class CKEditor5

class CKEditor5 extends EditorBase implements ContainerFactoryPluginInterface {
    
    /**
     * The CKEditor plugin manager.
     *
     * @var \Drupal\ckeditor5\Plugin\CKEditor5PluginManagerInterface
     */
    protected $ckeditor5PluginManager;
    
    /**
     * The language manager.
     *
     * @var \Drupal\Core\Language\LanguageManagerInterface
     */
    protected $languageManager;
    
    /**
     * The module handler.
     *
     * @var \Drupal\Core\Extension\ModuleHandlerInterface
     */
    protected $moduleHandler;
    
    /**
     * Smart default settings utility.
     *
     * @var \Drupal\ckeditor5\SmartDefaultSettings
     */
    protected $smartDefaultSettings;
    
    /**
     * The set of configured CKEditor 5 plugins.
     *
     * @var \Drupal\ckeditor5\Plugin\CKEditor5PluginInterface[]
     */
    private $plugins = [];
    
    /**
     * The submitted editor.
     *
     * @var \Drupal\editor\EditorInterface
     */
    private $submittedEditor;
    
    /**
     * The cache.
     *
     * @var \Drupal\Core\Cache\CacheBackendInterface
     */
    protected $cache;
    
    /**
     * A logger instance.
     *
     * @var \Psr\Log\LoggerInterface
     */
    protected $logger;
    
    /**
     * Constructs a CKEditor 5 editor plugin.
     *
     * @param array $configuration
     *   A configuration array containing information about the plugin instance.
     * @param string $plugin_id
     *   The plugin ID for the plugin instance.
     * @param mixed $plugin_definition
     *   The plugin implementation definition.
     * @param \Drupal\ckeditor5\Plugin\CKEditor5PluginManagerInterface $ckeditor5_plugin_manager
     *   The CKEditor 5 plugin manager.
     * @param \Drupal\Core\Language\LanguageManagerInterface $language_manager
     *   The language manager
     * @param \Drupal\Core\Extension\ModuleHandlerInterface $module_handler
     *   The module handler.
     * @param \Drupal\ckeditor5\SmartDefaultSettings $smart_default_settings
     *   The smart default settings utility.
     * @param \Drupal\Core\Cache\CacheBackendInterface $cache
     *   The cache.
     * @param \Psr\Log\LoggerInterface $logger
     *   A logger instance.
     */
    public function __construct(array $configuration, $plugin_id, $plugin_definition, CKEditor5PluginManagerInterface $ckeditor5_plugin_manager, LanguageManagerInterface $language_manager, ModuleHandlerInterface $module_handler, SmartDefaultSettings $smart_default_settings, CacheBackendInterface $cache, LoggerInterface $logger) {
        parent::__construct($configuration, $plugin_id, $plugin_definition);
        $this->ckeditor5PluginManager = $ckeditor5_plugin_manager;
        $this->languageManager = $language_manager;
        $this->moduleHandler = $module_handler;
        $this->smartDefaultSettings = $smart_default_settings;
        $this->cache = $cache;
        $this->logger = $logger;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
        return new static($configuration, $plugin_id, $plugin_definition, $container->get('plugin.manager.ckeditor5.plugin'), $container->get('language_manager'), $container->get('module_handler'), $container->get('ckeditor5.smart_default_settings'), $container->get('cache.default'), $container->get('logger.channel.ckeditor5'));
    }
    
    /**
     * {@inheritdoc}
     */
    public function getDefaultSettings() {
        return [
            'toolbar' => [
                'items' => [
                    'heading',
                    'bold',
                    'italic',
                ],
            ],
            'plugins' => [
                'ckeditor5_heading' => Heading::DEFAULT_CONFIGURATION,
            ],
        ];
    }
    
