Promise源码实现
Promise使用
let promise = new Promise((resolve, reject) => {
setTimeout(() => {
resolve('promise')
}, 1000);
})
promise.then((data) => {
console.log(data)
}, (err) => {
console.log(err)
})
Promise类实现实现
class Promise {
constructor(executor) {
// 默认状态是等待态
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 = (data) => {
if (this.status === 'pending') {
this.value = data;
this.status = 'resolved';
this.onResolvedCallbacks.forEach(fn => fn());
}
}
let reject = (reason) => {
if (this.status === 'pending') {
this.reason = reason;
this.status = 'rejected';
this.onRejectedCallbacks.forEach(fn => fn());
}
}
try { // 执行时可能会发生异常
executor(resolve, reject);
} catch (e) {
reject(e); // promise失败了
}
}
}
Promise原型then方法实现
class Promise {
constructor(executor) {
...
}
then(onFulFilled, onRejected) {
if (this.status === 'resolved') {
onFulFilled(this.value);
}
if (this.status === 'rejected') {
onRejected(this.reason);
}
// 当前既没有完成 也没有失败
if (this.status === 'pending') {
// 存放成功的回调
this.onResolvedCallbacks.push(() => {
onFulFilled(this.value);
});
// 存放失败的回调
this.onRejectedCallbacks.push(() => {
onRejected(this.reason);
});
}
}
}
- then方法可以返回值或者Promise, resolvePromise方法处理
function resolvePromise(promise2, x, resolve, reject) {
// 判断x是不是promise
// 规范里规定了一段代码，这个代码可以实现我们的promise和别人的promise可以进行交互
if (promise2 === x) { // 不能自己等待自己完成
return reject(new TypeError('循环引用'));
}
// x不是null或者是对象或者函数
if (x !== null && (typeof x === 'object' || typeof x === 'function')) {
let called; // 防止成功后调用失败
try { // 防止取then是出现异常 Object.defineProperty
let then = x.then; // 取x的then方法 {then:{}}
if (typeof then === 'function') { // 如果then是函数我就认为它是promise
// call 第一个参数是this ，后面的是成功的回调和失败的回调
then.call(x, y => { // 如果y是promise就继续递归解析promise
if(called) return;
called = true;
resolvePromise(promise2,y,resolve,reject);
}, r => { // 只要失败了就失败了
if (called) return;
called = true;
reject(r);
});
}else{ // then是一个普通对象，就直接成功即可1
resolve(x);
}
} catch (e) {
if (called) return;
called = true;
reject(e);
}
} else { // x = 123
resolve(x); // x就是一个普通值
}
}
class Promise {
constructor(executor) {
...
}
then(onFulFilled, onRejected) {
let promise2;
if (this.status === 'resolved') {
promise2 = new Promise((resolve, reject) => {
// 成功的逻辑 失败的逻辑
let x = onFulFilled(this.value);
// 看x是不是promise 如果是promise 取他的结果 作为promise2,成功的结果
// 如果要是返回一个普通值 作为promise2,成功的结果
// resolvePromise可以解析x和promise2之间的关系
resolvePromise(promise2, x, resolve, reject);
});
}
if (this.status === 'rejected') {
promise2 = new Promise((resolve, reject) => {
let x = onRejected(this.reason);
resolvePromise(promise2, x, resolve, reject)
});
}
// 当前既没有完成 也没有失败
if (this.status === 'pending') {
// 存放成功的回调
promise2 = new Promise((resolve, reject) => {
this.onResolvedCallbacks.push(() => {
let x = onFulFilled(this.value);
resolvePromise(promise2, x, resolve, reject)
});
// 存放失败的回调
this.onRejectedCallbacks.push(() => {
let x = onRejected(this.reason);
resolvePromise(promise2, x, resolve, reject);
});
})
}
return promise2; // 调用then后返回一个新的promise
}
}
- then方法穿透的处理
class Promise {
constructor(executor) {
...
}
then(onFulFilled, onRejected) {
onFulFilled = typeof onFulFilled === 'function' ? onFulFilled : y => y;
onRejected = typeof onRejected === 'function' ? onRejected : err => { throw err; };
let promise2;
if (this.status === 'resolved') {
promise2 = new Promise((resolve, reject) => {
setTimeout(() => {
try {
let x = onFulFilled(this.value);
resolvePromise(promise2, x, resolve, reject);
} catch (e) {
reject(e);
}
}, 0);
});
}
if (this.status === 'rejected') {
promise2 = new Promise((resolve, reject) => {
setTimeout(() => {
try {
let x = onRejected(this.reason);
resolvePromise(promise2, x, resolve, reject)
} catch (e) {
reject(e);
}
}, 0);
});
}
if (this.status === 'pending') {
promise2 = new Promise((resolve, reject) => {
this.onResolvedCallbacks.push(() => {
setTimeout(() => {
try {
let x = onFulFilled(this.value);
resolvePromise(promise2, x, resolve, reject)
} catch (e) {
reject(e);
}
}, 0)
});
// 存放失败的回调
this.onRejectedCallbacks.push(() => {
setTimeout(() => {
try {
let x = onRejected(this.reason);
resolvePromise(promise2, x, resolve, reject);
} catch (e) {
reject(e);
}
}, 0);
});
})
}
return promise2; // 调用then后返回一个新的promise
}
// catch接收的参数 只用错误
catch(onRejected) {
// catch就是then的没有成功的简写
return this.then(null, onRejected);
}
}
Promose静态方法resolve, reject, all, race实现
Promise.resolve = function (val) {
return new Promise((resolve, reject) => resolve(val))
}
Promise.reject = function (val) {
return new Promise((resolve, reject) => reject(val));
}
Promise.race = function (promises) {
return new Promise((resolve, reject) => {
for (let i = 0; i < promises.length; i++) {
promises[i].then(resolve, reject);
}
});
}
Promise.all = function (promises) {
return new Promise((resolve,reject)=>{
let arr = [];
let i = 0; // i的目的是为了保证获取全部成功，来设置的索引
function processData(index,data) {
arr[index] = data;
i++;
if (i === promises.length){
resolve(arr);
}
}
for(let i = 0;i<promises.length;i++){
promises[i].then(data=>{
processData(i,data);
}, reject);
}
})
}
Promise源码实现大概就这些内容，作为学习的一个记录，希望对大家有所帮助
