class ConfigFormBase

abstract class ConfigFormBase extends FormBase {
    use ConfigFormBaseTrait;
    
    /**
     * The $form_state key which stores a map of config keys to form elements.
     *
     * This map is generated and stored by ::storeConfigKeyToFormElementMap(),
     * which is one of the form's #after_build callbacks.
     *
     * @see ::storeConfigKeyToFormElementMap()
     *
     * @var string
     */
    protected const CONFIG_KEY_TO_FORM_ELEMENT_MAP = 'config_targets';
    
    /**
     * Constructs a \Drupal\system\ConfigFormBase object.
     *
     * @param \Drupal\Core\Config\ConfigFactoryInterface $config_factory
     *   The factory for configuration objects.
     * @param \Drupal\Core\Config\TypedConfigManagerInterface $typedConfigManager
     *   The typed config manager.
     */
    public function __construct(ConfigFactoryInterface $config_factory, TypedConfigManagerInterface $typedConfigManager) {
        $this->setConfigFactory($config_factory);
    }
    
    /**
     * {@inheritdoc}
     */
    public static function create(ContainerInterface $container) {
        return new static($container->get('config.factory'), $container->get('config.typed'));
    }
    
    /**
     * Returns the typed config manager service.
     *
     * @return \Drupal\Core\Config\TypedConfigManagerInterface
     *   The typed config manager service.
     */
    protected function typedConfigManager() : TypedConfigManagerInterface {
        if ($this->typedConfigManager instanceof TypedConfigManagerInterface) {
            return $this->typedConfigManager;
        }
        return \Drupal::service('config.typed');
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildForm(array $form, FormStateInterface $form_state) {
        $form['actions']['#type'] = 'actions';
        $form['actions']['submit'] = [
            '#type' => 'submit',
            '#value' => $this->t('Save configuration'),
            '#button_type' => 'primary',
        ];
        // By default, render the form using system-config-form.html.twig.
        $form['#theme'] = 'system_config_form';
        // Load default values from config into any element with a #config_target
        // property.
        $form['#process'][] = '::loadDefaultValuesFromConfig';
        $form['#after_build'][] = '::storeConfigKeyToFormElementMap';
        return $form;
    }
    
    /**
     * Process callback to recursively load default values from #config_target.
     *
     * @param array $element
     *   The form element.
     *
     * @return array
     *   The form element, with its default value populated.
     */
    public function loadDefaultValuesFromConfig(array $element) : array {
        if (array_key_exists('#config_target', $element) && !array_key_exists('#default_value', $element)) {
            $target = $element['#config_target'];
            if (is_string($target)) {
                $target = ConfigTarget::fromString($target);
            }
            $config = $this->configFactory()
                ->getEditable($target->configName);
            $element['#default_value'] = $target->getValue($config);
        }
        foreach (Element::children($element) as $key) {
            $element[$key] = $this->loadDefaultValuesFromConfig($element[$key]);
        }
        return $element;
    }
    
    /**
     * #after_build callback which stores a map of element names to config keys.
     *
     * This will store an array in the form state whose keys are strings in the
     * form of `CONFIG_NAME:PROPERTY_PATH`, and whose values are instances of
     * \Drupal\Core\Form\ConfigTarget.
     *
     * This callback is run in the form's #after_build stage, rather than
     * #process, to guarantee that all of the form's elements have their final
     * #name and #parents properties set.
     *
     * @param array $element
     *   The element being processed.
     * @param \Drupal\Core\Form\FormStateInterface $form_state
     *   The current form state.
     *
     * @return array
     *   The processed element.
     *
     * @see \Drupal\Core\Form\ConfigFormBase::buildForm()
     */
    public function storeConfigKeyToFormElementMap(array $element, FormStateInterface $form_state) : array {
        // Empty the map to ensure the information is always correct after
        // rebuilding the form.
        $form_state->set(static::CONFIG_KEY_TO_FORM_ELEMENT_MAP, []);
        return $this->doStoreConfigMap($element, $form_state);
    }
    