    /**
     * Validates a Text Editor + Text Format pair.
     *
     * Drupal is designed to only verify schema conformity (and validation) of
     * individual config entities. The Text Editor module layers a tightly coupled
     * Editor entity on top of the Filter module's FilterFormat config entity.
     * This inextricable coupling is clearly visible in EditorInterface:
     * \Drupal\editor\EditorInterface::getFilterFormat(). They are always paired.
     * Because not every text editor is guaranteed to be compatible with every
     * text format, the pair must be validated.
     *
     * @param \Drupal\editor\EditorInterface $text_editor
     *   The paired text editor to validate.
     * @param \Drupal\filter\FilterFormatInterface $text_format
     *   The paired text format to validate.
     * @param bool $all_compatibility_problems
     *   Get all compatibility problems (default) or only fundamental ones.
     *
     * @return \Symfony\Component\Validator\ConstraintViolationListInterface
     *   The validation constraint violations.
     *
     * @throws \InvalidArgumentException
     *   Thrown when the text editor is not configured to use CKEditor 5.
     *
     * @see \Drupal\editor\EditorInterface::getFilterFormat()
     * @see ckeditor5.pair.schema.yml
     */
    public static function validatePair(EditorInterface $text_editor, FilterFormatInterface $text_format, bool $all_compatibility_problems = TRUE) : ConstraintViolationListInterface {
        if ($text_editor->getEditor() !== 'ckeditor5') {
            throw new \InvalidArgumentException('This text editor is not configured to use CKEditor 5.');
        }
        $typed_config_manager = \Drupal::getContainer()->get('config.typed');
        $typed_config = $typed_config_manager->createFromNameAndData('ckeditor5_valid_pair__format_and_editor', [
            // A mix of:
            // - editor.editor.*.settings — note that "settings" is top-level in
            //   editor.editor.*, and so it is here, so all validation constraints
            //   will continue to work fine.
'settings' => $text_editor->toArray()['settings'],
            // - filter.format.*.filters — note that "filters" is top-level in
            //   filter.format.*, and so it is here, so all validation constraints
            //   will continue to work fine.
'filters' => $text_format->toArray()['filters'],
            // - editor.editor.*.image_upload — note that "image_upload" is
            //   top-level in editor.editor.*, and so it is here, so all validation
            //   constraints will continue to work fine.
'image_upload' => $text_editor->toArray()['image_upload'],
        ]);
        $violations = $typed_config->validate();
        // Only consider validation constraint violations covering the pair, so not
        // irrelevant details such as a PrimitiveTypeConstraint in filter settings,
        // which do not affect CKEditor 5 anyway.
        foreach ($violations as $i => $violation) {
            assert($violation instanceof ConstraintViolation);
            if (explode('.', $violation->getPropertyPath())[0] === 'filters' && is_a($violation->getConstraint(), PrimitiveTypeConstraint::class)) {
                $violations->remove($i);
            }
        }
        if (!$all_compatibility_problems) {
            foreach ($violations as $i => $violation) {
                // Remove all violations that are not fundamental — these are at the
                // root (property path '').
                if ($violation->getPropertyPath() !== '') {
                    $violations->remove($i);
                }
            }
        }
        return $violations;
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildConfigurationForm(array $form, FormStateInterface $form_state) {
        $editor = $form_state->get('editor');
        assert($editor instanceof EditorEntity);
        $language = $this->languageManager
            ->getCurrentLanguage();
        // When enabling CKEditor 5, generate sensible settings from the
        // pre-existing text editor/format rather than the hardcoded defaults
        // whenever possible.
        // @todo Remove after https://www.drupal.org/project/drupal/issues/3226673.
        $format = $form_state->getFormObject()
            ->getEntity();
        assert($format instanceof FilterFormatInterface);
        if ($editor->isNew() && !$form_state->get('ckeditor5_is_active') && $form_state->get('ckeditor5_is_selected')) {
            assert($editor->getSettings() === $this->getDefaultSettings());
            if (!$format->isNew()) {
                [
                    $editor,
                    $messages,
                ] = $this->smartDefaultSettings
                    ->computeSmartDefaultSettings($editor, $format);
                $form_state->set('used_smart_default_settings', TRUE);
                foreach ($messages as $type => $messages_per_type) {
                    foreach ($messages_per_type as $message) {
                        $this->messenger()
                            ->addMessage($message, $type);
                    }
                }
                if (isset($messages[MessengerInterface::TYPE_WARNING]) || isset($messages[MessengerInterface::TYPE_ERROR])) {
                    $this->messenger()
                        ->addMessage($this->t('Check <a href=":handbook">this handbook page</a> for details about compatibility issues of contrib modules.', [
                        ':handbook' => 'https://www.drupal.org/node/3273985',
                    ]), MessengerInterface::TYPE_WARNING);
                }
            }
            $eventual_editor_and_format = $this->getEventualEditorWithPrimedFilterFormat($form_state, $editor);
            // Provide the validated eventual pair in form state to
            // ::getGeneratedAllowedHtmlValue(), to update filter_html's
            // "allowed_html".
            $form_state->set('ckeditor5_validated_pair', $eventual_editor_and_format);
            // Ensure that CKEditor 5 plugins that need to interact with the Editor
            // config entity are able to access the computed Editor, which was cloned
            // from $form_state->get('editor').
            // @see \Drupal\ckeditor5\Plugin\CKEditor5Plugin\Image::buildConfigurationForm
            $form_state->set('editor', $editor);
        }
        // AJAX validation errors should appear visually close to the text editor
        // since this is a very long form: otherwise they would not be noticed.
        $form['real_time_validation_errors_location'] = [
            '#type' => 'container',
            '#id' => 'ckeditor5-realtime-validation-messages-container',
        ];
        $form['toolbar'] = [
            '#type' => 'container',
            '#title' => $this->t('CKEditor 5 toolbar configuration'),
            '#theme' => 'ckeditor5_settings_toolbar',
            '#attached' => [
                'library' => $this->ckeditor5PluginManager
                    ->getAdminLibraries(),
                'drupalSettings' => [
                    'ckeditor5' => [
                        'language' => [
                            'dir' => $language->getDirection(),
                            'langcode' => $language->getId(),
                        ],
                    ],
                ],
            ],
        ];
        $form['available_items_description'] = [
            '#type' => 'container',
            '#markup' => $this->t('Press the down arrow key to add to the toolbar.'),
            '#id' => 'available-button-description',
            '#attributes' => [
                'class' => [
                    'visually-hidden',
                ],
            ],
        ];
        $form['active_items_description'] = [
            '#type' => 'container',
            '#markup' => $this->t('Move this button in the toolbar by pressing the left or right arrow keys. Press the up arrow key to remove from the toolbar.'),
            '#id' => 'active-button-description',
            '#attributes' => [
                'class' => [
                    'visually-hidden',
                ],
            ],
        ];
        // The items are encoded in markup to provide a no-JS fallback.
        // Although CKEditor 5 is useless without JS it would still be possible
        // to see all the available toolbar items provided by plugins in the format
        // that needs to be entered in the textarea. The UI app parses this list.
        $form['toolbar']['available'] = [
            '#type' => 'container',
            '#title' => 'Available items',
            '#id' => 'ckeditor5-toolbar-buttons-available',
            'available_items' => [
                '#markup' => Json::encode($this->ckeditor5PluginManager
                    ->getToolbarItems()),
            ],
        ];
        $editor_settings = $editor->getSettings();
        // This form field requires a JSON-style array of valid toolbar items.
        // e.g. ["bold","italic","|","drupalInsertImage"].
        // CKEditor 5 config for toolbar items takes an array of strings which
        // correspond to the keys under toolbar_items in a plugin yml or annotation.
        // @see https://ckeditor.com/docs/ckeditor5/latest/features/toolbar/toolbar.html
        $form['toolbar']['items'] = [
            '#type' => 'textarea',
            '#title' => $this->t('Toolbar items'),
            '#rows' => 1,
            '#default_value' => Json::encode($editor_settings['toolbar']['items']),
            '#id' => 'ckeditor5-toolbar-buttons-selected',
            '#attributes' => [
                'tabindex' => '-1',
                'aria-hidden' => 'true',
            ],
        ];
        $form['plugin_settings'] = [
            '#type' => 'vertical_tabs',
            '#title' => $this->t('CKEditor 5 plugin settings'),
            // Add an ID to the editor settings vertical tabs wrapper so it can be
            // easily targeted by JavaScript.
'#wrapper_attributes' => [
                'id' => 'plugin-settings-wrapper',
            ],
        ];
        $this->injectPluginSettingsForm($form, $form_state, $editor);
        // Allow reliable detection of switching to CKEditor 5 from another text
        // editor (or none at all).
        $form['is_already_using_ckeditor5'] = [
            '#type' => 'hidden',
            '#default_value' => TRUE,
        ];
        return $form;
    }
    
