Sign up a user with a unique username in Firebase

SignupViewModel.m

- (void)signUpUser:(NSString *)username password:(NSString *)password email:(NSString *)email completion:(void (^)(FIRUser *user,  NSError *error))completion {
    [[FIRAuth auth]
     createUserWithEmail:email
     password:password
     completion:^(FIRUser *user, NSError *error) {
         FIRDatabaseReference *ref = [[FIRDatabase database] reference];
         FIRDatabaseReference *userRef = [[ref child:@"users"] child:username];
         [userRef observeSingleEventOfType:FIRDataEventTypeValue withBlock:^(FIRDataSnapshot *snapshot) {
             if (snapshot && snapshot.value[@"uid"] == nil) {
                 [userRef setValue:user.uid forKey:@"uid"];
             }
         }
         withCancelBlock:^(NSError * _Nonnull error) {
             NSLog(@"%@", error.localizedDescription);
         }];
     }];
}