1. Event

/**
 * 请实现一个MyEvent类, 继承自此类的对象都会拥有两个方法on, off, once, trigger;
 */
function MyEvent() {
  if (!(this instanceof MyEvent)) {
    return new MyEvent();
  }
  this._callbacks = {};
}
MyEvent.prototype.on = function(type, handler) {
  this_callbacks = this._callbacks || {};
  this._callbacks[type] = this.callbacks[type] || [];
  this._callbacks[type].push(handler);

  return this;
};

MyEvent.prototype.off = function(type, handler) {
  var list = this._callbacks[type];

  if (list) {
    for (var i = list.length; i >= 0; --i) {
      if (list[i] === handler) {
        list.splice(i, 1);
      }
    }
  }

  return this;
};

MyEvent.prototype.trigger = function(type, data) {
  var list = this._callbacks[type];

  if (list) {
    for (var i = 0, len = list.length; i < len; ++i) {
      list[i].call(this, data);
    }
  }
};

MyEvent.prototype.once = function(type, handler) {
  var self = this;

  function wrapper() {
    handler.apply(self, arguments);
    self.off(type, wrapper);
  }
  this.on(type, wrapper);
  return this;
};