FormValidator::doValidateForm

protected FormValidator::doValidateForm(&$elements, FormStateInterface &$form_state, $form_id = NULL)

protected function doValidateForm(&$elements, FormStateInterface &$form_state, $form_id = NULL) {
  // Recurse through all children.
  foreach (Element::children($elements) 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
      // string '0', which could be a valid value.
      $is_empty_multiple = (!count($elements['#value']));
      $is_empty_string = (is_string($elements['#value']) && Unicode::strlen(trim($elements['#value'])) == 0);
      $is_empty_value = ($elements['#value'] === 0);
      if ($is_empty_multiple || $is_empty_string || $is_empty_value) {
        // Flag this element as #required_but_empty to allow #element_validate
        // handlers to set a custom required error message, but without having
        // to re-implement the complex logic to figure out whether the field
        // value is empty.
        $elements['#required_but_empty'] = TRUE;
      }
    }

    // Call user-defined form level validators.
    if (isset($form_id)) {
      $this->executeValidateHandlers($elements, $form_state);
    }
    // Call any element-specific validators. These must act on the element
    // #value data.
    elseif (isset($elements['#element_validate'])) {
      foreach ($elements['#element_validate'] as $callback) {
        $complete_form = &$form_state->getCompleteForm();
        call_user_func_array($form_state->prepareCallback($callback), array(&$elements, &$form_state, &$complete_form));
      }
    }

    // Ensure that a #required form error is thrown, regardless of whether
    // #element_validate handlers changed any properties. If $is_empty_value
    // is defined, then above #required validation code ran, so the other
    // variables are also known to be defined and we can test them again.
    if (isset($is_empty_value) && ($is_empty_multiple || $is_empty_string || $is_empty_value)) {
      if (isset($elements['#required_error'])) {
        $form_state->setError($elements, $elements['#required_error']);
      }
      // A #title is not mandatory for form elements, but without it we cannot
      // set a form error message. So when a visible title is undesirable,
      // form constructors are encouraged to set #title anyway, and then set
      // #title_display to 'invisible'. This improves accessibility.
      elseif (isset($elements['#title'])) {
        $form_state->setError($elements, $this->t('@name field is required.', array('@name' => $elements['#title'])));
      }
      else {
        $form_state->setError($elements);
      }
    }

    $elements['#validated'] = TRUE;
  }

  // Done validating this element, so turn off error suppression.
  // self::doValidateForm() turns it on again when starting on the next
  // element, if it's still appropriate to do so.
  $form_state->setLimitValidationErrors(NULL);
}