    /**
     * Determines whether the plugin settings form should be visible.
     *
     * @param \Drupal\ckeditor5\Plugin\CKEditor5PluginDefinition $definition
     *   The configurable CKEditor 5 plugin to assess the visibility for.
     * @param \Drupal\editor\EditorInterface $editor
     *   A configured text editor object.
     *
     * @return bool
     *   Whether this configurable plugin's settings form should be visible.
     */
    private function shouldHaveVisiblePluginSettingsForm(CKEditor5PluginDefinition $definition, EditorInterface $editor) : bool {
        assert($definition->isConfigurable());
        $enabled_plugins = $this->ckeditor5PluginManager
            ->getEnabledDefinitions($editor);
        $plugin_id = $definition->id();
        // Enabled plugins should be configurable.
        if (isset($enabled_plugins[$plugin_id])) {
            return TRUE;
        }
        // There are two circumstances where a plugin not listed in $enabled_plugins
        // due to isEnabled() returning false, that should still have its config
        // form provided:
        // 1 - A conditionally enabled plugin that does not depend on a toolbar item
        // to be active AND the plugins it depends on are enabled (if any) AND the
        // filter it depends on is enabled (if any).
        // 2 - A conditionally enabled plugin that does depend on a toolbar item,
        // and that toolbar item is active.
        if ($definition->hasConditions()) {
            $conditions = $definition->getConditions();
            if (!array_key_exists('toolbarItem', $conditions)) {
                $conclusion = TRUE;
                // The filter this plugin depends on must be enabled.
                if (array_key_exists('filter', $conditions)) {
                    $required_filter = $conditions['filter'];
                    $format_filters = $editor->getFilterFormat()
                        ->filters();
                    $conclusion = $conclusion && $format_filters->has($required_filter) && $format_filters->get($required_filter)->status;
                }
                // The CKEditor 5 plugins this plugin depends on must be enabled.
                if (array_key_exists('plugins', $conditions)) {
                    $all_plugins = $this->ckeditor5PluginManager
                        ->getDefinitions();
                    $dependencies = array_intersect_key($all_plugins, array_flip($conditions['plugins']));
                    $unmet_dependencies = array_diff_key($dependencies, $enabled_plugins);
                    $conclusion = $conclusion && empty($unmet_dependencies);
                }
                return $conclusion;
            }
            elseif (in_array($conditions['toolbarItem'], $editor->getSettings()['toolbar']['items'], TRUE)) {
                return TRUE;
            }
        }
        return FALSE;
    }
    
