Download single Object of Firestore and save it into an struct/class object - swift

Question

I am coding since January 2019 and this is my first post here.
I am using Swift and Firestore. In my App is a tableView where I display events loaded out of a single Document with an array of events inside as [String: [String:Any]]. If the user wants to get more infos about an event he taps on it. In the background the TableViewController will open a new "DetailEventViewController" with a segue and give it the value of the eventID in the tapped cell.
When the user is on the DetailViewController Screen the app will download a new Document with the EventID as key for the document.
I wanna save this Data out of Firestore in a Struct called Event. For this example just with Event(eventName: String).
When I get all the data I can print it directly out but I can't save it in a variable and print it out later. I really don't know why. If I print the struct INSIDE the brackets where I get the data its working but if I save it into a variable and try to use this variable it says its nil.
So how can I fetch data out of Firestore and save in just a Single ValueObject (var currentEvent = Event? -> currentEvent = Event.event(for: data as [String:Any]) )
I search in google, firebaseDoc and stackoverflow but didn't find anything about it so I tried to save all the singe infos of the data inside a singe value.
// Struct
struct Event {
var eventName: String!
static func event(for eventData: [String:Any]) -> Event? {
guard let _eventName = eventData["eventName"] as? String
else {
print("error")
return nil
}
return Event(eventName: _eventName)
}
// TableView VC this should work
override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
if segue.identifier == "ShowEventDetailSegue" {
if let ShowEvent = segue.destination as? DetailEventViewController, let event = eventForSegue{
ShowEvent.currentEventId = event.eventID
}
}
}
// DetailViewController
var currentEvent = Event()
var currentEventId: String?
override func viewWillAppear(_ animated: Bool) {
super.viewWillAppear(animated)
guard let _eventID = currentEventId else {
print("error in EventID")
return}
setupEvent(eventID: _eventID) /* currentEvent should be set here */
setupView(event: currentEvent) /* currentEvent has after "setupEvent" the value of nil */
}
func setupEvent(eventID: String) {
let FirestoreRef = Firestore.firestore().collection("events").document(eventID)
FirestoreRef.getDocument { (document, error) in
if let err = error {
debugPrint("Error fetching docs: \(err)")
SVProgressHUD.showError(withStatus: "Error in Download")
}else {
if let document = document, document.exists {
guard let data = document.data() else {return}
let eventData = Event.event(for: data as [String:Any])
print(eventData)
//here all infos are printed out - so I get them
self.currentEvent = eventData!
//Here is the error.. I can't save the fetched Data in my single current Event
} else {
SVProgressHUD.showError(withStatus: "Error")
}
}
}
}
func setupView(event: Event) {
self.titleLabel.text = event.eventName
}
I expect that the function setupEvents will give the currentEvent in the DetailViewController a SINGLEvalue cause its a SINGLE document not an array. So I can use this single Eventvalue for further actions. Like starting a new segue for a new ViewController and just push the Event there not