Promise对Executor中异步操作与then方法多次调用的处理

一、Executor中异步操作的处理
executor中异步操作，then方法中callbacks需要在promise.status改变后再执行
二、then方法的多次调用
多次调用then方法，callbacks会按照顺序来执行。
此处用到了发布订阅模式，收集then方法中回调时用到了订阅，resolve和reject方法执行时用到了发布。

const PENDING = 'pending';
const FULFILLED = 'fulfilled';
const REJECTED = 'rejected';
class MPromise{
    constructor(excuter){
      this.status = PENDING;
    this.value = undefined;
    this.reason = undefined;
    this.onFulfilledCallbacks = []
    this.onRejectedCallbacks = []
    //声明resole和reject函数，这里用箭头函数可以避免this指向改变，单独方法调用，this指向window或undefined
    const resolve = (val) =>{
        if(this.status === PENDING){
          this.value = val
        this.status = FULFILLED
        this.onFulfilledCallbacks.forEach((fn)=> fn())
      }
    }
    const reject = (reason) =>{
        if(this.status === PENDING){
          this.reason = reason
        this.status = REJECTED
        this.onRejectedCallbacks.forEach((fn)=> fn())
      }
    }
    try{
        executor(resolve, reject)
    }catch(error){
        reject(error)
    }
  }
  //then方法核心部分是，判断promise当前status，做出不同操作
  //function class里面添加原型方法，不用写function关键字
  then(onFulfilled, onRejected){
    //如果是pending状态，收集相关方法，等状态改变时再调用
      if(this.status === PENDING){
      //之前搜集回调时，没有传value和reason
      this.onFulfilledCallbacks.push(()=>{
          onFulfilled(this.value)
      })
      this.onRejectedCallbacks.push(()=>{
          onRejected(this.reason)
      })
    }
    if(this.status === FULFIILED){
        onFulfilled(this.value)
    }
    if(this.status === REJECTED){
        onRjected(this.reason)
    }
  }
}