    /**
     * Helper method for #after_build callback ::storeConfigKeyToFormElementMap().
     *
     * @param array $element
     *   The element being processed.
     * @param \Drupal\Core\Form\FormStateInterface $form_state
     *   The current form state.
     *
     * @return array
     *   The processed element.
     *
     * @see \Drupal\Core\Form\ConfigFormBase::storeConfigKeyToFormElementMap()
     */
    protected function doStoreConfigMap(array $element, FormStateInterface $form_state) : array {
        if (array_key_exists('#config_target', $element)) {
            $map = $form_state->get(static::CONFIG_KEY_TO_FORM_ELEMENT_MAP) ?? [];
            
            /** @var \Drupal\Core\Form\ConfigTarget|string $target */
            $target = $element['#config_target'];
            if (is_string($target)) {
                $target = ConfigTarget::fromString($target);
            }
            elseif ($target->toConfig instanceof \Closure || $target->fromConfig instanceof \Closure) {
                // If the form is using closures as toConfig or fromConfig callables
                // then form cannot be cached.
                $form_state->disableCache();
            }
            foreach ($target->propertyPaths as $property_path) {
                if (isset($map[$target->configName][$property_path])) {
                    throw new \LogicException(sprintf('Two #config_targets both target "%s" in the "%s" config: `%s` and `%s`.', $property_path, $target->configName, '$form[\'' . implode("']['", $map[$target->configName][$property_path]) . '\']', '$form[\'' . implode("']['", $element['#array_parents']) . '\']'));
                }
                $map[$target->configName][$property_path] = $element['#array_parents'];
            }
            $form_state->set(static::CONFIG_KEY_TO_FORM_ELEMENT_MAP, $map);
        }
        foreach (Element::children($element) as $key) {
            $element[$key] = $this->doStoreConfigMap($element[$key], $form_state);
        }
        return $element;
    }
    
    /**
     * {@inheritdoc}
     */
    public function validateForm(array &$form, FormStateInterface $form_state) {
        $map = $form_state->get(static::CONFIG_KEY_TO_FORM_ELEMENT_MAP) ?? [];
        foreach (array_keys($map) as $config_name) {
            $config = $this->configFactory()
                ->getEditable($config_name);
            static::copyFormValuesToConfig($config, $form_state, $form);
            $typed_config = $this->typedConfigManager()
                ->createFromNameAndData($config_name, $config->getRawData());
            $violations = $typed_config->validate();
            // Rather than immediately applying all violation messages to the
            // corresponding form elements, first collect the messages. The structure
            // of the form may cause a single form element to contain multiple config
            // property paths thanks to `type: sequence`. Common example: a <textarea>
            // with one line per sequence item.
            // @see \Drupal\Core\Config\Schema\Sequence
            // @see \Drupal\Core\Config\Schema\SequenceDataDefinition
            $violations_per_form_element = [];
            
