stepEngine - simplify everyauth asynchronous resolution

stepEngine.js

var fs = require('fs'),
  http = require('http');

var Promise = function(values) {
  this._callbacks = [];
  this._errbacks = [];
  if (arguments.length > 0) {
    this.fulfill.apply(this, values);
  }
}
Promise.prototype = {
    callback: function(fn, scope) {
      //已有values表示promise已fulfill，立即执行
      if (this.values) {
        fn.apply(scope, this.values);
        return this;
      }
      this._callbacks.push([fn, scope]);
      return this;
    }
  , errback: function(fn, scope) {
      if (this.err) {
        fn.call(scope, this.err);
        return this;
      }
      this._errbacks.push([fn, scope]);
      return this;
    }
  , fulfill: function () {
      if (this.isFulfilled || this.err) return;
      this.isFulfilled = true;
      var callbacks = this._callbacks;
      this.values = arguments;
      for (var i = 0, l = callbacks.length; i < l; i++) {
        callbacks[i][0].apply(callbacks[i][1], arguments);
      }
      return this;
    }
  , fail: function (err) {
      var errbacks = this._errbacks;
      for (var i = 0, l = errbacks.length; i < l; i++) {
        errbacks[i][0].call(errbacks[i][1], err);
      }
      return this;
    }
}

var Step = function(name, _engine) {
  this.name = name;
  this.engine = _engine;
}

Step.prototype = {
    exec : function(seq) {
      var args = this._unwrapArgs(seq)
        ,  promises = this.promises

      var ret = this.engine[this.name]().apply(this.engine, args);

      //若函数返回不是Promise，即是函数里直接返回值无异步操作。为流程一致，包装一个立即执行的promise
      ret = (ret instanceof Promise)
          ?  ret
          : this.engine.Promise([ret]);

      ret.callback(function() {
        var returned = arguments
          , vals = seq.values;
        //step执行结束后把返回值写入seq.values供下一个step使用
        if (promises !== null) promises.forEach( function (valName, i) {
          vals[valName] = returned[i];
        });
      })

      //加上默认的错误回调方法
      ret.errback(this.engine.errback(), this.engine);
      return ret;
    }
  , _unwrapArgs: function (seq) {
      if (!this.accepts) return [];
      return this.accepts.reduce( function (args, accept) {
      	//根据accept名取出对应变量
        args.push(seq.values[accept]);
        return args;
      }, []);
    }
}

var Sequence = function(name, engine) {
  this.name = name;
  this.engine = engine;
  this.stepNames = [];
  this.values = {};
}

Sequence.prototype = {
    _bind : function(priorPromise, nextStep) {
      var nextPromise = new Promise()
        , seq = this;

      priorPromise.callback( function () {
        var resultPromise = nextStep.exec(seq);
        resultPromise.callback(function () {
          nextPromise.fulfill();
        });
      });
      return nextPromise;
    }

  , start : function() {
      var steps = this.steps;
      var priorStepPromise = steps[0].exec(this);

      for (var i = 1, l = steps.length; i < l; i++) {
      	//绑定step链
        priorStepPromise = this._bind(priorStepPromise, steps[i]);
      }
      return priorStepPromise;
    }
}

Object.defineProperty(Sequence.prototype, 'steps', {
  get: function () {
      var allSteps = this.engine._steps;
      return this.stepNames.map(function (stepName) {
        return allSteps[stepName];
      })
  }
});


var engine = {
    configurable : function(name) {
      this[name] = function(setTo) {
        var k = '_' + name;
        if (arguments.length) {
          this[k] = setTo;
          return this;
        }
        return this[k];
      }
      return this;
    }
  , step : function(name) {
      var steps = this._steps
        ,  sequence = this._currSeq;

      sequence.stepNames.push(name);
      this._currentStep = 
        steps[name] || (steps[name] = new Step(name, this));

      this.configurable(name);  
      return this;
    }
  , accepts : function(input) {
      this._currentStep.accepts = input && input.split(/\s+/)  || null;
      return this;
    }
  , promises : function(output) {
      this._currentStep.promises = output && output.split(/\s+/)  || null;
      return this;
    }
  , do : function(name) {
      this._currSeq =
        this._stepSequences[name] || (this._stepSequences[name] = new Sequence(name, this));
      return this;
    }
  , Promise : function(values) {
      return values ? new Promise(values) : new Promise();
    }
  , _stepSequences: {}
  , _steps: {}

  , start : function(seqName) {
      var seq = this._stepSequences[seqName];
      seq.start();
    }
}

engine
  .configurable('errback')
  .errback(function(err) {
    console.log('errback', err);
  });

engine
  .do('fetchHtml')
    .step('readUrl')
      .accepts('')
      .promises('url')
    .step('getHtml')
      .accepts('url')
      .promises('html')
    .step('saveHtml')
      .accepts('url html')
      .promises(null)
  
  .readUrl(function(){
    var p = this.Promise();
    //url.txt保存了一个网址
    fs.readFile('./url.txt', 'utf8', function (err,data) {
      if (err) p.fail(err);
      else p.fulfill(data);
    });
    return p;
  })

  .getHtml(function(url){
    var p = this.Promise();
    http.get(url, function(res){
      var body = '';
      res.on('data', function(c){
        body += c;
      }).on('end', function(){
        p.fulfill(body);
      });
    }).on('error', function(err){
      p.fail(err)
    });
    return p;
  })

  .saveHtml(function(url, html){
    fs.writeFile('./fetchResult', url + html, function(e) {
      if (e) console.log('error', e);
      else console.log('done');
    });
  })


  .start('fetchHtml')