Promise的原理

export default class Promise {
     constructor (handle) {
       // 初始值  默认的状态
        this['[[PromiseState]]'] = 'pending';
        this['[[PromiseResult]]'] = 'undefined';

       //给then定义数组
       this.resolveFnQueue = [];
       this.rejectFnQueue = [];
       //执行下面的两个函数，并且改变this的状态
       handle (this._resolve.bind(this), this._reject.bind(this))
     }
    
     _resolve(val){
        //1.改变状态 2，改变value
        this ['[[PromiseState]]'] = 'fulfilled'
        this ['[[PromiseResult]]'] = val
        //模拟微任务
        // let run = () => {
        //    let cb;
        //    //删除数组中第一个数组
        //    while (cb = this.resolveFnQueue.shift()) {
        //      cb (val)
        //    }
        // }
        // let mo = new MutationObserver(run);
        // let box = document.querySelector(".box");
        // mo.observe(box,{
        //   attributes:true
        // })
        // box.setAttribute("kkb","kkb")

        setTimeout (()=>{
          //  this.resolveFn(val)
          let cb;
          while(cb = this.resolveFnQueue.shift()){
              cb(val);
          }
        })
     }
     _reject(err){
      //改变状态
      this ['[[PromiseState]]'] = "rejected";
      this ['[[PromiseResult]]'] = err;
      // let run = () => {
      //    let cb;
      //    while(cb = this.rejectFnQueue.shift()){
      //       cb(ell)
      //    }
      // }
      // let mo = new MutationObserver(run);
      // let box = document.querySelector(".box");
      // mo.observe(box,{
      //   attributes:true
      // })
      // box.setAttribute("kkb","kkb");

      setTimeout(()=>{
        //  this.rejectFn(err)
        let cb;
        while(cb = this.rejectFnQueue.shift()){
            cb(err);
        }
      })
     }
     then(onResolved, onRejected) {
      // 创建一个新的promise对象 并且返回
       return new Promise ((resolve,reject)=>{
           //封装函数
           let onResolvedFn = (val)=>{
              let res = onRejected && onResolved(val);
              //判断执行结果是不是promise对象
              if(res instanceof Promise){
                //如果是promise就返回resolve
                 res.then(resolve);
              }else{
                //否则就返回res
                resolve(res)
              }
           }
           let onRejectedFn = (err)=>{
             onResolved && onRejected(err);
             reject(err);
           }
           //执行把onRejectedFn和onResolvedFn里的数添加到 数组里面
           this.resolveFnQueue.push(onRejectedFn);
           this.rejectFnQueue.push(onResolvedFn)
       })
     }
    static resolve(val){
        return new KPromise(resolve=>{
            resolve(val);
        })
    }
    static reject(err){
        return new KPromise((undefined,reject)=>{
            reject(err);
        })
    }
    static race(list){
        return new KPromise((resolve,reject)=>{
            list.forEach(item=>{
                item.then(res=>{
                    resolve(res);
                },err=>{
                    reject(err)
                })
            })   
        })
    }
    static all(list){
        return new KPromise(resolve=>{
            let resArr = [];
            let num = 0;
            list.forEach(item=>{
                item.then(res=>{
                    num++;
                    resArr.push(res); 
                    if(num===list.length){
                        resolve(resArr);
                    }
                })
            })
        })
    }

    static allSettled(list){
        let resArr = new Array(list.length);
        let num = 0
        return new KPromise(resolve=>{
            list.forEach((item,key)=>{
                let obj = {};
                item.then(res=>{
                    obj['status'] = this['[[PromiseState]]'];
                    obj.value = res;
                    resArr[key] = obj;
                    num++
                    if(num===list.length){
                        resolve(resArr); 
                    }
                },err=>{
                    obj['status'] = this['[[PromiseState]]'];
                    obj.reson = err;
                    resArr[key] = obj;
                    num++
                    if(num===list.length){
                        resolve(resArr); 
                    }
                })
            })
        });

    }
       finally(cb){
           this.then(cb,cb);
       }
       catch(cb){
           this.then(undefined,err=>{
               cb && cb(err);
           })
       }
}