#pragma mark - UITextFieldDelegate
- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string {
        // 四位加一个空格
        if ([string isEqualToString:@""]) { // 删除字符
            if ((textField.text.length - 2) % 5 == 0) {
                textField.text = [textField.text substringToIndex:textField.text.length - 1];
            }
            return YES;
        } else {
            if (textField.text.length % 5 == 0) {
                textField.text = [NSString stringWithFormat:@"%@ ", textField.text];
            }
        }
        return YES;
}