Promise原理与实现
Promise标准
Promise规范有很多，如Promise/A，Promise/B，Promise/D以及Promise/A
的升级版Promise/A+。ES6中采用了Promise/A+规范。 英文版规范:
Promises/A+规范 中文版规范:
Promises/A+规范(中文)Promise标准解读
(1)一个promise的当前状态只能是pending、fulfilled和rejected三种之一。状态改变只能是pending到fulfilled或者pending到rejected。状态改变不可逆。
(2)promise的then方法接收两个可选参数，表示该promise状态改变时的回调(promise.then(onFulfilled,onRejected))。then方法返回一个promise。then方法可以被同一个promise调用多次。
实现Promise
Promise标准仅描述了then方法的行为，未对catch、all、race方法进行描述，也未规范如何创建一个Promise对象。ES6中Promise提供了以下API。
(1) 构造函数
function Promise(resolver) {}
(2) 原型方法
Promise.prototype.then = function() {}
Promise.prototype.catch = function() {}
(3) 静态方法
Promise.resolve = function() {}
Promise.reject = function() {}
Promise.all = function() {}
Promise.race = function() {}
构造函数
Promise/A+标准并没有指定如何构造一个Promise对象，我们以ES6原生Promise模块里通过构造函数创建Promise对象的方式实现Promise构造函数。ES6中通过构造函数创建Promise对象的简单用法如下：
const promise = new Promise((resolve) => {
setTimeout(()=> {
resolve(1);
}, 2000);
});
promise.then(a=> alert(a));
promise.then(a => alert(a+1));
构造函数用法总结：
(1) 构造函数接收一个executor立即执行函数
(2) executor立即执行函数接收一个resolve函数
(3) promise对象的then方法绑定状态变为fulfilled时的回调
(4) resolve函数被调用时会触发then方法中的回调
- 构造函数的简单实现
function Promise(executor) {
var self = this;
self.status = 'pending'; //promise当前的状态
self.data = undefined; //promise的值
self.onResolvedCallback = [];
//promise状态变为resolve时的回调函数集，可能有多个
function resolve(value) {
if(self.status === 'pending') {
self.status = 'resolved';
self.data = value;
for(var i = 0; i < self.onResolvedCallback.length; i++) {
self.onResolvedCallback[i](value);
}
}
}
executor(resolve);
};
Promise.prototype.then = function (resolve) {
this.onResolvedCallback.push(resolve);
};
executor自执行函数接收的第二个参数为reject函数且reject函数在promise对象状态变为rejected时或executor抛出异常时触发。
function Promise(executor) {
var self = this;
self.status = 'pending'; //promise当前的状态
self.data = undefined; //promise的值
self.onResolvedCallback = [];
//promise状态变为resolve时的回调函数集，可能有多个
self.onRejectedCallback = [];
//promise状态变为reject时的回调函数集，可能有多个
function resolve(value) {
if(self.status === 'pending') {
self.status = 'resolved';
self.data = value;
for(var i = 0; i < self.onResolvedCallback.length; i++) {
self.onResolvedCallback[i](value);
}
}
}
function reject(reason) {
if(self.status === 'pending') {
self.status = 'rejected';
self.data = reason;
for(var i = 0; i < self.onRejectedCallback.length; i++) {
self.onRejectedCallback[i](reason);
}
}
}
try {
executor(resolve, reject);
} catch (e){
reject(e);
}
};
Promise.prototype.then = function (onResolve, onReject) {
this.onResolvedCallback.push(onResolve);
this.onRejectedCallback.push(onReject);
};
总结：①executor函数作为实参在创建Promise对象时传入Promise构造函数。②resolve和reject函数作为实参传入executor函数。③value作为实参传入resolve和reject函数。
