructor(executor) {
this.status = PENDING;
this.value = undefined;
this.reason = undefined;
this.onResolvedCallbacks = [];
this.onRejectedCallbacks= [];
let resolve = (value) => {
if(this.status === PENDING) {
this.status = FULFILLED;
this.value = value;
this.onResolvedCallbacks.forEach(fn=>fn());
}
}
let reject = (reason) => {
if(this.status === PENDING) {
this.status = REJECTED;
this.reason = reason;
this.onRejectedCallbacks.forEach(fn=>fn());
}
}
try {
executor(resolve,reject)
} catch (error) {
reject(error)
}
}
then(onFulfilled, onRejected) {
//解决 onFufilled,onRejected 没有传值的问题
//Promise/A+ 2.2.1 / Promise/A+ 2.2.5 / Promise/A+ 2.2.7.3 / Promise/A+ 2.2.7.4
onFulfilled = typeof onFulfilled === 'function' ? onFulfilled : v => v;
//因为错误的值要让后面访问到,所以这里也要跑出个错误,不然会在之后 then 的 resolve 中捕获
onRejected = typeof onRejected === 'function' ? onRejected : err => { throw err };
// 每次调用 then 都返回一个新的 promise Promise/A+ 2.2.7
let promise2 = new Promise((resolve, reject) => {
if (this.status === FULFILLED) {
//Promise/A+ 2.2.2
//Promise/A+ 2.2.4 --- setTimeout
setTimeout(() => {
try {
//Promise/A+ 2.2.7.1
let x = onFulfilled(this.value);
// x可能是一个proimise
resolvePromise(promise2, x, resolve, reject);
} catch (e) {
//Promise/A+ 2.2.7.2
reject(e)
}
}, 0);
}
if (this.status === REJECTED) {
//Promise/A+ 2.2.3
setTimeout(() => {
try {
let x = onRejected(this.reason);
resolvePromise(promise2, x, resolve, reject);
} catch (e) {
reject(e)
}
}, 0);
}
if (this.status === 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;
}
}
Promise.all
核心就是有一个失败则失败,全成功才进行 resolve
Promise.all = function(values) {
if (!Array.isArray(values)) {
const type = typeof values;
return new TypeError(`TypeError: ${type} ${values} is not iterable`)
}
return new Promise((resolve, reject) => {
let resultArr = [];
let orderIndex = 0;
const processResultByKey = (value, index) => {
resultArr[index] = value;
if (++orderIndex === values.length) {
resolve(resultArr)
}
}
for (let i = 0; i < values.length; i++) {
let value = values[i];
if (value && typeof value.then === 'function') {
value.then((value) => {
processResultByKey(value, i);
}, reject);
} else {
processResultByKey(value, i);
}
}
});
}
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