Promise标准了解一下
传送门1⃣️ 👉Promises/A+规范
传送门2⃣️ 👉阮一峰前辈ECMAScript 6入门——Promise对象
Promise重点难点
Promise的状态不受外界影响，只由Promise内的异步操作结果决定。
Promise的状态一旦改变就不会再变。
- pending 👉 fulfilled
- pending 👉 rejected
Promise的构造函数是同步的，then()方法中的函数是异步的。
then()方法或者catch()方法的参数期望是函数，传入非函数则会发生值穿透
Promise属于microtask，同一次事件循环中，microtask永远在macrotask之前执行。
手写promise
- 实现基本状态（pending, fulfilled, rejected）
- 实现then方法
- 返回一个promise，实现链式调用
- 实现状态判断
- 实现resolve函数
- 接受一个Promise作为参数时的情况，实现链式Promise
- 执行回调函数时使用setTimeout，保证在注册then方法后触发
- 实现Promise.all,Promise.race方法
(function (window) {
var Promise = function (fn) {
var value = null;
var callbacks = [];
var state = 'pending'; // pending, fulfilled, rejected
var promise = this;
// 注册then事件，供resolve后调用
promise.then = function (onFulfilled, onRejected) {
// 返回promise实现链式promise调用
return new Promise(function (resolve, reject) {
handle({
onFulfilled: onFulfilled || null,
onRejected: onRejected || null,
resolve: resolve,
reject: reject
})
})
}
promise.catch = function(onRejected) {
return promise.then(undefined, onRejected)
}
function handle (callback) {
// 状态变化前，事件推进队列里；状态一旦变化后不再变动，直接执行结果
if (state === 'pending') {
callbacks.push(callback);
} else {
var cb = state === 'fulfilled' ? callback.onFulfilled : callback.onRejected
// then方法没有传递任何参数的情况下，返回结果值
if (!cb) {
cb = state === 'fulfilled' ? callback.resolve : callback.reject;
cb(value)
} else {
try {
var ret = cb(value);
callback.resolve(ret);
} catch (e) {
callback.reject(e)
}
}
}
}
function resolve(newValue) {
// 状态一旦改变便不再改变
if (state !== 'pending') return
// 假如resolve了一个promise的话（链式promise）
if (newValue && (typeof newValue === 'object' || typeof newValue === 'function')) {
var then = newValue.then;
if (typeof then === 'function') {
// 调用第二个promise中的then，递归直到不是一个promise值为止
then.call(newValue, resolve, reject);
return
}
}
value = newValue;
state = 'fulfilled';
execute()
}
function reject(reason) {
// 状态一旦改变便不再改变
if (state !== 'pending') return
state = 'rejected'
value = reason
execute()
}
function execute() {
// 使用setTimeOut保证resolve一定在then事件注册后执行
setTimeout(() => {
callbacks.forEach(function (callback) {
handle(callback);
})
}, 0);
}
fn(resolve, reject);
}
Promise.all = function (promises) {
if (!Array.isArray(promises)) {
throw new TypeError('请传入promise数组')
}
return new Promise(function (resolve, reject) {
var result = [];
var count = promises.length
function reslover (index) {
return function(value) {
resloveAll(index, value)
}
}
function rejecter (reason) {
reject(reason)
}
function resloveAll (index, value) {
result[index] = value
// 等待全部promise执行完才执行resolve一个数组
if (--count === 0) {
resolve(result)
}
}
promises.forEach(function (promise, index) {
promise.then(reslover(index), rejecter)
})
})
}
Promise.race = function (promises) {
if (!Array.isArray(promises)) {
throw new TypeError('请传入promise数组')
}
return new Promise(function (resolve, reject) {
function reslover (value) {
resolve(value)
}
function rejecter (reason) {
reject(reason)
}
promises.forEach(function (promise, index) {
promise.then(reslover, rejecter)
})
})
}
window.Promise = Promise
})(window)
/******************************************实例 */
function test(i) {
return new Promise(function (resolve) {
setTimeout(() => {
resolve(i);
}, 1000);
})
}
function test2(i) {
return new Promise(function (resolve, reject) {
setTimeout(() => {
if (i % 2) {
resolve(i);
} else {
reject(i);
}
}, 2000);
})
}
// 链式Promise
test(1).then(test2).then(function (something) {
console.log('case1: success!' + something);
}).catch(function (something) {
console.log('case1: failed!' + something);
})
// Promise.all
Promise.all([test(2), test2(4)]).then(function (something) {
console.log('case2: success!' + something);
}).catch(function (something) {
console.log('case2: failed!' + something)
})
// Promise.race
Promise.race([test(3), test2(5)]).then(function (something) {
console.log('case3: success!' + something);
}).catch(function (something) {
console.log('case3: failed!' + something)
})
// 多次改变状态，只有第一次生效
function test3 () {
return new Promise((resolve, reject) => {
setTimeout(() => {
resolve('success1')
reject('error')
resolve('success2')
},0)
})
}
test3().then((res) => {
console.log('then: ', res)
})
// 一旦改变状态后，多次调用then()方法也只会马上得到Promise返回的值，Promise构造函数不会多次执行
function test4 () {
return new Promise((resolve, reject) => {
setTimeout(() => {
resolve('success')
}, 1000)
})
}
const start = Date.now()
test4().then((res) => {
console.log(res, Date.now() - start)
})
test4().then((res) => {
console.log(res, Date.now() - start)
})