如果executor自执行函数中的resolve函数立即触发时，发现Promise失效，例如：
const promise = new Promise((resolve) => {
resolve(1);
});
promise.then((a) => alert(a));
解决办法：需要将promise的resolve和reject异步执行。
function resolve(value) {
setTimeout(function () {
if(self.status === 'pending') {
self.status = 'resolved';
self.data = value;
for(var i = 0; i < self.onResolvedCallback.length; i++) {
self.onResolvedCallback[i](value);
}
}
})
}
function reject(reason) {
setTimeout(function () {
if(self.status === 'pending') {
self.status = 'rejected';
self.data = reason;
for(var i = 0; i < self.onRejectedCallback.length; i++) {
self.onRejectedCallback[i](reason);
}
}
})
}
then方法
- 在构造函数中的示例中，
then方法并没有返回一个promise对象，而Promise/A+
规范中规定then方法用来注册promise对象状态改变时的回调，且返回一个新的promise对象。
(1)then方法返回一个新的promise对象。
(2)executor自执行函数中的resolve参数调用时执行then方法的第一个回调函数onResolved。
(3)executor自执行函数中的reject参数调用时执行then方法的第二个回调函数onRejected。
Promise.prototype.then = function (onResolved, onRejected) {
var self = this;
var promise2;
onResolved = typeof onResolved === 'function'
? onResolved
: function (value) {return value};
onRejected = typeof onRejected === 'function'
? onRejected
: function (reason) {throw reason};
//promise对象当前状态为resolved
if(self.status === 'resolved') {
return promise2 = new Promise(function (resolve, reject) {
try {
//调用onResolve回调函数
var x = onResolved(self.data);
//如果onResolve回调函数返回值为一个promise对象
if(x instanceof Promise) {
//将它的结果作为promise2的结果
x.then(resolve, reject);
} else {
resolve(x);//执行promise2的onResolve回调
}
} catch (e) {
reject(e); //执行promise2的onReject回调
}
})
}
//promise对象当前状态为rejected
if(self.status === 'rejected') {
return promise2 = new Promise(function (resolve, reject) {
try {
var x = onRejected(self.data);
if (x instanceof Promise) {
x.then(resolve, reject)
} else {
resolve(x)
}
} catch (e) {
reject(e)
}
})
}
//promise对象当前状态为pending
//此时并不能确定调用onResolved还是onRejected，需要等当前Promise状态确定。
//所以需要将callBack放入promise1的回调数组中
if(self.status === 'pending') {
return promise2 = new Promise(function (resolve, reject) {
self.onResolvedCallback.push(function (value) {
try {
var x = onResolved(self.data);
if (x instanceof Promise) {
x.then(resolve, reject);
} else {
resolve(x);
}
} catch (e) {
reject(e);
}
})
self.onRejectedCallback.push(function(reason) {
try {
var x = onRejected(self.data);
if (x instanceof Promise) {
x.then(resolve, reject)
} else {
resolve(x);
}
} catch (e) {
reject(e)
}
})
})
}
};
- 参照
Promise/A+标准对promise进行改写
function resolvePromise(promise2, x, resolve, reject) {
var then
var thenCalledOrThrow = false
if (promise2 === x) {
return reject(new TypeError('Chaining cycle detected for promise!'))
