performRequiredValidation example

    $elements_sorted = $elements;
    foreach (Element::children($elements_sorted, TRUE) as $key) {
      if (isset($elements[$key]) && $elements[$key]) {
        $this->doValidateForm($elements[$key], $form_state);
      }
    }

    // Validate the current input.     if (!isset($elements['#validated']) || !$elements['#validated']) {
      // The following errors are always shown.       if (isset($elements['#needs_validation'])) {
        $this->performRequiredValidation($elements, $form_state);
      }

      // Set up the limited validation for errors.       $form_state->setLimitValidationErrors($this->determineLimitValidationErrors($form_state));

      // Make sure a value is passed when the field is required.       if (isset($elements['#needs_validation']) && $elements['#required']) {
        // A simple call to empty() will not cut it here as some fields, like         // checkboxes, can return a valid value of '0'. Instead, check the         // length if it's a string, and the item count if it's an array.         // An unchecked checkbox has a #value of integer 0, different than