Server-side controller for Lightning component enabling partner users to follow and unfollow topics

CommunityTopicListItemController.cls

public class CommunityTopicListItemController {

    @AuraEnabled
    public static void subscribe(Id entityId) {
        ConnectApi.Subscription subscription =
            	ConnectApi.ChatterUsers.follow(Network.getNetworkId(), 'me', entityId);
    }
    
    @AuraEnabled
    public static void unsubscribe(Id subscriptionId) {
        ConnectApi.Chatter.deleteSubscription(Network.getNetworkId(), subscriptionId);
    }
    
    @AuraEnabled
    public static EntitySubscription getSubscription(Id entityId) {
        List<EntitySubscription> subscriptions = [
            SELECT Id
            FROM EntitySubscription
            WHERE ParentId = :entityId
            AND SubscriberId = :UserInfo.getUserId()
            AND NetworkId = :Network.getNetworkId()
            LIMIT 1
        ];
        
        return subscriptions.isEmpty() ? null : subscriptions.get(0);
    }
}