조건식과 함께 AngularJS ngClass를 효과적으로 사용하려면 어떻게 해야 합니까?

<span ng-class="{test: <expression>}">test</span>

<span ng-class="{test: 'obj.value1 == \'someothervalue\''}">test</span>

<span ng-class="{test: obj.value1 == 'someothervalue'}">test</span>

<span ng-class="{test: checkValue()}">test</span>

$scope.checkValue = function() {
  return $scope.obj.value === 'somevalue';
}

myApp.directive('customConditional', function() {
  return {
    restrict: 'A',
    compile: function(tElement, tAttrs) {
      var expression = tAttrs.customConditional;
      return function postLink(scope, element) {
        scope.$watch(expression, function(newValue) {
          if (newValue) {
            element.addClass('my-class');
          } else {
            element.removeClass('my-class');
          }
        });
      };
    }
  };
});