FullCalendarDemo.java

global class FullCalendarDemo{

    @RemoteAction
    global static List<User_Calendar__c> getEvents() {
        String userId = UserInfo.getUserId() ;
        List<User_Calendar__c> userCalendar = [SELECT Id, Name, User__r.Name,
                                               Event__c, Event__r.Name, Event__r.Start_Time__c, Event__r.End_Time__c
                                               FROM User_Calendar__c
                                               WHERE User__c=:userId];
        if(userCalendar.size()>0){
	        return userCalendar;
        } else{
            return null;
        }
    }
    
    @RemoteAction
    global static List<User_Calendar__c> createOrUpdateEvents(String calendarEvent) {
        //Geting Calendar_Event__c object from received Json String
        Calendar_Event__c eventObject = ((Calendar_Event__c) JSON.deserialize(calendarEvent, Calendar_Event__c.class));
        //you need to put proper regex to validate recordId
        Boolean isOldRecord = eventObject.Id != null;
        upsert eventObject;
        //If its new record, create User Calendar record as well
        if(!isOldRecord){
            User_Calendar__c userCalendar = new User_Calendar__c(Event__c=eventObject.Id, User__c=UserInfo.getUserId());
            insert userCalendar;
        }
        return getEvents();
    }
}