    /**
     * Injects the CKEditor plugins settings forms as a vertical tabs subform.
     *
     * @param array &$form
     *   A reference to an associative array containing the structure of the form.
     * @param \Drupal\Core\Form\FormStateInterface $form_state
     *   The current state of the form.
     * @param \Drupal\editor\EditorInterface $editor
     *   A configured text editor object.
     */
    private function injectPluginSettingsForm(array &$form, FormStateInterface $form_state, EditorInterface $editor) : void {
        $definitions = $this->ckeditor5PluginManager
            ->getDefinitions();
        $eventual_editor_and_format = $this->getEventualEditorWithPrimedFilterFormat($form_state, $editor);
        foreach ($definitions as $plugin_id => $definition) {
            if ($definition->isConfigurable() && $this->shouldHaveVisiblePluginSettingsForm($definition, $eventual_editor_and_format)) {
                $plugin = $this->ckeditor5PluginManager
                    ->getPlugin($plugin_id, $editor);
                $plugin_settings_form = [];
                $form['plugins'][$plugin_id] = [
                    '#type' => 'details',
                    '#title' => $definition->label(),
                    '#open' => TRUE,
                    '#group' => 'editor][settings][plugin_settings',
                    '#attributes' => [
                        'data-ckeditor5-plugin-id' => $plugin_id,
                    ],
                ];
                $form['plugins'][$plugin_id] += $plugin->buildConfigurationForm($plugin_settings_form, $form_state);
            }
        }
    }
    
    /**
     * Form #after_build callback: provides text editor state changes.
     *
     * Updates the internal $this->entity object with submitted values when the
     * form is being rebuilt (e.g. submitted via AJAX), so that subsequent
     * processing (e.g. AJAX callbacks) can rely on it.
     *
     * @see \Drupal\Core\Entity\EntityForm::afterBuild()
     */
    public static function assessActiveTextEditorAfterBuild(array $element, FormStateInterface $form_state) : array {
        // The case of the form being built initially, and the text editor plugin in
        // use is already CKEditor 5.
        if (!$form_state->isProcessingInput()) {
            $editor = $form_state->get('editor');
            $already_using_ckeditor5 = $editor && $editor->getEditor() === 'ckeditor5';
        }
        else {
            // Whenever there is user input, this cannot be the initial build of the
            // form and hence we need to inspect user input.
            $already_using_ckeditor5 = FALSE;
            NestedArray::getValue($form_state->getUserInput(), [
                'editor',
                'settings',
                'is_already_using_ckeditor5',
            ], $already_using_ckeditor5);
        }
        $form_state->set('ckeditor5_is_active', $already_using_ckeditor5);
        $form_state->set('ckeditor5_is_selected', $form_state->getValue([
            'editor',
            'editor',
        ]) === 'ckeditor5');
        return $element;
    }
    
    /**
     * Validate callback to inform the user of CKEditor 5 compatibility problems.
     */
    public static function validateSwitchingToCKEditor5(array $form, FormStateInterface $form_state) : void {
        if (!$form_state->get('ckeditor5_is_active') && $form_state->get('ckeditor5_is_selected')) {
            $minimal_ckeditor5_editor = EditorEntity::create([
                'format' => NULL,
                'editor' => 'ckeditor5',
            ]);
            $submitted_filter_format = CKEditor5::getSubmittedFilterFormat($form_state);
            $fundamental_incompatibilities = CKEditor5::validatePair($minimal_ckeditor5_editor, $submitted_filter_format, FALSE);
            foreach ($fundamental_incompatibilities as $violation) {
                // If the violation uses the nonAllowedElementsMessage template, it can
                // be skipped because this is a violation that automatically fixed
                // within SmartDefaultSettings, but SmartDefaultSettings does not
                // execute until this validator passes.
                if ($violation->getMessageTemplate() === $violation->getConstraint()->nonAllowedElementsMessage) {
                    continue;
                }
                // @codingStandardsIgnoreLine
                $form_state->setErrorByName('editor][editor', t($violation->getMessageTemplate(), $violation->getParameters()));
            }
        }
    }
    
