手写基于Promise A+规范的Promise

const PENDING =  'pending';//初始态
const FULFILLED = 'fulfilled';//初始态
const REJECTED = 'rejected';//初始态
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(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
//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);
}
});
});
}

}
//catch原理就是只传失败的回调
Promise.prototype.catch = function(onRejected){
this.then(null,onRejected);
}
Promise.deferred = Promise.defer = function(){
let defer = {};
defer.promise = new Promise(function(resolve,reject){
defer.resolve = resolve;
defer.reject = reject;
});
return defer;
}
function gen(times,cb){
let result = [],count=0;
return function(i,data){
result[i] = data;
if(++count==times){
cb(result);
}
}
}
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 = function(promises){
return new Promise(function(resolve,reject){
for(let i=0;i<promises.length;i++){
promises[i].then(resolve,reject);
}
});
}
//返回一个立刻成功的promise
//别人提供 给你一个方法,需要你传入一个promise,但你只有一个普通的值,你就可以通过这个方法把这个普通的值(string number object)转成一个promise对象
Promise.resolve = function(value){
return new Promise(function(resolve){
resolve(value);
});
}
//返回一个立刻失败的promise
Promise.reject = function(reason){
return new Promise(function(resolve,reject){
reject(reason);
});
}
module.exports = Promise;
posted @ 2018-01-14 00:43  Macchiato  阅读(...)  评论(... 编辑 收藏