Promise的用法 ：
let p = new Promise(function(resolve, reject){
console.log(111)
setTimeout(function(){
resolve(2)
}, 1000)
})
p.then(function(res){
console.log('suc',res)
},function(err){
console.log('err',err)
})可以看到new了一个Promise,里面有一个回调函数，回调函数里有2个参数，分别又是另外2个函数。其实里面很多也都是回调函数的封装调用。
const [PENDING,RESOLVED,REJECTED]=["PENDING","RESOLVED","REJECTED"];`
class Promise1 {
constructor(executor){
state = PENDING;
value = undefined;
reason = undefined;
resolve = (value) => {
if(this.state === PENDING){
this.state = RESOLVED;
this.value = value;
}
}
reject = (reason) => {
if(this.state === PENDING){
this.state = REJECTED;
this.value = reason;
}
}
try{
executor(this.resolve,this.reject)
}catch(e){
reject(e)
}
}
}测试一下执行情况：
let p1 = new Promise1((resolve,reject)=>{//function:executor
console.log(111)
setTimeout(()=>{
resolve(111)//this.resolve,直接传参调用
}, 1000)
})下面就是then了
class Promise1 {
constructor(executor){
...
onResolvedCallbacks=[]; //then,pending里先存起来
onRejectedCallbacks=[];
...
resolve = (value) => {
if(this.state === PENDING){
...
//executor里如果是异步会把then里存起来,然后executor走完了resolved的话,会再调then里回调函数
this.onResolvedCallbacks.forEach(fn => fn(value))
}
}
reject = (reason) => {
if(this.state === PENDING){
...
this.onRejectedCallbacks.forEach(fn => fn(reason))
}
}
}
then(onResolved,onRejected){
onResolved = typeof onResolved === 'function' ? onResolved : value => value;
onRejected = typeof onRejected === 'function' ? onRejected : reason => {
throw reason
};
switch(this.state){
case RESOLVED : //状态变了,就直接把结果给出去
onResolved(this.value);
case REJECTED :
onRejected(this.reason);
case PENDING :
//如果是pending的话,说明executor还没走完,先存起来,走完再调用
this.onResolvedCallbacks.push(onResolved);
this.onRejectedCallbacks.push(onRejected)
}
}
}测试一下 ：
let p2 = new Promise1((resolve, reject) => {
setTimeout(()=>{
resolve(222)
}, 1000)
})
p2.then(res => console.log(res, 'res')) //222链式调用：
class Promise1 {
...
then(onResolved,onRejected){
...
//返回一个promise1
let newPromise=new Promise1((resolve,reject)=>{
switch(this.state){
case RESOLVED : //状态变了,就直接把结果给出去
setTimeout(()=>{//then,放后面处理
try{
let reValue=onResolved(this.value);
rePromise(newPromise,reValue,resolve,reject)
}catch(e){reject(e)}
},0)
case REJECTED :
setTimeout(()=>{
try{
let reValue=onRejected(this.reason);
rePromise(newPromise,reValue,resolve,reject)
}catch(e){reject(e)}
},0)
case PENDING :
//如果是pending的话,说明executor还没走完,先存起来,走完再调用
this.onResolvedCallbacks.push(()=>{
setTimeout(()=>{
try{
let reValue=onResolved(this.value);
rePromise(newPromise,reValue,resolve,reject)
}catch(e){reject(e)}
},0)
});
this.onRejectedCallbacks.push(()=>{
setTimeout(()=>{
try{
let reValue=onRejected(this.reason);
rePromise(newPromise,reValue,resolve,reject)
}catch(e){reject(e)}
},0)
})
}
})
return newPromise;
}
}
//newPromise:新的promise1对象,reValue:上一个then的返回值,newPromise.resolve,newPromise.reject
function rePromise(newPromise,reValue,resolve,reject){
//相当于把自己return出去了,let p2 = p.then(res => p2);
if(newPromise === reValue){
reject(new TypeError('循环了'))
}
if(reValue !== null && (typeof reValue === 'object' || typeof reValue === 'function')){
try{
//判断一下reValue是不是promise1,是个对象,还带有then函数
//如果有then并是个函数,就调then;
let then = reValue.then;
if(typeof then === 'function'){
then.call(reValue, res=>{
resolve(res)
},err=>{
reject(err)
})
}else{//then不是函数,直接用resolve返回值
resolve(reValue)
}
}catch(e){
reject(e)
}
}else{//只是个普通值
resolve(reValue)
}
}测试一下：
p3.then(res => {
console.log(res)
return new Promise1((resolve, reject) => {
resolve('p3')
})
}).then(res => console.log(res, 'pp3'))//p3但还有个问题
...
function rePromise(newPromise,reValue,resolve,reject){
...
let called;//是否调用过,防止多次调用
if(reValue !== null && (typeof reValue === 'object' || typeof reValue === 'function')){
try{
let then = reValue.then;
if(typeof then === 'function'){
then.call(reValue, res=>{
if(called) return;
called = true;
//promise1就继续下一个,直到then走完,变成普通的值
rePromise(newPromise,res,resolve,reject)
},err=>{
if(called) return;
called = true;
reject(err)
})
}else{//then不是函数,直接用resolve返回值
resolve(reValue)
}
}catch(e){
if(called) return;
called = true;
reject(e)
}
}else{//只是个普通值
resolve(reValue)
}
}测试一下
let p1=new Promise1((resolve,reject)=>{
resolve(111)
})
let p2=p1.then(res=>{console.log(res,'p1')},err=>console.log(err,'p1'));
p2.then(res=>console.log(res,'p2'))
.then(res=>console.log(res,'p3))还有其它属性：
class Promise1 {
...
catch(onReject){
return this.then(null,onReject)
}
defer(){
let defer={};
defer.promise=new Promise1((resolve,reject)=>{
defer.resolve=resolve;
defer.reject=reject;
})
return defer;
}
deffered(){
return this.defer;
}
all(promises){
return new Promise1((resolve,reject)=>{
let done=gen(promises.length,resolve);
for(let i=0;i<promises.length,i++){
promises[i].then(res=>{
done(i,res)
},reject)
}
})
}
race(promises){
return new Promise1((resolve,reject)=>{
for(let i=0;i<promises.length;i++){
promises[i].then(resolve,reject)
}
})
}
resolve(value){
return new Promise1((resolve,reject)=>{
resolve(value)
})
}
reject(reason){
return new Promise1((resolve,reject)=>{
reject(reason)
})
}
}
function gen(times,cb){
const result=[];
let count=0;
return (i,data)=>{
result[i]=data;
count+=1;
if(count===times){
cb(result)
}
}
}