            /** @var \Symfony\Component\Validator\ConstraintViolationInterface $violation */
            foreach ($violations as $violation) {
                $property_path = $violation->getPropertyPath();
                // Default to index 0.
                $index = 0;
                // Detect if this is a sequence item property path, and if so, attempt
                // to fall back to the containing sequence's property path.
                if (!isset($map[$config_name][$property_path]) && preg_match("/.*\\.(\\d+)\$/", $property_path, $matches) === 1) {
                    $index = intval($matches[1]);
                    // The property path as known in the config key-to-form element map
                    // will not have the sequence index in it.
                    $property_path = rtrim($property_path, '0123456789.');
                }
                if (isset($map[$config_name][$property_path])) {
                    $config_target = ConfigTarget::fromForm($map[$config_name][$property_path], $form);
                    $form_element_name = implode('][', $config_target->elementParents);
                }
                else {
                    // We cannot determine where to place the violation. The only option
                    // is the entire form.
                    $form_element_name = '';
                }
                $violations_per_form_element[$form_element_name][$index] = $violation;
            }
            // Now that we know how many constraint violation messages exist per form
            // element, set them. This is crucial because FormState::setErrorByName()
            // only allows a single validation error message per form element.
            // @see \Drupal\Core\Form\FormState::setErrorByName()
            foreach ($violations_per_form_element as $form_element_name => $violations) {
                // When only a single message exists, just set it.
                if (count($violations) === 1) {
                    $form_state->setErrorByName($form_element_name, reset($violations)->getMessage());
                    continue;
                }
                // However, if multiple exist, that implies it's a single form element
                // containing a `type: sequence`.
                $form_state->setErrorByName($form_element_name, $this->formatMultipleViolationsMessage($form_element_name, $violations));
            }
        }
    }
    
    /**
     * Formats multiple violation messages associated with a single form element.
     *
     * Validation constraints only know the internal data structure (the
     * configuration schema structure), but this need not be a disadvantage:
     * rather than informing the user some values are wrong, it is possible
     * guide them directly to the Nth entry in the sequence.
     *
     * To further improve the user experience, it is possible to override
     * method in subclasses to use specific knowledge about the structure of the
     * form and the nature of the data being validated, to instead generate more
     * precise and/or shortened violation messages.
     *
     * @param string $form_element_name
     *   The form element for which to format multiple violation messages.
     * @param \Symfony\Component\Validator\ConstraintViolationListInterface $violations
     *   The list of constraint violations that apply to this form element.
     *
     * @return \Drupal\Core\StringTranslation\TranslatableMarkup
     */
    protected function formatMultipleViolationsMessage(string $form_element_name, array $violations) : TranslatableMarkup {
        $transformed_message_parts = [];
        foreach ($violations as $index => $violation) {
            // Note that `@validation_error_message` (should) already contain a
            // trailing period, hence it is intentionally absent here.
            $transformed_message_parts[] = $this->t('Entry @human_index: @validation_error_message', [
                // Humans start counting from 1, not 0.
'@human_index' => $index + 1,
                // Translators may not necessarily know what "violation constraint
                // messages" are, but they definitely know "validation errors".
'@validation_error_message' => $violation->getMessage(),
            ]);
        }
        return $this->t(implode("\n", $transformed_message_parts));
    }
    
    /**
     * {@inheritdoc}
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
        $map = $form_state->get(static::CONFIG_KEY_TO_FORM_ELEMENT_MAP) ?? [];
        foreach (array_keys($map) as $config_name) {
            $config = $this->configFactory()
                ->getEditable($config_name);
            static::copyFormValuesToConfig($config, $form_state, $form);
            $config->save();
        }
        $this->messenger()
            ->addStatus($this->t('The configuration options have been saved.'));
    }
    
    /**
     * Copies form values to Config keys.
     *
     * This should not change existing Config key-value pairs that are not being
     * edited by this form.
     *
     * @param \Drupal\Core\Config\Config $config
     *   The configuration being edited.
     * @param \Drupal\Core\Form\FormStateInterface $form_state
     *   The current state of the form.
     * @param array $form
     *   The form array.
     *
     * @see \Drupal\Core\Entity\EntityForm::copyFormValuesToEntity()
     */
    private static function copyFormValuesToConfig(Config $config, FormStateInterface $form_state, array $form) : void {
        $map = $form_state->get(static::CONFIG_KEY_TO_FORM_ELEMENT_MAP);
        foreach ($map[$config->getName()] as $array_parents) {
            $target = ConfigTarget::fromForm($array_parents, $form);
            if ($target->configName !== $config->getName()) {
                continue;
            }
            $value = $form_state->getValue($target->elementParents);
            $target->setValue($config, $value, $form_state);
        }
    }

}