}
if (x instanceof Promise) {
if (x.status === 'pending') { //because x could resolved by a Promise Object
x.then(function(v) {
resolvePromise(promise2, v, resolve, reject)
}, reject)
} else { //but if it is resolved, it will never resolved by a Promise Object but a static value;
x.then(resolve, reject)
}
return
}
if ((x !== null) && ((typeof x === 'object') || (typeof x === 'function'))) {
try {
then = x.then //because x.then could be a getter
if (typeof then === 'function') {
then.call(x, function rs(y) {
if (thenCalledOrThrow) return
thenCalledOrThrow = true
return resolvePromise(promise2, y, resolve, reject)
}, function rj(r) {
if (thenCalledOrThrow) return
thenCalledOrThrow = true
return reject(r)
})
} else {
resolve(x)
}
} catch (e) {
if (thenCalledOrThrow) return
thenCalledOrThrow = true
return reject(e)
}
} else {
resolve(x)
}
}
Promise.prototype.then = function(onResolved, onRejected) {
var self = this
var promise2
onResolved = typeof onResolved === 'function' ? onResolved : function(v) {
return v
}
onRejected = typeof onRejected === 'function' ? onRejected : function(r) {
throw r
}
if (self.status === 'resolved') {
return promise2 = new Promise(function(resolve, reject) {
setTimeout(function() { // 异步执行onResolved
try {
var x = onResolved(self.data)
resolvePromise(promise2, x, resolve, reject)
} catch (reason) {
reject(reason)
}
})
})
}
if (self.status === 'rejected') {
return promise2 = new Promise(function(resolve, reject) {
setTimeout(function() { // 异步执行onRejected
try {
var x = onRejected(self.data)
resolvePromise(promise2, x, resolve, reject)
} catch (reason) {
reject(reason)
}
})
})
}
if (self.status === 'pending') {
// 这里之所以没有异步执行，是因为这些函数必然会被resolve或reject调用，而resolve或reject函数里的内容已是异步执行，构造函数里的定义
return promise2 = new Promise(function(resolve, reject) {
self.onResolvedCallback.push(function(value) {
try {
var x = onResolved(value)
resolvePromise(promise2, x, resolve, reject)
} catch (r) {
reject(r)
}
})
self.onRejectedCallback.push(function(reason) {
try {
var x = onRejected(reason)
resolvePromise(promise2, x, resolve, reject)
} catch (r) {
reject(r)
}
})
})
}
}
完整代码
var Promise = (function() {
function Promise(resolver) {
if (typeof resolver !== 'function') { //resolver必须是函数
throw new TypeError('Promise resolver ' + resolver + ' is not a function')
}
if (!(this instanceof Promise)) return new Promise(resolver)
var self = this //保存this
self.callbacks = [] //保存onResolve和onReject函数集合
self.status = 'pending' //当前状态
function resolve(value) {
setTimeout(function() { //异步调用
if (self.status !== 'pending') {
return
}
self.status = 'resolved' //修改状态
self.data = value
for (var i = 0; i < self.callbacks.length; i++) {
self.callbacks[i].onResolved(value)
}
})
}
function reject(reason) {
setTimeout(function(){ //异步调用
if (self.status !== 'pending') {
return
}
self.status = 'rejected' //修改状态
self.data = reason
for (var i = 0; i < self.callbacks.length; i++) {
self.callbacks[i].onRejected(reason)
}
})
}
try{
resolver(resolve, reject) //执行resolver函数
} catch(e) {
reject(e)
}
}
function resolvePromise(promise, x, resolve, reject) {
var then
var thenCalledOrThrow = false
if (promise === x) {
return reject(new TypeError('Chaining cycle detected for promise!'))
}
if ((x !== null) && ((typeof x === 'object') || (typeof x === 'function'))) {
try {
then = x.then
if (typeof then === 'function') {
then.call(x, function rs(y) {
if (thenCalledOrThrow) return
thenCalledOrThrow = true
return resolvePromise(promise, y, resolve, reject)
}, function rj(r) {
if (thenCalledOrThrow) return
thenCalledOrThrow = true
return reject(r)
})
} else {
return resolve(x)
}
} catch(e) {
if (thenCalledOrThrow) return
thenCalledOrThrow = true
return reject(e)
}
} else {
return resolve(x)
}
}
Promise.prototype.then = function(onResolved, onRejected) {
//健壮性处理，处理点击穿透