    /**
     * Value callback to set the CKEditor 5-generated "allowed_html" value.
     *
     * Used to set the value of filter_html's "allowed_html" form item if the form
     * has been validated and hence `ckeditor5_validated_pair` is available
     * in form state. This allows setting a guaranteed to be valid value.
     *
     * `ckeditor5_validated_pair` can be set from two places:
     * - When switching to CKEditor 5, this is populated by
     *   CKEditor5::buildConfigurationForm().
     * - When making filter or editor settings changes, it is populated by
     *  CKEditor5::validateConfigurationForm().
     *
     * @param array $element
     *   An associative array containing the properties of the element.
     * @param mixed $input
     *   The incoming input to populate the form element. If this is FALSE,
     *   the element's default value should be returned.
     * @param \Drupal\Core\Form\FormStateInterface $form_state
     *   The current state of the form.
     *
     * @return string
     *   The value to assign to the element.
     */
    public static function getGeneratedAllowedHtmlValue(array &$element, $input, FormStateInterface $form_state) : string {
        if ($form_state->isValidationComplete()) {
            $validated_format = $form_state->get('ckeditor5_validated_pair')
                ->getFilterFormat();
            $configuration = $validated_format->filters()
                ->get('filter_html')
                ->getConfiguration();
            return $configuration['settings']['allowed_html'];
        }
        else {
            if ($input !== FALSE) {
                return $input;
            }
            return $element['#default_value'];
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function validateConfigurationForm(array &$form, FormStateInterface $form_state) {
        $json = $form_state->getValue([
            'toolbar',
            'items',
        ]);
        $toolbar_items = Json::decode($json);
        // This basic validation must live in the form logic because it can only
        // occur in a form context.
        if (!$toolbar_items) {
            $form_state->setErrorByName('toolbar][items', $this->t('Invalid toolbar value.'));
            return;
        }
        // Construct a Text Editor config entity with the submitted values for
        // validation. Do this on a clone: do not manipulate form state.
        $submitted_editor = clone $form_state->get('editor');
        $settings = $submitted_editor->getSettings();
        // Update settings first to match the submitted toolbar items. This is
        // necessary for ::shouldHaveVisiblePluginSettingsForm() to work.
        $settings['toolbar']['items'] = $toolbar_items;
        $submitted_editor->setSettings($settings);
        $eventual_editor_and_format_for_plugin_settings_visibility = $this->getEventualEditorWithPrimedFilterFormat($form_state, $submitted_editor);
        $settings['plugins'] = [];
        $default_configurations = [];
        foreach ($this->ckeditor5PluginManager
            ->getDefinitions() as $plugin_id => $definition) {
            if (!$definition->isConfigurable()) {
                continue;
            }
            // Create a fresh instance of this CKEditor 5 plugin, not tied to a text
            // editor configuration entity.
            $plugin = $this->ckeditor5PluginManager
                ->getPlugin($plugin_id, NULL);
            // If this plugin is configurable but it has empty default configuration,
            // that means the configuration must be stored out of band.
            // @see \Drupal\ckeditor5\Plugin\CKEditor5Plugin\Image
            // @see editor_image_upload_settings_form()
            $default_configuration = $plugin->defaultConfiguration();
            $configuration_stored_out_of_band = empty($default_configuration);
            // If this plugin is configurable but has not yet had user interaction,
            // the default configuration will still be active and may trigger
            // validation errors. Do not trigger those validation errors until the
            // form is actually saved, to allow the user to first configure other
            // CKEditor 5 functionality.
            $default_configurations[$plugin_id] = $default_configuration;
            if ($form_state->hasValue([
                'plugins',
                $plugin_id,
            ])) {
                $subform = $form['plugins'][$plugin_id];
                $subform_state = SubformState::createForSubform($subform, $form, $form_state);
                $plugin->validateConfigurationForm($subform, $subform_state);
                $plugin->submitConfigurationForm($subform, $subform_state);
                // If the configuration is stored out of band, ::submitConfigurationForm
                // will already have stored it. If it is not stored out of band,
                // populate $settings, to populate $submitted_editor.
                if (!$configuration_stored_out_of_band) {
                    $settings['plugins'][$plugin_id] = $plugin->getConfiguration();
                }
            }
            elseif ($this->shouldHaveVisiblePluginSettingsForm($definition, $eventual_editor_and_format_for_plugin_settings_visibility)) {
                if (!$configuration_stored_out_of_band) {
                    $settings['plugins'][$plugin_id] = $default_configuration;
                }
            }
        }
        // All plugin settings have been collected, including defaults that depend
        // on visibility. Store the collected settings, throw away the interim state
        // that allowed determining which defaults to add.
        // Create a new clone, because the plugins whose data is being stored
        // out-of-band may have modified the Text Editor config entity in the form
        // state.
        // @see \Drupal\editor\EditorInterface::setImageUploadSettings()
        // @see \Drupal\ckeditor5\Plugin\CKEditor5Plugin\Image::submitConfigurationForm()
        unset($eventual_editor_and_format_for_plugin_settings_visibility);
        $submitted_editor = clone $form_state->get('editor');
        $submitted_editor->setSettings($settings);
        // Validate the text editor + text format pair.
        // Note that the eventual pair is computed and validated, not the received
        // pair: if the filter_html filter is in use, the CKEditor 5 configuration
        // dictates the filter_html's filter plugin's "allowed_html" setting.
        // @see ckeditor5_form_filter_format_form_alter()
        // @see ::getGeneratedAllowedHtmlValue()
        $eventual_editor_and_format = $this->getEventualEditorWithPrimedFilterFormat($form_state, $submitted_editor);
        $violations = CKEditor5::validatePair($eventual_editor_and_format, $eventual_editor_and_format->getFilterFormat());
        foreach ($violations as $violation) {
            $property_path_parts = explode('.', $violation->getPropertyPath());
            // Special case: AJAX updates that do not submit the form (that cannot
            // result in configuration being saved).
            if (in_array('editor_form_filter_admin_format_editor_configure', $form_state->getSubmitHandlers(), TRUE)) {
                // Ensure that plugins' validation constraints do not immediately
                // trigger a validation error: the user may choose to configure other
                // CKEditor 5 aspects first.
                if ($property_path_parts[0] === 'settings' && $property_path_parts[1] === 'plugins') {
                    $plugin_id = $property_path_parts[2];
                    // This CKEditor 5 plugin settings form was just added: the user has
                    // not yet had a chance to configure it.
                    if (!$form_state->hasValue([
                        'plugins',
                        $plugin_id,
                    ])) {
                        continue;
                    }
                    // This CKEditor 5 plugin settings form was added recently, the user
                    // is triggering AJAX rebuilds of the configuration UI because they're
                    // configuring other functionality first. Only require these to be
                    // valid at form submission time.
                    if ($form_state->getValue([
                        'plugins',
                        $plugin_id,
                    ]) === $default_configurations[$plugin_id]) {
                        continue;
                    }
                }
            }
            $form_item_name = static::mapPairViolationPropertyPathsToFormNames($violation->getPropertyPath(), $form);
            // When adding a toolbar item, it is possible that not all conditions for
            // using it have been met yet. FormBuilder refuses to rebuild forms when a
            // validation error is present. But to meet the condition for the toolbar
            // item, configuration must be set in a vertical tab that must still
            // appear. Work-around: reduce the validation error to a warning message.
            // @see \Drupal\ckeditor5\Plugin\Validation\Constraint\ToolbarItemConditionsMetConstraintValidator
            if ($form_state->isRedirectDisabled() && $form_item_name === 'editor][settings][toolbar][items') {
                $this->messenger()
                    ->addWarning($violation->getMessage());
                continue;
            }
            $form_state->getCompleteFormState()
                ->setErrorByName($form_item_name, $violation->getMessage());
        }
        // Pass it on to ::submitConfigurationForm().
        $form_state->get('editor')
            ->setSettings($settings);
        // Provide the validated eventual pair in form state to
        // ::getGeneratedAllowedHtmlValue(), to update filter_html's
        // "allowed_html".
        $form_state->set('ckeditor5_validated_pair', $eventual_editor_and_format);
    }
    
