如何在此块中返回布尔值？

Question

如果用户输入无效的登录凭据，我不希望该函数返回false。我做了__block BOOL但是没有用。我看了一下这篇文章Return value for function inside a block 我不明白如何实现回调。在这种情况下，它会如何工作？

- (BOOL)shouldPerformSegueWithIdentifier:(NSString *)identifier sender:(id)sender {

__block BOOL ret=YES;

NSString *phoneNumberField = [self.phoneNumberField.text stringByTrimmingCharactersInSet:
                              [NSCharacterSet whitespaceAndNewlineCharacterSet]];
NSString *passwordField = [self.passwordField.text stringByTrimmingCharactersInSet:
                           [NSCharacterSet whitespaceAndNewlineCharacterSet]];

PFUser *newUser = [PFUser user];
newUser.username = phoneNumberField;
newUser.password = passwordField;

if ([identifier isEqualToString:@"Verify"]) {

    if([newUser.username length] == 0 || [newUser.password length] == 0) {
        UIAlertView *alertView = [[UIAlertView alloc] initWithTitle:@"Oops! Sorry"
                                                            message:@"Please make sure you enter both a phone number and a password!" delegate: nil cancelButtonTitle:@"OK" otherButtonTitles:nil];
        [alertView show];
        return NO;
    }
    else
    {
        //PROBLEM IS BELOW
        [newUser signUpInBackgroundWithBlock:^(BOOL succeeded, NSError * _Nullable error) {
            if(!error)
            {
                ret = YES;
                NSLog(@"Successful login");
            }
            else
            {
                ret = NO;
                UIAlertView *alertView = [[UIAlertView alloc] initWithTitle:@"Sorry!" message:@"That phone number is already in use" delegate:nil cancelButtonTitle:@"Ok" otherButtonTitles:nil];
                [alertView show];
            }
        }];
    }
    return ret;
}  else {
    return YES;
}
}

Return value for function inside a block