如何让textField的输入框只能输入特定字符

- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string   // return NO to not change text

{
    

    
    NSCharacterSet *cs;
    cs = [[NSCharacterSet characterSetWithCharactersInString:@"123"]invertedSet];
    
    NSString *filtered = [[string componentsSeparatedByCharactersInSet:cs]componentsJoinedByString:@""]; //按cs分离出数组,数组按@""分离出字符串
    BOOL canChange = [string isEqualToString:filtered];
    
    if (![string isEqualToString:@""]) {
        if ([filtered isEqualToString:@""]) {

            //弹出一个提示，提示只能输入123，不能输入以外的字符
            
        }
    }
    
    return canChange;

}