Marks invalid UITextFields red in iOS. If valid, restores original border.

InvalidTextFields

#import <QuartzCore/QuartzCore.h>

// Form validation. You'll probably place this in your buttonClicked action
if ([textField.text isEqualToString:@""])  // checks if field is empty, add other form validation here
{
  textField.layer.cornerRadius = 8.0f;
  textField.layer.masksToBounds = YES;
  textField.layer.borderColor = [[UIColor redColor] CGColor];
  textField.layer.borderWidth = 1.0f;
}
else
{
  textField.layer.borderColor = [[UIColor clearColor] CGColor];
}


// To check if valid after each character is entered, implement UITextField delegate 
// in your interface file and add this code to your implementation file
#pragma mark - UITextField Delegate
- (BOOL) textField:(UITextField*)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString*)textEntered {
  
  // Do not check if the textField was previously blank
  if (![textField.text isEqualToString:@""]) {
    // perform from validation here instead of on button click
  }

  return YES;
}