Koa "yield next" simple implement

app.js

'use strict';

var co = require('co');

function App () {
  this.gens = [];
}

App.prototype.use = function (gen) {
  this.gens.push(gen);
};

App.prototype.init = function () {
  var self = this;

  var gens = self.gens;

  var fn = gens[gens.length - 1];
  for (var i = gens.length - 2; i >= 0; i--) {
    fn = gens[i].call(self, fn);
  }
  
  var p = co(fn);

  p.then(function () {
    console.log('do something after all done...');
  });
};

// use
var app = new App();

app.use(function* fn1 (next){
  console.log('>> Run A');
  yield next;
  console.log('<< Run A');
});

app.use(function* fn2 (next){
  console.log('>> Run B');
  yield next;
  console.log('<< Run B');
});

app.use(function* fn3 (next){
  console.log('>> Run C');
  yield next;
  console.log('<< Run C');
});

app.use(function* fn4 (next){
  console.log('>> Run D');
});

// run
app.init();
// return:
// >> Run A
// >> Run B
// >> Run C
// >> Run D
// << Run C
// << Run B
// << Run A