How can I change my value with Firebase

My code give me an infinite loop he never go in if or else, I'm pretty sure that because firebase get only asynchronous function.
I want to check if "random_hexa" exist, and get new random until I get a value who don't exist on my database

while (bool_check_while_exist == false)
    {
        ref.child("Salons").child(random_hexa).observeSingleEventOfType(.Value, withBlock: { (snapshot) in
            if (snapshot.exists())
            {
                random_hexa = self.randomAlphaNumericString(5)
            }
            else
            {
                bool_check_while_exist = true
            }
        })