Code after guard is called later than expected - ios

Question

Hi I am new in iOS development and I am having hard time to understand the following issue. Basically I am trying to get user's name by passing current user's id to Cloud Firestore. However I am having hard time to understand a bug in the code. I can successfully pass the name of user to name variable, while the function returns default value of name which is "" (empty string). It seems that the block of code inside
if let data = snapshot?.data() {
guard let userName = data["name"] as? String else { return }
name = userName
print("after guard") // this line
}
happens later than
print("name") // this line
return name
Full code:
private func returnCurrentUserName() -> String {
// User is signed in.
var name = ""
if let user = Auth.auth().currentUser {
let db = Firestore.firestore()
db.collection("users").document(user.uid).getDocument { (snapshot, error) in
if error == nil {
if let data = snapshot?.data() {
guard let userName = data["name"] as? String else { return }
name = userName
print("after guard") // this line
}
}
}
print("name") // this line
return name
}else {
return ""
}
}
(Note: the query from Cloud Firestore is successful and I can get users name on the console but "name" is printed after "after guard".)

Answer 1

In addition to the other answer:
If you would like to execute code after your operation is done, you could use a completion block (that's just a closure which gets called upon completion):
private func returnCurrentUserName(completion: #escaping () -> ()) -> String {
// User is signed in.
var name = ""
if let user = Auth.auth().currentUser {
let db = Firestore.firestore()
db.collection("users").document(user.uid).getDocument { (snapshot, error) in
if error == nil {
if let data = snapshot?.data() {
guard let userName = data["name"] as? String else { return }
name = userName
completion()//Here you call the closure
print("after guard") // this line
}
}
}
print("name") // this line
return name
}else {
return ""
}
}
How you would call returnCurrentUserName:
returnCurrentUserName {
print("runs after the operation is done")
}
Simplified example:
func returnCurrentUserName(completion: #escaping () -> ()) -> String {
DispatchQueue.main.asyncAfter(deadline: .now() + 4) {
completion() //runs after 4 seconds
}
return "xyz"
}
let test = returnCurrentUserName {
print("runs after the operation is done")
}
print(test)

Answer 2

The reason is your getDocument is an asynchronous operation. It takes a callback, and that callback will be invoked when the operation is done. Because of the asynchronous operation, the program will continue process the next line without waiting for the async operation to be completed. That's why you see your print("name") getting executed before the print("after guard")