One login view, sign up or login - iphone

Question

I want to be able to have just one login screen. The user will enter the username, email and password every time they login. I am having the problem where I sign up, logout and then try to login using the same credentials and I just get an error back saying that the username is already taken. How can I just log them in if they give me the correct credentials?
PFUser *user = [PFUser user];
user.email = emailEntry;
user.username = nickNameEntry;
user.password = passwordEntry;
[user signUpInBackgroundWithBlock:^(BOOL succeeded, NSError *error)
{
if (!error)
{
[MYAlertView showAlertWithTitle:#"Successful login"
message:#"success"
cancelButtonTitle:#"OK"];
}
else
{
NSString *errorString = [[error userInfo] objectForKey:#"error"];
[MyAlertView showAlertWithTitle:#"There was an error signing up."
message:errorString
cancelButtonTitle:#"OK"];
}
}];

Answer 1

If user needs to sign up (first time), you need to use –signUpInBackgroundWithBlock:, as you are doing.
However, if he is already signed up, I guess you should use +logInWithUsernameInBackground:password:block:.
I'm getting this infos from this page, however, I never used this SDK.
The easier way to do that is :
Having two buttons, that pushes different view controllers, one for signing up, the other for logging in
The same form, and two buttons, one which calls a methods that calls –signUpInBackgroundWithBlock:, and the other that calls +logInWithUsernameInBackground:password:block:.
With the second solution, the problem is that if you have the same form, you can't ask special informations (name, date of birth, etc.) on sign up.
EDIT :
You might want to have a look at :
PFSignUpViewControllerDelegate,
PFLoginViewControllerDelegate,
Parse API Documentation (where I've found the two links above).