tinyEmitter

function tinyEmitter() {
}

tinyEmitter.prototype.on = function(evtype, fn) {
  this._events = this._events || {};
  this._events[evtype] = this._events[evtype] || [];
  if (this._events[evtype].indexOf(fn) !== -1) return;
  this._events[evtype].push(fn);
}
tinyEmitter.prototype.off = function(evtype, fn) {
  this._events = this._events || {};
  if (evtype in this._events === false)  return;
  fn && this._events[evtype].splice(this._events[evtype].indexOf(fn), 1);
  !fn && delete this._events[evtype];
}
tinyEmitter.prototype.emit = function(evtype) {
  this._events = this._events || {};
  var self = this, args = [].slice.call(arguments, 1);
  if (evtype in this._events === false) return;
  this._events[evtype].forEach(function(fn, i) {
      fn.apply(self, args);
  });
}

tinyEmitter.eventify = function(Klass) {
    Klass.prototype = Object.create(Klass.prototype);
    for (var attr in tinyEmitter.prototype) {
        Klass.prototype[attr] = tinyEmitter.prototype[attr]
    }
    return Klass;
}

tinyScroll

function tinyScroll() {
}
tinyEmitter.eventify(tinyScroll);

tinyScroll.prototype.init = function() {
  var self  = this;
  self.checkDir();
}

tinyScroll.prototype.checkDir = function(e) {
  var self = this, scrollTop = self.scrollTop = self.scrollTop || document.body.scrollTop;

  if (scrollTop > document.body.scrollTop) {
    self.emit('up', e);
  }
  if (scrollTop < document.body.scrollTop) {
    self.emit('down', e);
  }
  self.emit('scroll', e);
}


// use
var s = new tinyScroll();

s.on('down', function() {
  console.log('down...');
});