Promise理解与实现

摘要：对象设置初始状态默认为等待状态用数组来保存成功函数设置为成功状态设置为失败状态调用回调函数增加方法根据状态执行成功失败方法执行成功方法执行失败方法将成功失败方法保存在数组里

promise的意思是承诺。承诺理解为某个时候一些条件满足后，会兑现一件事情。

//为了方便理解我编一个小故事
//先假装我有一个女朋友
//她承诺如果她爸妈不回来就给我就可以去帮她修电脑 否则...
let promise = new Promise(function (success,fail) {
    //她说三秒后会告诉我今晚爸妈回不回来
    let herParentBack = Math.random() > 0.5 ? true : false;
    setTimeout(function(){
        if(herParentBack){
            success("我马上过来来帮你修你电脑");
        }else{
            fail("你要早点休息");
        }
    },3000);

});
 
 //第一个函数接收sucess函数传来的成功信息
 //第二个函数接收fail函数传来的失败信息

promise.then(function (sucessMsg) {
    console.log(sucessMsg);
},function (fail) {
    console.log(failMsg);
})
//我马上过来帮你修电脑

new Promise会传入一个回调函数，会伴着对象创建被立即调用。
这个function是承诺的主体内容。一般我们会做一些判断或者异步请求。（在这里要等女友回消息，我想好怎么回答她。）

promise.then方法在promise执行了success或fail后会执行对应的成功失败方法。 （这里理解为吧想好告诉她）

根据promis规范promise Promise/A

Promise设置了三个状态，"pending"、"resolved"、"rejected"。

//Promise对象设置初始状态
function Promise(callback) {
    var self = this;
    //默认为等待状态
    self.status = "pending";
    self.value = undefined;
    self.reason = undefined;
    //用数组来保存成功函数
    self.onResolvedCallBacks = [];
    self.onRejectedCallbacks = [];

    function resolve(value){
        if(self.status === "pending"){
            //设置为成功状态
            self.status = "resolved";
            self.value = value;
            self.onResolvedCallBacks.forEach(item=>item(self.value));
        }
    }
    function reject(reason) {
        if(self.status === "pending"){
            //设置为失败状态
            self.status = "rejected";
            self.reason = reason;
            self.onRejectedCallbacks.forEach(item=>item(self.reason));
        }
    }

    callback(resolve,reject);
    //调用Promise回调函数
}

module.exports = Promise;

增加Promise.then方法

//根据Promise状态执行成功失败方法
Promise.prototype.then = function (onFulfilled,onRejected) {
    let self = this;
    onFulfilled = typeof onFulfilled == "function"?onFulfilled:function(value){return value};
  onReject = typeof onReject=="function"?onReject:function(reason){throw reason;}
    if(self.status === "resolved"){
        return new Promise(function (resolve,reject) {
            try {
                let x = onFullFilled(self.value);
                if(x instanceof Promise){
                    x.then(resolve,reject);
                }else{
                    resolve(x);
                }
            }
            catch(e) {
                reject(e);
            }
        })
        //执行成功方法
    }else if(self.status == "rejected"){
        return new Promise(function (resolve,reject) {
            try {
                let x = onRejected(self.reason);
                if(x instanceof Promise){
                    x.then(resolve,reject);
                }else{
                    resolve(x);
                }
            }
            catch(e) {
                reject(e)
            }
        })
        //执行失败方法
    }
    if(self.status === "pending"){
        return new Promise(function (reslove,reject) {
            self.onResolvedCallBacks.push(function () {
                let x = onFullFilled(self.value);
                if(x instanceof Promise){
                    x.then(resolve,reject);
                }else{
                    resolve(x);
                }
            })
            self.onRejectedCallbacks.push(function () {
                let x = onRejected(self.reason);
                if(x instanceof Promise){
                    x.then(resolve,reject);
                }else{
                    resolve(x);
                }
            })
        })
        //将成功失败方法保存在数组里
    }
}

Promise.all

Promise.all = all;
function all(iterable) {
  var self = this;
  if (!isArray(iterable)) {
    return this.reject(new TypeError("must be an array"));
  }

  var len = iterable.length;
  var called = false;
  if (!len) {
    return this.resolve([]);
  }

  var values = new Array(len);
  var resolved = 0;
  var i = -1;
  var promise = new this(INTERNAL);

  while (++i < len) {
    allResolver(iterable[i], i);
  }
  return promise;
  function allResolver(value, i) {
    self.resolve(value).then(resolveFromAll, function (error) {
      if (!called) {
        called = true;
        doReject(promise, error);
      }
    });
    function resolveFromAll(outValue) {
      values[i] = outValue;
      if (++resolved === len && !called) {
        called = true;
        doResolve(promise, values);
      }
    }
  }
}

Promise.race

Promise.race = function(iterable) {
  var self = this;
  if (!isArray(iterable)) {
    return this.reject(new TypeError("must be an array"));
  }

  var len = iterable.length;
  var called = false;
  if (!len) {
    return this.resolve([]);
  }

  var i = -1;
  var promise = new this(INTERNAL);

  while (++i < len) {
    resolver(iterable[i]);
  }
  return promise;
  function resolver(value) {
    self.resolve(value).then(function (response) {
      if (!called) {
        called = true;
        doResolve(promise, response);
      }
    }, function (error) {
      if (!called) {
        called = true;
        doReject(promise, error);
      }
    });
  }
}