var isGettingGroup = false
func getGroup(completion: (group?, error?)) {

    if isGettingGroup {
        wait for notification
    }
    if let group = groupFromCache() {
        completion(group, nil)
    } else {
        isGettingGroup = true
        callGetGroupAPI() { group, error in
            completion(group, error)
            cacheGroup(group)
            isGettingGroup = false
            send notification to continue
        }
    }
}