    /**
     * Gets the submitted text format config entity from form state.
     *
     * Needed for validation.
     *
     * @param \Drupal\Core\Form\FormStateInterface $filter_format_form_state
     *   The text format configuration form's form state.
     *
     * @return \Drupal\filter\FilterFormatInterface
     *   A FilterFormat config entity representing the current filter form state.
     */
    protected static function getSubmittedFilterFormat(FormStateInterface $filter_format_form_state) : FilterFormatInterface {
        $submitted_filter_format = clone $filter_format_form_state->getFormObject()
            ->getEntity();
        assert($submitted_filter_format instanceof FilterFormatInterface);
        // Get only the values of the filter_format form state that are relevant for
        // checking compatibility. This logic is copied from FilterFormatFormBase.
        // @see \Drupal\ckeditor5\Plugin\Validation\Constraint\FundamentalCompatibilityConstraintValidator
        // @see \Drupal\filter\FilterFormatFormBase::submitForm()
        $filter_format_form_values = array_intersect_key($filter_format_form_state->getValues(), array_flip([
            'filters',
            'filter_settings',
        ]));
        foreach ($filter_format_form_values as $key => $value) {
            if ($key !== 'filters') {
                $submitted_filter_format->set($key, $value);
            }
            else {
                foreach ($value as $instance_id => $config) {
                    $submitted_filter_format->setFilterConfig($instance_id, $config);
                }
            }
        }
        return $submitted_filter_format;
    }
    
