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理解Promise (4)
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then 方法必须 返回一个新的promise
promise2 = promise1.then(onFulfilled, onRejected);
新的Promise 必须返回传递两个方法 onFulfilled, onRejected
If either onFulfilled or onRejected returns a value x, run the Promise Resolution Procedure [[Resolve]](promise2, x).
onFulfilled, onRejected 其中的某个都要返回 一个value
if either
onFulfilled
or
onRejected
returns a value
x
, run the Promise Resolution Procedure
[[Resolve]](promise2, x)
如果onfulled或onrejected返回值x,请运行承诺解决过程[[解决]](promise2,x)。
let x = onfulfilled(this.value);
// x是普通值还是promise 如果是普通值 直接调用promise2的resolve
// 如果是promise 那应该让x这个promise执行 x.then
resolvePromise(promise2,x,resolve,reject);
这段代码是 resolvePromise 承诺解决过程
resolvePromise 这个函数里面 有Promise 这个参数,所以我们要拿到这个值 但是我们直接打印出来这个值 是 underfind 所以我们要用 promise2
这样,我们做到这里的时候 then 方法的大概结构就出来了
then(onfulfilled,onrejected){
let promise2;
promise2 = new Promise((resolve,reject)=>{
if(this.status === 'fulfilled'){
setTimeout(()=>{
try{
let x = onfulfilled(this.value);
// x是普通值还是promise 如果是普通值 直接调用promise2的resolve
// 如果是promise 那应该让x这个promise执行 x.then
resolvePromise(promise2,x,resolve,reject);
}catch(e){
reject(e);
}
},);
}
if(this.status === 'rejected'){
setTimeout(()=>{
try{
let x = onrejected(this.reason);
resolvePromise(promise2,x,resolve,reject);
}catch(e){
reject(e);
}
},)
}
if(this.status === 'pending'){
this.resolveCallbacks.push(()=>{
setTimeout(()=>{
try{
let x = onfulfilled(this.value);
resolvePromise(promise2,x,resolve,reject);
}catch(e){
reject(e);
}
},)
});
this.rejectCallbacks.push(()=>{
setTimeout(()=>{
try{
let x = onrejected(this.reason);
resolvePromise(promise2,x,resolve,reject);
}catch(e){
reject(e);
}
},)
})
}
});
return promise2 }
接下来我们来处理 The Promise Resolution Procedure 的问题
If
promise
and
x
refer to the same object, reject
promise
with a
TypeError
as the reason.
let resolvePromise = (promise2,x,resolve,reject) => {
if(promise2 === x){
return reject(new TypeError('循环引用'))
}
}
if
x
is an object or function,
if(typeof x === 'function' || (typeof x === 'object' && x !== null)){ }
Let
then
be
x.then
let then = x.then; // 如果取then方法出错 那就用错误拒绝promise2
-
f/when
resolvePromise
is called with a valuey
, run[[Resolve]](promise, y)
. -
If/when
rejectPromise
is called with a reasonr
, rejectpromise
withr
. -
If both
resolvePromise
andrejectPromise
are called, or multiple calls to the same argument are made, the first call takes precedence, and any further calls are ignored.
if(typeof then === 'function'){ // 我就认为他是一个promise
then.call(x,y=>{ // 让当前的promise执行 ,不用多次取then方法了
// y 有可能还是一个promise , 继续调用resolvePromise方法,直到解析出一个常量为止,最终把常量传递下去
if(called) return; // 放置此方法多次被调用
called = true;
resolvePromise(promise2,y,resolve,reject);
},r=>{
if(called) return;
called = true;
reject(r); // 让当前的promise变成失败态即可
})
}else{
// x就是一个普通的对象 并没有then方法
resolve(x);
}
全部的代码:
let resolvePromise = (promise2,x,resolve,reject) => {
// 判断x的类型 来处理promise2是成功还是失败
// 所有的promise都遵循这个规范,不同的人写的promise可能会混用
// 尽可能考虑的周全 要考虑别人promise可能出错的情况
if(promise2 === x){
return reject(new TypeError('循环引用'))
}
// 判断x是不是一个promise ,这个x 可能不是自己的promise 所以为了安全 需要在进行校验,放置调一起用成功和失败
if(typeof x === 'function' || (typeof x === 'object' && x !== null)){
// 尝试取当前x的then方法, 这个then方法可能别人定义的时候 用的Object.defineProperty
let called;
try{
let then = x.then; // 如果取then方法出错 那就用错误拒绝promise2
if(typeof then === 'function'){ // 我就认为他是一个promise
then.call(x,y=>{ // 让当前的promise执行 ,不用多次取then方法了
// y 有可能还是一个promise , 继续调用resolvePromise方法,直到解析出一个常量为止,最终把常量传递下去
if(called) return; // 放置此方法多次被调用
called = true;
resolvePromise(promise2,y,resolve,reject);
},r=>{
if(called) return;
called = true;
reject(r); // 让当前的promise变成失败态即可
})
}else{
// x就是一个普通的对象 并没有then方法
resolve(x);
}
}catch(e){
if(called) return;
called = true;
reject(e);
}
}else{
// x肯定一个常量
resolve(x);
}
} class Promise{
constructor(executor){
this.status = 'pending';
this.value;
this.reason;
this.resolveCallbacks = []; // 当then是pending 我希望吧成功的方法都放到数组中
this.rejectCallbacks = [];
let resolve = (value)=>{
if(this.status == 'pending'){
this.status = 'fulfilled';
this.value = value;
this.resolveCallbacks.forEach(fn=>fn()); // 发布
}
}
let reject = (reason)=>{
if(this.status === 'pending'){
this.status = 'rejected';
this.reason = reason;
this.rejectCallbacks.forEach(fn=>fn())
}
}
try{
executor(resolve,reject);
}catch(e){
reject(e);
}
}
then(onfulfilled,onrejected){
let promise2;
promise2 = new Promise((resolve,reject)=>{
if(this.status === 'fulfilled'){
setTimeout(()=>{
try{
let x = onfulfilled(this.value);
// x是普通值还是promise 如果是普通值 直接调用promise2的resolve
// 如果是promise 那应该让x这个promise执行 x.then
resolvePromise(promise2,x,resolve,reject);
}catch(e){
reject(e);
}
},);
}
if(this.status === 'rejected'){
setTimeout(()=>{
try{
let x = onrejected(this.reason);
resolvePromise(promise2,x,resolve,reject);
}catch(e){
reject(e);
}
},)
}
if(this.status === 'pending'){
this.resolveCallbacks.push(()=>{
setTimeout(()=>{
try{
let x = onfulfilled(this.value);
resolvePromise(promise2,x,resolve,reject);
}catch(e){
reject(e);
}
},)
});
this.rejectCallbacks.push(()=>{
setTimeout(()=>{
try{
let x = onrejected(this.reason);
resolvePromise(promise2,x,resolve,reject);
}catch(e){
reject(e);
}
},)
})
}
});
return promise2 }
}
module.exports = Promise;
https://promisesaplus.com/