回调嵌套
let fs = require('fs');
fs.readFile('./template.txt', 'utf8', function (err, template) {
fs.readFile('./data.txt', 'utf8', function (err, data) {
console.log(template + ' ' + data);
})
})
如果我们需要template中的数据，又需要data中的数据。 这样就会出现回调嵌套的问题。如果还有很多这样的请求，将会一层一层嵌套下去，这样代码将变得非常难看和难以维护。
如何解决这个问题？
promise的简单使用
promise为承诺，承诺的是未来的事情。可能成功，也可能失败。
let p1 = new Promise(function(resolve,reject){
setTimeout(function(){
resolve('成功');
},1000);
});
p1.then(function(value){
console.log('成功',value);
},function(reason){
console.log('失败',reason);
});
new一个promise的实例p1，传入一个函数，这个函数中有两个参数，resolve和reject这是两个方法，resolve为成功执行的方法，reject为失败执行的方法。
promise原理分析
简单实现promise
先简单实现一个promise，然后逐步完善。
Promise是一个构造函数，所以先创建一个构造函数。里面有resolve和reject两个方法。因为then方法是通过promise的实例调用的。在Promise的实例的原型上有一个then方法。
//构造函数的参数是一个异步任务
function Promise(task) {
let that = this;//缓存this
//默认状态为pending
that.status = 'pending';
//此变量里放着此promise的结果
that.value = undefined;
//存放的着所有成功的回调函数
that.onResolvedCallbacks = [];
//存放着所有的失败的回调函数
that.onRejectedCallbacks = [];
//调用此方法可以把promise变成成功态
function resolve(value) {
if (that.status == 'pending') {
that.status = 'fulfilled';
that.value = value;
that.onResolvedCallbacks.forEach(item=>item(that.value));
}
}
//调用此方法可以把当前的promise变成失败态
function reject(reason) {
//如果当前状态是初始态，则转成失败态
if (that.status == 'pending') {
that.status = 'rejected';
that.value = reason;
that.onRejectedCallbacks.forEach(item=>item(that.value));
}
}
//立即执行传入的任务
try {
task(resolve, reject);
} catch (e) {
reject(e);
}
}
//onFulfilled成功的回调，onReject失败的回调
Promise.prototype.then = function (onFulfilled, onReject) {
let that = this;
if(that.status == 'fulfilled'){
onFulfilled(that.value);
}
if(that.status == 'rejected'){
onReject(that.value);
}
if(that.status == 'pending'){
that.onResolvedCallbacks.push(onFulfilled);
that.onRejectedCallbacks.push(onReject);
}
}
module.exports = Promise;
完善then方法
//onFulfilled 是用来接收promise成功的值或者失败的原因
Promise.prototype.then = function(onFulfilled,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
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);
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);
}
});
});
}
}
添加resolvePromise方法
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
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);
}
}
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);
}
});
}
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);
}
});
Promise.resolve
Promise.resolve = function(value){
return new Promise(function(resolve){
resolve(value);
});
}
Promise.reject
Promise.reject = function(reason){
return new Promise(function(resolve,reject){
reject(reason);
});
}
