Как вводить только цифры в UITextField и ограничить длину строки ввода

#define NUMBERS_ONLY @"1234567890"
#define CHARACTER_LIMIT 4

-(BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string
{
    NSUInteger newLength = [textField.text length] + [string length] - range.length;
    NSCharacterSet *cs = [[NSCharacterSet characterSetWithCharactersInString:NUMBERS_ONLY] invertedSet];
    NSString *filtered = [[string componentsSeparatedByCharactersInSet:cs] componentsJoinedByString:@""];
    return (([string isEqualToString:filtered])&&(newLength <= CHARACTER_LIMIT));
}