观察者模式

var emitter = {
  list: {},
  on: function(key, fn) {
    if(!this.list[key]) {
       this.list[key] = [];
    }
    this.list[key].push(fn);
  },
  emit: function() {
    var key = Array.prototype.shift.call(arguments),
          fns = this.list[key];
    if(!fns && fns.length === 0) {
        for(var i = 0, fn; fn = fns[i++];) {
          fn.apply(this, arguments);
        }
    }
  }
}
var installEmitter = function(obj) {
  for(var item in obj) {
    obj[item] = emitter[item];
  }
}