PFUser *currentUser = [PFUser currentUser];

if (currentUser) {

// do stuff with the user
} else {

// show the signup or login screen
}

[PFUser logInWithUsernameInBackground:@"myname" password:@"mypass" block:(PFUser user, NSError error) {

if (user) {
    // Do stuff after successful login.
} else {
    // The login failed. Check error to see why.
}
}];

// Sent to the delegate when a PFUser is logged in.
- (void)logInViewController:(PFLogInViewController *)logInController didLogInUser:(PFUser    *)user {
[self dismissViewControllerAnimated:YES completion:NULL];
[self startMyApplication];  //example