docs: function observe($scope, $attrs, key, defValue, callbackFunc) {
          if (callbackFunc === void 0) { callbackFunc = null; }
          $attrs.$observe(key, function (value) {
              if (!angular.isDefined(value)) {
                  $scope[key] = defValue;
              }
              else {
                  $scope[key] = value;
              }
              if (angular.isDefined(callbackFunc) && callbackFunc) {
                  callbackFunc($scope[key]);
              }
          });
      }