1.Promise理解

• Promise本質(zhì)是一個狀態(tài)機(jī)，每個Promise只能是三種狀態(tài)中的一種：pending，fulfilled，rejected
  狀態(tài)轉(zhuǎn)變只能是 pending -> fulfilled 或者 pending -> rejected，狀態(tài)轉(zhuǎn)變不可逆
• then方法可以被同一個Promise鏈?zhǔn)秸{(diào)用多次
• then方法必須返回一個新的Promise

2.Promise實(shí)現(xiàn)代碼

function Promise(fn){
    //安全的構(gòu)造函數(shù)
    if(!(this instanceof Promise)){
        return new Promise(fn);
    }

    var self=this;
    //先執(zhí)行函數(shù)本身,然后調(diào)用resolve的時候運(yùn)行存儲在doneList中的回調(diào)函數(shù)
    this.doneList=[];        //成功回調(diào)函數(shù)隊列
    this.failList=[];        //失敗回調(diào)函數(shù)隊列
    this.status='PENDING';   //狀態(tài)
    this.value=null;
    this.error=null;

    //Promise改變狀態(tài)，執(zhí)行其成功回調(diào)函數(shù)隊列
    function resolve(value){
        setTimeout(function(){
            self.status='FULFILLED';
            self.value=value;
            self.doneList.forEach(function(done){
                self.value=done(self.value);   //將計算結(jié)果依次傳遞下去
            });
        });
    }

    //Promise改變狀態(tài)，執(zhí)行其失敗回調(diào)函數(shù)隊列
    function reject(error){
        setTimeout(function(){
            self.status='REJECTED';
            self.error=error;
            self.failList.forEach(function(fail){
                self.error=fail(self.error);   //將錯誤處理結(jié)果依次傳遞下去
            });
        });
    }
    fn(resolve,reject);
}

Promise.prototype={
    constructor: Promise,

    then: function(done,fail){
        var self=this;
        return new Promise(function(resolve,reject){
            //對then中的回調(diào)函數(shù)進(jìn)行包裝，并執(zhí)行resolve
            function handle(value){
                //成功和錯誤處理函數(shù)必須是函數(shù)才調(diào)用，否則直接返回上一個值
                var res=typeof done==='function' && done(value) || value;

                //如果返回一個Promise對象，則調(diào)用其then方法，等待獲取其中的返回值
                if(res && typeof res['then']==='function'){
                    res.then(function(value){
                        //調(diào)用resolve時，會執(zhí)行其成功回調(diào)函數(shù)列表
                        resolve(value);
                    });
                } else {
                    resolve(res);
                }
            }

            function errback(error){
                error=typeof fail==='function' && fail(error) || error;
                reject(error);
            }

            if(self.status==='PENDING'){
                self.doneList.push(handle);
                self.failList.push(errback);
            } else if(self.status==='FULFILLED'){
                //已完成后調(diào)用then則直接傳入計算結(jié)果
                handle(self.value);
            } else if(self.status==='REJECTED'){
                //已完成后調(diào)用則直接傳入異常
                errback(self.error);
            }
        });
    },

    resolve: function(arg){
        return new Promise(function(resolve,reject){
            resolve(arg);
        });
    },

    reject: function(arg){
        return new Promise(function(resolve,reject){
            reject(arg);
        });
    },

    catch: function(fail){
        return this.then(undefined,fail);
    }
}

//用Promise.then封裝串行異步函數(shù)
var res=new Promise(function(resolve){
    console.log(1);
    resolve(2);
}).then(function(num){
    return new Promise(function(resolve,reject){
        setTimeout(function(){
            console.log(num);
            resolve(num);
        },1000);
    });
}).then(function(num){
    return new Promise(function(resolve,reject){
        setTimeout(function(){
            console.log(num);
            resolve(num);
        },1000);
    });
});

//狀態(tài)已改變，則直接傳入結(jié)果或錯誤
res.then(function(num){
    console.log(num);
});

console.log(3);