Wait for completion handler to finish before returning

Question

    func checkUsername(username: String) -> Bool {
        var avalible = true
        
        Database.database().reference().child("usernames").child(username).observeSingleEvent(of: .value, with: { snapshot in
            if snapshot.exists() {
                print("exists")
                avalible = false
            }
        })
        
        return available
    }

I'm trying to check if a username exists.
I've already tried multiple things, but it seems like the function always returns before the completion handler even finishes and always gives the same output (true) even if the username is already taken.

Does somebody have an idea how to „wait“ for the completion handler first to finish before returning the „available“ variable?

An explanation with the answer would be excellent.
Thanks.

score 2 · Accepted Answer · answered Sep 29 '22 at 14:34

You cannot return something from an asynchronous task. Your code – with fixed typos – returns true even before the database request has been started.

Maybe there is a native async/await version of observeSingleEvent but in any case you can use a Continuation

func checkUsername(username: String) async -> Bool {
    return await withCheckedContinuation { continuation in
        Database.database().reference().child("usernames").child(username).observeSingleEvent(of: .value, with: { snapshot in
            continuation.resume(returning: !snapshot.exists())
        })
    }
}

And call it

Task {
   let isAvailable = await checkUsername(username: "Foo")

}

Thanks, I did try this before but with two different continuations, that's why I was confused and it didn't work. — David, Sep 29 '22 at 14:55

score 0 · Answer 2 · answered Sep 29 '22 at 14:16

This is because the Firebase operation is asynchronous, you cannot simply turn that into a synchronous operation without blocking the thread.

Have a look at How could I create a function with a completion handler in Swift?

The correct way to approach this, without the use of the new async/await syntax, is to use a completion handler yourself.

func checkUsername(username: String, completion: @escaping (Bool) -> ()) {
    Database.database().reference().child("usernames").child(username).observeSingleEvent(of: .value) { snapshot in
         completion(!snapshot.exists())
    }
}

// To use:
checkUsername(username: "foo") { exists in 
    print("exists? ", exists)
}

One does not simply return a result from an async function: https://imgflip.com/i/6v4w6c — Duncan C, Sep 29 '22 at 15:03

Wait for completion handler to finish before returning

2 Answers2