- Promise对象有以下几种状态:
- pending: 初始状态, 既不是 fulfilled 也不是 rejected.
- fulfilled: 成功的操作.
- rejected: 失败的操作.
1).Promise的初步框架
function Promise(executor) {
let self = this; //缓存this，当前promise的实例
self.status = 'pending'; //初始状态
self.value = 'undefined';//默认成功的值
self.reason = 'undefined';//默认失败的原因
function resolve(value) { //成功状态
if (self.status === 'pending') {
self.status = 'resolved';
self.value = value;
}
}
function reject(reason) { //失败状态
if (self.status === 'pending') {
self.status = 'rejected';
self.reason = reason;
}
}
try{
//因为此函数执行可能会异常，所以需要捕获，如果出错了，需要用错误对象reject
executor(resolve, reject)
}catch(e){
reject(e); // 捕获的时候发生异常,就直接失败了
}
}
Promise.prototype.then = function (onFulfilled, onRjected) {
let self = this;
if (self.status === 'resloved') {
onFulfilled(self.value)
}
if (self.status === 'rejected') {
onRjected(self.reason)
}
}
2).promise实例可以多次then,当成功后会将then中的成功方法按顺序执行，我们可以先将then中的成功的回调和失败的回调存到数组内，当成功时调用成功的数组即可。
function Promise(executor) {
let self = this;
self.status = 'pending';
self.value = 'undefined';
self.reason = 'undefined';
------------------------------------------------------------------------------
//此处为增加的代码
self.onResolvedCallbacks = []; // 存放then成功的回调
self.onRejectedCallbacks = []; // 存放then失败的回调
------------------------------------------------------------------------------
function resolve(value) { //成功状态
if (self.status === 'pending') {
self.status = 'resolved';
self.value = value;
------------------------------------------------------------------------------
//此处为增加的代码
self.onResolvedCallbacks.forEach(function (fn) {
fn();
});
------------------------------------------------------------------------------
}
}
function reject(reason) { //失败状态
if (self.status === 'pending') {
self.status = 'rejected';
self.reason = reason;
------------------------------------------------------------------------------
//此处为增加的代码
self.onRejectedCallbacks.forEach(function (fn) {
fn();
});
------------------------------------------------------------------------------
}
}
try{
executor(resolve, reject)
}catch(e){
reject(e)
}
}
Promise.prototype.then = function (onFulfilled, onRjected) {
let self = this;
if (self.status === 'resloved') {
onFulfilled(self.value)
}
if (self.status === 'rejected') {
onRjected(self.reason)
}
------------------------------------------------------------------------------
//此处为增加的代码
if(self.status === 'pending'){
self.onResolvedCallbacks.push(function(){
onFulfilled(self.value)
})
self.onRejectedCallbacks.push(function(){
onRjected(self.reason)
})
}
------------------------------------------------------------------------------
}
3).链式调用 jquery,jquery能实现链式调用靠的就是返回this,promise不能返回this,promise实现链式调用靠的是返回一个新的promise
function Promise(executor) {
let self = this;
self.status = 'pending';
self.value = 'undefined';
self.reason = 'undefined';
self.onResolvedCallbacks = []; // 存放then成功的回调
self.onRejectedCallbacks = []; // 存放then失败的回调
function resolve(value) { //成功状态
if (self.status === 'pending') {
self.status = 'resolved';
self.value = value;
self.onResolvedCallbacks.forEach(function (fn) {
fn();
});
}
}
function reject(reason) { //失败状态
if (self.status === 'pending') {
self.status = 'rejected';
self.reason = reason;
self.onRejectedCallbacks.forEach(function (fn) {
fn();
})
}
}
try {
executor(resolve, reject)
} catch (e) {
reject(e)
}
}
Promise.prototype.then = function (onFulfilled, onRjected) {
------------------------------------------------------------------------------
//我们的代码可以在then中什么都不传promise中值的穿透(实际使用)
//如果成功和失败默认不传，给一个函数（规范规定）
onFulfilled = typeof onFulfilled === 'function' ? onFulfilled : function (value) {
return value
};
onRjected = typeof onRjected === 'function' ? onRjected : function (err) {
throw err;
}
------------------------------------------------------------------------------
let self = this;
------------------------------------------------------------------------------
//增加的代码
let promise2; //返回的promise
------------------------------------------------------------------------------
if (self.status === 'resolved') {
------------------------------------------------------------------------------
//增加的代码
promise2 = new Promise(function (resolve, reject) {
// 当成功或者失败执行时有异常那么返回的promise应该处于失败状态
// x可能是一个promise 也有可能是一个普通的值
try {
let x = onFulfilled(self.value);
// x可能是别人promise，写一个方法统一处理
resolvePromise(promise2, x, resolve, reject);
} catch (e) {
reject(e);
}
})
------------------------------------------------------------------------------
}
if (self.status === 'rejected') {
------------------------------------------------------------------------------
//增加的代码
promise2 = new Promise(function (resolve, reject) {
try {
let x = onRjected(self.reason);
resolvePromise(promise2, x, resolve, reject);
} catch (e) {
reject(e);
}
})
