Make UI Wait for Method Response - swift

Question

I am a new swift developer. I am using Swift 4.2 and Xcode 10.2.
I need my UI to wait until a method has finished so I can use the result to display a balance. I am trying to use a dispatchGroup for this, but it does not appear to be waiting because the value of user?.userId below is nil. Here is my code:
// Load the local user data. Must wait until this is done to continue.
let dispatchGroup = DispatchGroup()
dispatchGroup.enter()
let user = LocalStorageService.loadCurrentUser()
dispatchGroup.leave()
// Display the current balance.
// Get a reference to the Firestore database.
let db = Firestore.firestore()
// Make sure we have a userId and then update the balance with a listener that keeps it updated.
// Only run this part when the dispatchGroup has completed (in this case, the user is loaded).
dispatchGroup.notify(queue: .main) {
if let userId = user?.userId {
db.collection("subs").whereField("ID", isEqualTo: userId)
.addSnapshotListener { querySnapshot, error in
// Make sure we have a document
guard let document = querySnapshot?.documents.first else {
print("Error fetching document: \(error!)")
return
}
// We have a document and it has data. Use it.
self.balance = document.get("balance") as! Double
// Format the balance
let currencyFormatter = NumberFormatter()
currencyFormatter.numberStyle = .currency
let balanceString = currencyFormatter.string(from: self.balance as NSNumber)
self.balanceLabel.setTitle(balanceString, for: .normal)
}
}
}
How can I make the UI wait until the method called in dispatchGroup.enter() has completed?
Here's what's in LoadCurrentUser....
static func loadCurrentUser() -> User? {
// Loads the current user in the UserDefaults if there is one
let defaults = UserDefaults.standard
let userId = defaults.value(forKey: Constants.LocalStorage.storedUserId) as? String
let phoneNumber = defaults.value(forKey: Constants.LocalStorage.storedPhoneNumber) as? String
let subscriberId = defaults.value(forKey: Constants.LocalStorage.storedDocumentId) as? String
guard userId != nil && phoneNumber != nil && subscriberId != nil else {
return nil
}
// Return the user
let u = User(userId:userId!, phoneNumber:phoneNumber!, subscriberId: subscriberId)
return u
}

Answer 1

Currently you do it correctly by setting vars inside the callback so no need for DispatchGroup , but to correctly use it then do ( notice the correct place where each line should be by numbers from 1 to 4 )
let dispatchGroup = DispatchGroup() /// 1
let user = LocalStorageService.loadCurrentUser()
// Display the current balance.
// Get a reference to the Firestore database.
let db = Firestore.firestore()
var balance = ""
// Make sure we have a userId and then update the balance with a listener that keeps it updated.
// Only run this part when the dispatchGroup has completed (in this case, the user is loaded).
if let userId = user?.userId {
dispatchGroup.enter() /// 2
db.collection("subs").whereField("ID", isEqualTo: userId)
.addSnapshotListener { querySnapshot, error in
// Make sure we have a document
guard let document = querySnapshot?.documents.first else {
print("Error fetching document: \(error!)")
return
}
// We have a document and it has data. Use it.
self.balance = document.get("balance") as! Double
dispatchGroup.leave() /// 3
}
}
dispatchGroup.notify(queue: .main) { /// 4
//update here
// Format the balance
let currencyFormatter = NumberFormatter()
currencyFormatter.numberStyle = .currency
let balanceString = currencyFormatter.string(from: self.balance as NSNumber)
self.balanceLabel.setTitle(balanceString, for: .normal)
}