1.Promise解决什么问题?
1.1回到地狱
比如实现一个动画，页面上有多个球，每个球要等到上一个球的运动停止之后开始动画。
function move(ball,target,cb){
let timer = setInterval(function(){
//先取到当前的left值
let index=0;
let timer = setInterval(function () {
if(index++<target){
ball.style.transform = `translateX(${index}px)`;
}else{
clearInterval(timer);
resolve();
cb();
}
}, 13)
}
},4)
}
move(ball1,100,function(){
move(ball2,100,function(){
move(ball3,100,function(){
alert('over');
});
});
});
问题：代码丑陋并且多层嵌套，Promise横空出世可以解决这个问题。下面用Promise进行实现这个动画
function move(ball, target) {
return new Promise(function (resolve, reject) {
let index=0;
let timer = setInterval(function () {
if(index++<target){
ball.style.transform = `translateX(${index}px)`;
}else{
clearInterval(timer);
resolve();
}
}, 13)
});
}
move(ball1, 100)
.then(function () {
return move(ball2, 100);
})
.then(function () {
return move(ball3, 100);
})
.then(function () {
alert('over');
})
2.实现Promise
实现一个符合 Promise/A+ 规范的 MyPromise，并实现 resolve、reject、catch、all、race、deferred等静态方法。
const PENDING = 'pending';//初始态
const FULFILLED = 'fulfilled';//初始态
const REJECTED = 'rejected';//初始态
2.1实现链式写法
function Promise(executor){
let self = this;//先缓存当前promise实例
self.status = PENDING;//设置状态
//定义存放成功的回调的数组
self.onResolvedCallbacks = [];
//定义存放失败回调的数组
self.onRejectedCallbacks = [];
//当调用此方法的时候，如果promise状态为pending,的话可以转成成功态,如果已经是成功态或者失败态了，则什么都不做
//2.1
function resolve(value){ //2.1.1
if(value!=null &&value.then&&typeof value.then == 'function'){
return value.then(resolve,reject);
}
//如果是初始态，则转成成功态
setTimeout(function(){
if(self.status == PENDING){
self.status = FULFILLED;
self.value = value;//成功后会得到一个值，这个值不能改
//调用所有成功的回调
self.onResolvedCallbacks.forEach(cb=>cb(self.value));
}
})
}
function reject(reason){ //2.1.2
setTimeout(function(){
//如果是初始态，则转成失败态
if(self.status == PENDING){
self.status = REJECTED;
self.value = reason;//失败的原因给了value
self.onRejectedCallbacks.forEach(cb=>cb(self.value));
}
});
}
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){
//如果这函数执行失败了，则用失败的原因reject这个promise
reject(e);
};
}
function resolvePromise(promise2,x,resolve,reject){
if(promise2 === x){
return reject(new TypeError('循环引用'));
}
let called = false;//promise2是否已经resolve 或reject了
if(x instanceof Promise){
if(x.status == PENDING){
x.then(function(y){
resolvePromise(promise2,y,resolve,reject);
},reject);
}else{
x.then(resolve,reject);
}
//x是一个thenable对象或函数，只要有then方法的对象，
}else if(x!= null &&((typeof x=='object')||(typeof x == 'function'))){
//当我们的promise和别的promise进行交互，编写这段代码的时候尽量的考虑兼容性，允许别人瞎写
try{
let then = x.then;
if(typeof then == 'function'){
//有些promise会同时执行成功和失败的回调
then.call(x,function(y){
//如果promise2已经成功或失败了，则不会再处理了
if(called)return;
called = true;
resolvePromise(promise2,y,resolve,reject)
},function(err){
if(called)return;
called = true;
reject(err);
});
}else{
//到此的话x不是一个thenable对象，那直接把它当成值resolve promise2就可以了
resolve(x);
}
}catch(e){
if(called)return;
called = true;
reject(e);
}
}else{
//如果X是一个普通 的值，则用x的值去resolve promise2
resolve(x);
}
}
//onFulfilled 是用来接收promise成功的值或者失败的原因
Promise.prototype.then = function(onFulfildeled,onRejected){
//如果成功和失败的回调没有传，则表示这个then没有任何逻辑，只会把值往后抛
//2.2.1
onFulfilled = typeof onFulfilled == 'function'?onFulfilled:function(value){return value};
onRejected = typeof onRejected == 'function'?onRejected:reason=>{throw reason};
//如果当前promise状态已经是成功态了，onFulfilled直接取值
let self = this;
let promise2;
if(self.status == FULFILLED){
return promise2 = new Promise(function(resolve,reject){
setTimeout(function(){
try{
let x =onFulfilled(self.value);
//如果获取到了返回值x,会走解析promise的过程
resolvePromise(promise2,x,resolve,reject);
}catch(e){
//如果执行成功的回调过程中出错了，用错误原因把promise2 reject
reject(e);
}
})
});
}
if(self.status == REJECTED){
return promise2 = new Promise(function(resolve,reject){
setTimeout(function(){
try{
let x =onRejected(self.value);
resolvePromise(promise2,x,resolve,reject);
}catch(e){
reject(e);
}
})
});
}
if(self.status == PENDING){
return promise2 = new Promise(function(resolve,reject){
self.onResolvedCallbacks.push(function(){
try{
let x =onFulfilled(self.value);
//如果获取到了返回值x,会走解析promise的过程
resolvePromise(promise2,x,resolve,reject);
}catch(e){
reject(e);
}
});
self.onRejectedCallbacks.push(function(){
try{
let x =onRejected(self.value);
resolvePromise(promise2,x,resolve,reject);
}catch(e){
reject(e);
}
});
});
}
}
2.2 Promise.prototype.catch
//catch原理就是只传失败的回调
Promise.prototype.catch = function(onRejected){
this.then(null,onRejected);
}
2.3 Promise.deferred
Promise.deferred = Promise.defer = function(){
let defer = {};
defer.promise = new Promise(function(resolve,reject){
defer.resolve = resolve;
defer.reject = reject;
});
return defer;
}
2.4 Promise.all
Promise.all = function(promises){
return new Promise(function(resolve,reject){
let done = gen(promises.length,resolve);
for(let i=0;i<promises.length;i++){
promises[i].then(function(data){
done(i,data);
},reject);
}
});
}
2.5 Promise.race
Promise.race = function(promises){
return new Promise(function(resolve,reject){
for(let i=0;i<promises.length;i++){
promises[i].then(resolve,reject);
}
});
}
2.6 Promise.resolve
Promise.resolve = function(value){
return new Promise(function(resolve){
resolve(value);
});
}
2.7 Promise.reject
Promise.reject = function(reason){
return new Promise(function(resolve,reject){
reject(reason);
});
}
参考资料