    /**
     * Gets the eventual text format config entity: from form state + editor.
     *
     * Needed for validation.
     *
     * @param \Drupal\Core\Form\SubformStateInterface $editor_form_state
     *   The text editor configuration form's form state.
     * @param \Drupal\editor\EditorInterface $submitted_editor
     *   The current text editor config entity.
     *
     * @return \Drupal\editor\EditorInterface
     *   A clone of the received Editor config entity , with a primed associated
     *   FilterFormat that corresponds to the current form state, to avoid the
     *   stored FilterFormat config entity being loaded.
     */
    protected function getEventualEditorWithPrimedFilterFormat(SubformStateInterface $editor_form_state, EditorInterface $submitted_editor) : EditorInterface {
        $submitted_filter_format = static::getSubmittedFilterFormat($editor_form_state->getCompleteFormState());
        $pair = static::createEphemeralPairedEditor($submitted_editor, $submitted_filter_format);
        // When CKEditor 5 plugins are disabled in the form-based admin UI, the
        // associated settings (if any) should be omitted too, except for plugins
        // that are enabled using `requiresConfiguration` (because whether they are
        // enabled or not depends on the associated settings).
        $original_settings = $pair->getSettings();
        $enabled_plugins = $this->ckeditor5PluginManager
            ->getEnabledDefinitions($pair);
        $config_enabled_plugins = [];
        foreach ($this->ckeditor5PluginManager
            ->getDefinitions() as $id => $definition) {
            if ($definition->hasConditions() && isset($definition->getConditions()['requiresConfiguration'])) {
                $config_enabled_plugins[$id] = TRUE;
            }
        }
        $updated_settings = [
            'plugins' => array_intersect_key($original_settings['plugins'], $enabled_plugins + $config_enabled_plugins),
        ] + $original_settings;
        $pair->setSettings($updated_settings);
        if ($pair->getFilterFormat()
            ->filters('filter_html')->status) {
            // Compute elements provided by the current CKEditor 5 settings.
            $restrictions = new HTMLRestrictions($this->ckeditor5PluginManager
                ->getProvidedElements(array_keys($enabled_plugins), $pair));
            // Compute eventual filter_html setting. Eventual as in: this is the list
            // of eventually allowed HTML tags.
            // @see \Drupal\filter\FilterFormatFormBase::submitForm()
            // @see ckeditor5_form_filter_format_form_alter()
            $filter_html_config = $pair->getFilterFormat()
                ->filters('filter_html')
                ->getConfiguration();
            $filter_html_config['settings']['allowed_html'] = $restrictions->toFilterHtmlAllowedTagsString();
            $pair->getFilterFormat()
                ->setFilterConfig('filter_html', $filter_html_config);
        }
        return $pair;
    }
    
    /**
     * Creates an ephemeral pair of text editor + text format config entity.
     *
     * Clones the given text editor config entity object and then overwrites its
     * $filterFormat property, to prevent loading the text format config entity
     * from entity storage in calls to Editor::hasAssociatedFilterFormat() and
     * Editor::getFilterFormat().
     * This is necessary to be able to evaluate unsaved text editor and format
     * config entities:
     * - for assessing which CKEditor 5 plugins are enabled and whose settings
     *   forms to show
     * - for validating them.
     *
     * @param \Drupal\editor\EditorInterface $editor
     *   The submitted text editor config entity, constructed from form values.
     * @param \Drupal\filter\FilterFormatInterface $filter_format
     *   The submitted text format config entity, constructed from form values.
     *
     * @return \Drupal\editor\EditorInterface
     *   A clone of the given text editor config entity, with its $filterFormat
     *   property set to a clone of the given text format config entity.
     *
     * @throws \ReflectionException
     *
     * @see \Drupal\ckeditor5\Plugin\CKEditor5PluginManager::isPluginDisabled()
     * @todo Remove this in https://www.drupal.org/project/drupal/issues/3231347
     */
    protected static function createEphemeralPairedEditor(EditorInterface $editor, FilterFormatInterface $filter_format) : EditorInterface {
        $paired_editor = clone $editor;
        // If the editor is still being configured, the configuration may not yet be
        // valid. Explicitly mark the ephemeral paired editor as new to allow other
        // code to treat this accordingly.
        // @see \Drupal\ckeditor5\Plugin\CKEditor5PluginManager::getProvidedElements()
        $paired_editor->enforceIsNew(TRUE);
        $reflector = new \ReflectionObject($paired_editor);
        $property = $reflector->getProperty('filterFormat');
        $property->setValue($paired_editor, clone $filter_format);
        return $paired_editor;
    }
    
