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p4.js
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p4.js
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function Promise(executor){
let _=this;
_.value=undefined;
_.reason=undefined;
_.state="pending"//大家一开始都是一样,等着吧
_.resolveCallbacks=[];//callbacks在pending中添加,fullfilled中执行
_.rejectCallbacks=[];//callbacks在pending中添加,rejected中执行
function resolve(value){
_.value=value//实现之后的感言
_.state="fulfilled"//实现啦!
_.resolveCallbacks.forEach((fn)=>fn())
}
function reject(reason){
_.reason=reason//给我一个被拒绝的理由
_.state="rejected"//被拒绝了!
_.rejectCallbacks.forEach((fn)=>fn())
}
executor(resolve,reject)
}
function resolvePromise(promise,x,resolve,reject){
//如果x可能是一个promise
if(x!==null&&(typeof x==="object"||typeof x==="function")){
let then=x.then;
//如果x是一个promise,因为promise都要有then函数的
if(typeof then === "function"){
//y表示x这个promise的值
then.call(x,y=>{
//继续遍历,直至返回值不是promise
resolvePromise(promise,y,resolve,reject)
},err=>{
reject(err)
})
}else{
//如果x是个普通对象,直接运行
resolve(x)
}
}else{
//如果x不是一个promise,也就是x是一个常量,直接运行
resolve(x)
}
}
Promise.prototype.then=function(onFulfilled, onRejected){
let _=this;
let promise2;
//将当前promise的值传递到下一次then的调用中
function resolveFunction(promise,resolve,reject){
let x=onFulfilled(_.value)
resolvePromise(promise,x,resolve,reject)
}
function rejectFunction(promise,resolve,reject){
let x=onRejected(_.reason)
resolvePromise(promise,x,resolve,reject)
}
promise2=new Promise((resolve,reject)=>{
if(_.state=="pending"){
//把回调方法塞进队列中
_.resolveCallbacks.push(()=>{
resolveFunction(promise2,resolve,reject)
})
_.rejectCallbacks.push(()=>{
rejectFunction(promise2,resolve,reject)
})
}
if(_.state=="fulfilled"){
resolveFunction(promise2,resolve,reject)
}
if(_.state=="rejected"){
rejectFunction(promise2,resolve,reject)
}
})
return promise2
}
//e.g
let Iagree=new Promise((resolve,reject)=>{
setTimeout(()=>{
resolve("我开心就同意了");
},1000)
})
//为了防止多次then,所以回调方法需要丢入队列中,防止方法被覆盖。
Iagree.then((data)=>{
console.log(data)
return new Promise((resolve,reject)=>{
setTimeout(()=>{
resolve("看心情干活");
},1000)
})
}).then((data)=>{
console.log("前方返回一个promise:"+data)
return data+",我是一个常量"
}).then((data)=>{
console.log("常量返回:"+data)
}).then((data)=>{
console.log("前方无法返回:"+data)
})