浅谈Node异步编程的机制(4)

var Deferred = function(){
  this.promise = new Promise()
}

//完成态
Deferred.prototype.resolve = function(obj){
  var promise = this.promise;
  var handler;
  while((handler = promise.queue.shift())){
    if(handler && handler.fulfilled){
      var ret = handler.fulfilled(obj);
      if(ret && ret.isPromise){
        ret.queue = promise.queue;
        this.promise = ret;
        return;
      }
    }
  }
}

//失败态
Deferred.prototype.reject = function(err){
  var promise = this.promise;
  var handler;
  while((handler = promise.queue.shift())){
    if(handler && handler.error){
      var ret = handler.error(err);
      if(ret && ret.isPromise){
        ret.queue = promise.queue;
        this.promise = ret;
        return
      }
    }
  }
}

//生成回调函数
Deferred.prototype.callback = function(){
  var that = this;
  return function(err,file){
    if(err){
      return that.reject(err);
    }
    that.resolve(file)
  }
}

var Promise = function(){
  this.queue = []; //队列用于存储待执行的回到函数
  this.isPromise = true;
};
Promise.prototype.then = function(fulfilledHandler,errorHandler,progressHandler){
  var handler = {};
  if(typeof fulfilledHandler === 'function'){
    handler.fulfilled = fulfilledHandler;
  }
  if(typeof errorHandler === 'function'){
    handler.error = errorHandler;
  }
  this.queue.push(handler);
  return this;
}

var readFile1 = function(file,encoding){
  var deferred = new Deferred();
  fs.readFile(file,encoding,deferred.callback());
  return deferred.promise;
}
var readFile2 = function(file,encoding){
  var deferred = new Deferred();
  fs.readFile(file,encoding,deferred.callback());
  return deferred.promise;
}

readFile1('file1.txt','utf8').then(function(file1){
  return readFile2(file1.trim(),'utf8')
}).then(function(file2){
  console.log(file2)
})