    /**
     * Maps Text Editor config object property paths to form names.
     *
     * @param string $property_path
     *   A config object property path.
     * @param array $subform
     *   The subform being checked.
     *
     * @return string
     *   The corresponding form name in the subform.
     */
    protected static function mapViolationPropertyPathsToFormNames(string $property_path, array $subform) : string {
        $parts = explode('.', $property_path);
        // The "settings" form element does exist, but one level above the Text
        // Editor-specific form. This is operating on a subform.
        $shifted = array_shift($parts);
        assert($shifted === 'settings');
        // It is not required (nor sensible) for the form structure to match the
        // config schema structure 1:1. Automatically identify the relevant form
        // name. Try to be specific. Worst case, an entire plugin settings vertical
        // tab is targeted. (Hence the minimum of 2 parts: the property path gets at
        // minimum mapped to 'toolbar.items' or 'plugins.<plugin ID>'.)
        while (count($parts) > 2 && !NestedArray::keyExists($subform, $parts)) {
            array_pop($parts);
        }
        assert(NestedArray::keyExists($subform, $parts));
        return implode('][', array_merge([
            'settings',
        ], $parts));
    }
    
    /**
     * Maps Text Editor + Text Format pair property paths to form names.
     *
     * @param string $property_path
     *   A config object property path.
     * @param array $form
     *   The form being checked.
     *
     * @return string
     *   The corresponding form name in the complete form.
     */
    protected static function mapPairViolationPropertyPathsToFormNames(string $property_path, array $form) : string {
        // Fundamental compatibility errors are at the root. Map these to the text
        // editor plugin dropdown.
        if ($property_path === '') {
            return 'editor][editor';
        }
        // Filters are top-level.
        if (preg_match('/^filters\\..*/', $property_path)) {
            return implode('][', array_merge(explode('.', $property_path), [
                'settings',
            ]));
        }
        // Image upload settings are stored out-of-band and may also trigger
        // validation errors.
        // @see \Drupal\ckeditor5\Plugin\CKEditor5Plugin\Image
        if (str_starts_with($property_path, 'image_upload.')) {
            $image_upload_setting_property_path = str_replace('image_upload.', '', $property_path);
            return 'editor][settings][plugins][ckeditor5_image][' . implode('][', explode('.', $image_upload_setting_property_path));
        }
        // Everything else is in the subform.
        return 'editor][' . static::mapViolationPropertyPathsToFormNames($property_path, $form);
    }
    
    /**
     * {@inheritdoc}
     */
    public function submitConfigurationForm(array &$form, FormStateInterface $form_state) {
        // @see ::validateConfigurationForm()
        $editor = $form_state->get('editor');
        // Prepare the editor settings for editor_form_filter_admin_format_submit().
        // This strips away unwanted form values too, because those never can exist
        // in the already validated Editor config entity.
        $form_state->setValues($editor->getSettings());
        parent::submitConfigurationForm($form, $form_state);
        if ($form_state->get('used_smart_default_settings')) {
            $format_name = $editor->getFilterFormat()
                ->get('name');
            $this->logger
                ->info($this->t('The migration of %text_format to CKEditor 5 has been saved.', [
                '%text_format' => $format_name,
            ]));
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function getJSSettings(EditorEntity $editor) {
        $toolbar_items = $editor->getSettings()['toolbar']['items'];
        $plugin_config = $this->ckeditor5PluginManager
            ->getCKEditor5PluginConfig($editor);
        $settings = [
            'toolbar' => [
                'items' => $toolbar_items,
                'shouldNotGroupWhenFull' => in_array('-', $toolbar_items, TRUE),
            ],
        ] + $plugin_config;
        if ($this->moduleHandler
            ->moduleExists('locale')) {
            $language_interface = $this->languageManager
                ->getCurrentLanguage();
            $settings['language']['ui'] = _ckeditor5_get_langcode_mapping($language_interface->getId());
        }
        return $settings;
    }
    
    /**
     * {@inheritdoc}
     */
    public function getLibraries(EditorEntity $editor) {
        $plugin_libraries = $this->ckeditor5PluginManager
            ->getEnabledLibraries($editor);
        if ($this->moduleHandler
            ->moduleExists('locale')) {
            $language_interface = $this->languageManager
                ->getCurrentLanguage();
            $plugin_libraries[] = 'core/ckeditor5.translations.' . _ckeditor5_get_langcode_mapping($language_interface->getId());
        }
        return $plugin_libraries;
    }

}