onResolved = typeof onResolved === 'function' ? onResolved : function(v){return v}
onRejected = typeof onRejected === 'function' ? onRejected : function(r){throw r}
var self = this
var promise2
//promise状态为resolved
if (self.status === 'resolved') {
return promise2 = new Promise(function(resolve, reject) {
setTimeout(function() {
try {
//调用then方法的onResolved回调
var x = onResolved(self.data)
//根据x的值修改promise2的状态
resolvePromise(promise2, x, resolve, reject)
} catch(e) {
//promise2状态变为rejected
return reject(e)
}
})
})
}
//promise状态为rejected
if (self.status === 'rejected') {
return promise2 = new Promise(function(resolve, reject) {
setTimeout(function() {
try {
//调用then方法的onReject回调
var x = onRejected(self.data)
//根据x的值修改promise2的状态
resolvePromise(promise2, x, resolve, reject)
} catch(e) {
//promise2状态变为rejected
return reject(e)
}
})
})
}
//promise状态为pending
//需要等待promise的状态改变
if (self.status === 'pending') {
return promise2 = new Promise(function(resolve, reject) {
self.callbacks.push({
onResolved: function(value) {
try {
//调用then方法的onResolved回调
var x = onResolved(value)
//根据x的值修改promise2的状态
resolvePromise(promise2, x, resolve, reject)
} catch(e) {
//promise2状态变为rejected
return reject(e)
}
},
onRejected: function(reason) {
try {
//调用then方法的onResolved回调
var x = onRejected(reason)
//根据x的值修改promise2的状态
resolvePromise(promise2, x, resolve, reject)
} catch(e) {
//promise2状态变为rejected
return reject(e)
}
}
})
})
}
}
//获取当前Promise传递的值
Promise.prototype.valueOf = function() {
return this.data
}
//由then方法实现catch方法
Promise.prototype.catch = function(onRejected) {
return this.then(null, onRejected)
}
//finally方法
Promise.prototype.finally = function(fn) {
return this.then(function(v){
setTimeout(fn)
return v
}, function(r){
setTimeout(fn)
throw r
})
}
Promise.prototype.spread = function(fn, onRejected) {
return this.then(function(values) {
return fn.apply(null, values)
}, onRejected)
}
Promise.prototype.inject = function(fn, onRejected) {
return this.then(function(v) {
return fn.apply(null, fn.toString().match(/\((.*?)\)/)[1].split(',').map(function(key){
return v[key];
}))
}, onRejected)
}
Promise.prototype.delay = function(duration) {
return this.then(function(value) {
return new Promise(function(resolve, reject) {
setTimeout(function() {
resolve(value)
}, duration)
})
}, function(reason) {
return new Promise(function(resolve, reject) {
setTimeout(function() {
reject(reason)
}, duration)
})
})
}
Promise.all = function(promises) {
return new Promise(function(resolve, reject) {
var resolvedCounter = 0
var promiseNum = promises.length
var resolvedValues = new Array(promiseNum)
for (var i = 0; i < promiseNum; i++) {
(function(i) {
Promise.resolve(promises[i]).then(function(value) {
resolvedCounter++
resolvedValues[i] = value
if (resolvedCounter == promiseNum) {
return resolve(resolvedValues)
}
}, function(reason) {
return reject(reason)
})
})(i)
}
})
}
Promise.race = function(promises) {
return new Promise(function(resolve, reject) {
for (var i = 0; i < promises.length; i++) {
Promise.resolve(promises[i]).then(function(value) {
return resolve(value)
}, function(reason) {
return reject(reason)
})
}
})
}
Promise.resolve = function(value) {
var promise = new Promise(function(resolve, reject) {
resolvePromise(promise, value, resolve, reject)
})
return promise
}
Promise.reject = function(reason) {
return new Promise(function(resolve, reject) {
reject(reason)
})
}
Promise.fcall = function(fn){
// 虽然fn可以接收到上一层then里传来的参数，但是其实是undefined，所以跟没有是一样的，因为resolve没参数啊
return Promise.resolve().then(fn)
}
Promise.done = Promise.stop = function(){
return new Promise(function(){})
}
Promise.deferred = Promise.defer = function() {
var dfd = {}
dfd.promise = new Promise(function(resolve, reject) {
dfd.resolve = resolve
dfd.reject = reject
})
return dfd
}
try { // CommonJS compliance
module.exports = Promise
} catch(e) {}
return Promise
})()