------------------------------------------------------------------------------
}
if (self.status === 'pending') {
self.onResolvedCallbacks.push(function () {
------------------------------------------------------------------------------
//增加的代码
try {
let x = onFulfilled(self.value);
resolvePromise(promise2, x, resolve, reject);
} catch (e) {
reject(e)
}
})
self.onRejectedCallbacks.push(function () {
try {
let x = onRjected(self.reason);
resolvePromise(promise2, x, resolve, reject);
} catch (e) {
reject(e);
}
------------------------------------------------------------------------------
})
}
------------------------------------------------------------------------------
//增加的代码
return promise2;
------------------------------------------------------------------------------
}
4).解析then中的结果
function resolvePromise(promise2, x, resolve, reject) {
// 有可能这里返回的x是别人的promise
// 尽可能允许其他乱写
//返回的结果和promise是同一个那么永远不会成功和失败
if (promise2 === x) { //这里应该报一个类型错误，有问题
return reject(new TypeError('循环引用了'))
}
// 看x是不是一个promise,promise应该是一个对象
//有些人写的promise可能会既调用成功 又调用失败，如果两个都调用先 调用谁另一个就忽略掉
let called; // 表示是否调用过成功或者失败
if (x !== null && (typeof x === 'object' || typeof x === 'function')) {
// 可能是promise {},看这个对象中是否有then方法，如果有then我就认为他是promise了
try { // {then:1}
let then = x.then;
if (typeof then === 'function') {
// 成功
then.call(x, function (y) {
if (called) return
called = true
// y可能还是一个promise，在去解析直到返回的是一个普通值
resolvePromise(promise2, y, resolve, reject)
}, function (err) { //失败
if (called) return
called = true
reject(err);
})
} else {
resolve(x)
}
} catch (e) {
if (called) return
called = true;
reject(e);
}
} else { // 说明是一个普通值1
resolve(x); // 表示成功了
}
}
如果then中无论是成功的回调还是失败的回调只要返回了结果就会走下一个then中的成功，如果有错误走下一个then的失败
如果第一个promise返回一个普通值,会进到下一次then的成功的回调,如果第一个promise返回了一个promise，需要等待返回的promise执行后的结果传递给下一次then中
6).捕获错误的方法
Promise.prototype.catch = function (callback) {
return this.then(null, callback)
}
7).Promise的静态方法
- all:全部成功才算成功
Promise.all = function (promises) {
//promises是一个promise的数组
return new Promise(function (resolve, reject) {
let arr = []; //arr是最终返回值的结果
let i = 0; // 表示成功了多少次
function processData(index, y) {
arr[index] = y;
if (++i === promises.length) {
resolve(arr);
}
}
for (let i = 0; i < promises.length; i++) {
promises[i].then(function (y) {
processData(i, y)
}, reject)
}
})
}
- race:只要有一个promise成功了 就算成功。如果第一个失败了就失败了
Promise.race = function (promises) {
return new Promise(function (resolve, reject) {
for (var i = 0; i < promises.length; i++) {
promises[i].then(resolve,reject)
}
})
}
- resolve:生成一个成功的promise
Promise.resolve = function(value){
return new Promise(function(resolve,reject){
resolve(value);
})
}
- reject:生成一个失败的promise
Promise.reject = function(reason){
return new Promise(function(resolve,reject){
reject(reason);
})
}
8).promise规范中要求，所有的onFufiled和onRjected都需要异步执行,setTimeout(在onFufiled和onRjected处添加setTimeout)
Promise.prototype.then = function (onFulfilled, onRjected) {
//成功和失败默认不穿给一个函数
onFulfilled = typeof onFulfilled === 'function' ? onFulfilled : function (value) {
return value;
}
onRjected = typeof onRjected === 'function' ? onRjected : function (err) {
throw err;
}
let self = this;
let promise2; //返回的promise
if (self.status === 'resolved') {
promise2 = new Promise(function (resolve, reject) {
// 当成功或者失败执行时有异常那么返回的promise应该处于失败状态
// x可能是一个promise 也有可能是一个普通的值
-------------------------------------------------------------------------------------------------
setTimeout(function () {
try {
let x = onFulfilled(self.value);
// x可能是别人promise，写一个方法统一处理
resolvePromise(promise2, x, resolve, reject);
} catch (e) {
reject(e);
}
})
})
}
if (self.status === 'rejected') {
promise2 = new Promise(function (resolve, reject) {
setTimeout(function () {
try {
let x = onRjected(self.reason);
resolvePromise(promise2, x, resolve, reject);
} catch (e) {
reject(e);
}
})
})
}
// 当调用then时可能没成功 也没失败
if (self.status === 'pending') {
promise2 = new Promise(function (resolve, reject) {
// 此时没有resolve 也没有reject
self.onResolvedCallbacks.push(function () {
setTimeout(function () {
try {
let x = onFulfilled(self.value);
resolvePromise(promise2, x, resolve, reject);
} catch (e) {
reject(e)
}
})
});
self.onRejectedCallbacks.push(function () {
setTimeout(function () {
try {
let x = onRjected(self.reason);
resolvePromise(promise2, x, resolve, reject);
} catch (e) {
reject(e);
}
})
});
})
}
-------------------------------------------------------------------------------------------------
return promise2;
}
9).promise测试
下载一个Promise的测试库,promises-aplus-tests
- npm install -g promises-aplus-tests
- promises-aplus-tests 文件名
测试需要添加的代码
Promise.defer = Promise.deferred = function () {
let dfd = {};
dfd.promise = new Promise(function (resolve, reject) {
dfd.resolve = resolve;
dfd.reject = reject;
});
return dfd
}