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Value of String won't change in Swift despite clear declaration [duplicate]

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I am making an app in which I have to return a String representing the existence of a certain reference to my firebase realtime database (I am not using a Bool for other reasons). Despite executing the print commands directly above and below, when I print the function, the value of the String won't change and prints its default value "Unchanged".

My code:

func checkIfMovieHasAlreadyBeenShown(movieID: String) -> String

var hasItBeenSeen = "Unchanged"
let uid = Auth.auth().currentUser?.uid
let ref = Database.database().reference()
ref.child("Seen Movies").child(uid!).observeSingleEvent(of: .value, with: (snapshot) in

if snapshot.hasChild(movieID)
print("The Result:")
self.changeStringHasItBeenSeen(YNString: "yes")
print("YES")


else
print("The Result:")
self.changeStringHasItBeenSeen(YNString: "no")
print("NO")

)

return hasItBeenSeen



It is printed here:

print(checkIfMovieHasAlreadyBeenShown(movieID: "39"))
print(checkIfMovieHasAlreadyBeenShown(movieID: "38"))


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return hasItBeenSeen

1 Answer
1

observeSingleEvent is called in a closure. The return hasItBeenSeen line will not be affected by anything in the closure, as it will return first.

observeSingleEvent

return hasItBeenSeen

Imagine this: you tell your friend to go buy some apples. Before he has even left, you ask him for the price. Of course, he won't know, because he hasn't gone to buy the apples yet.

You need some way of performing something AFTER the closure has been called (or after the apples have been bought). Usually this can be done with a completion handler, where you pass a closure into your checkIfMovieHasAlreadyBeenShown function, such as, "do this AFTER you buy the apples".

checkIfMovieHasAlreadyBeenShown

Then you would call your function this way: checkIfMovieHasAlreadyBeenShown(movieID: "39", completion: seen in print(seen) )

checkIfMovieHasAlreadyBeenShown(movieID: "39", completion: seen in print(seen) )