How to update angular ng-class when a variable changes?

I want my ng-class to update when the variable in my controller changes. For some reason, it is only triggered on the page load and whilst the variable change, the ng-class does not update to reflect this.

Here is the code that triggers the variable change:

<a class="cs-mobile-menu__close-icon" ng-click="switchMobileMenu('right')">
</a>

Here is the code which I want to change depending on the variable:

<div class="right-menu__user"
        ng-class="{
                  'animate': true,
                  'animate--leave': !isRightMenuOpen}">
  {{user.firstName}} {{user.lastName}}
</div>

Here is the corresponding code from the controller:

$rootScope.isLeftMenuOpen = false;
$rootScope.isRightMenuOpen = false;

$scope.switchMobileMenu = function(direction) {
       if (direction === 'left') {
           $scope.isLeftMenuOpen = !($scope.isLeftMenuOpen);
       } else {
           $scope.isRightMenuOpen = !($scope.isRightMenuOpen);
       }
};

It seems that ng-class is only set on the initial ng-click. I believe I need to wrap the above function in a $scope.apply() function so that the ng-class watches for changes and applies them as the ng-click toggles the variable. Am I on the right track? If so, how would I do this?