whole_func_string: function(event, func) {
      this._events = this._events || {};
      this._events[event] = this._events[event] || [];
      this._events[event].push(func);
    }
func_code_string: function(event, func) {
      this._events = this._events || {};
      this._events[event] = this._events[event] || [];
      this._events[event].push(func);
    }
func_documentation_string: Add custom event listener
  @param  {String} event Event type
  @param  {Function} func   Callback
  @return {Void}