class ConfigSingleExportForm

class ConfigSingleExportForm extends FormBase {
    
    /**
     * The entity type manager.
     *
     * @var \Drupal\Core\Entity\EntityTypeManagerInterface
     */
    protected $entityTypeManager;
    
    /**
     * The config storage.
     *
     * @var \Drupal\Core\Config\StorageInterface
     */
    protected $configStorage;
    
    /**
     * Tracks the valid config entity type definitions.
     *
     * @var \Drupal\Core\Entity\EntityTypeInterface[]
     */
    protected $definitions = [];
    
    /**
     * Constructs a new ConfigSingleImportForm.
     *
     * @param \Drupal\Core\Entity\EntityTypeManagerInterface $entity_type_manager
     *   The entity type manager.
     * @param \Drupal\Core\Config\StorageInterface $config_storage
     *   The config storage.
     */
    public function __construct(EntityTypeManagerInterface $entity_type_manager, StorageInterface $config_storage) {
        $this->entityTypeManager = $entity_type_manager;
        $this->configStorage = $config_storage;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function create(ContainerInterface $container) {
        return new static($container->get('entity_type.manager'), $container->get('config.storage'));
    }
    
    /**
     * {@inheritdoc}
     */
    public function getFormId() {
        return 'config_single_export_form';
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildForm(array $form, FormStateInterface $form_state, $config_type = NULL, $config_name = NULL) {
        $form['#prefix'] = '<div id="js-config-form-wrapper">';
        $form['#suffix'] = '</div>';
        foreach ($this->entityTypeManager
            ->getDefinitions() as $entity_type => $definition) {
            if ($definition->entityClassImplements(ConfigEntityInterface::class)) {
                $this->definitions[$entity_type] = $definition;
            }
        }
        $entity_types = array_map(function (EntityTypeInterface $definition) {
            return $definition->getLabel();
        }, $this->definitions);
        // Sort the entity types by label, then add the simple config to the top.
        uasort($entity_types, 'strnatcasecmp');
        $config_types = [
            'system.simple' => $this->t('Simple configuration'),
        ] + $entity_types;
        $form['config_type'] = [
            '#title' => $this->t('Configuration type'),
            '#type' => 'select',
            '#options' => $config_types,
            '#default_value' => $config_type,
            '#ajax' => [
                'callback' => '::updateConfigurationType',
                'wrapper' => 'js-config-form-wrapper',
            ],
        ];
        $default_type = $form_state->getValue('config_type', $config_type);
        $form['config_name'] = [
            '#title' => $this->t('Configuration name'),
            '#type' => 'select',
            '#options' => $this->findConfiguration($default_type),
            '#default_value' => $config_name,
            '#prefix' => '<div id="edit-config-type-wrapper">',
            '#suffix' => '</div>',
            '#ajax' => [
                'callback' => '::updateExport',
                'wrapper' => 'edit-export-wrapper',
            ],
        ];
        $form['export'] = [
            '#title' => $this->t('Here is your configuration:'),
            '#type' => 'textarea',
            '#rows' => 24,
            '#prefix' => '<div id="edit-export-wrapper">',
            '#suffix' => '</div>',
        ];
        if ($config_type && $config_name) {
            $fake_form_state = (new FormState())->setValues([
                'config_type' => $config_type,
                'config_name' => $config_name,
            ]);
            $form['export'] = $this->updateExport($form, $fake_form_state);
        }
        return $form;
    }
    
    /**
     * Handles switching the configuration type selector.
     */
    public function updateConfigurationType($form, FormStateInterface $form_state) {
        $form['config_name']['#options'] = $this->findConfiguration($form_state->getValue('config_type'));
        $form['export']['#value'] = NULL;
        return $form;
    }
    
    /**
     * Handles switching the export textarea.
     */
    public function updateExport($form, FormStateInterface $form_state) {
        // Determine the full config name for the selected config entity.
        if ($form_state->getValue('config_type') !== 'system.simple') {
            $definition = $this->entityTypeManager
                ->getDefinition($form_state->getValue('config_type'));
            $name = $definition->getConfigPrefix() . '.' . $form_state->getValue('config_name');
        }
        else {
            $name = $form_state->getValue('config_name');
        }
        // Read the raw data for this config name, encode it, and display it.
        $form['export']['#value'] = Yaml::encode($this->configStorage
            ->read($name));
        $form['export']['#description'] = $this->t('Filename: %name', [
            '%name' => $name . '.yml',
        ]);
        return $form['export'];
    }
    
    /**
     * Handles switching the configuration type selector.
     */
    protected function findConfiguration($config_type) {
        $names = [
            '' => $this->t('- Select -'),
        ];
        // For a given entity type, load all entities.
        if ($config_type && $config_type !== 'system.simple') {
            $entity_storage = $this->entityTypeManager
                ->getStorage($config_type);
            foreach ($entity_storage->loadMultiple() as $entity) {
                $entity_id = $entity->id();
                if ($label = $entity->label()) {
                    $names[$entity_id] = new TranslatableMarkup('@label (@id)', [
                        '@label' => $label,
                        '@id' => $entity_id,
                    ]);
                }
                else {
                    $names[$entity_id] = $entity_id;
                }
            }
        }
        else {
            // Gather the config entity prefixes.
            $config_prefixes = array_map(function (EntityTypeInterface $definition) {
                return $definition->getConfigPrefix() . '.';
            }, $this->definitions);
            // Find all config, and then filter our anything matching a config prefix.
            $names = $this->configStorage
                ->listAll();
            $names = array_combine($names, $names);
            foreach ($names as $config_name) {
                foreach ($config_prefixes as $config_prefix) {
                    if (strpos($config_name, $config_prefix) === 0) {
                        unset($names[$config_name]);
                    }
                }
            }
        }
        return $names;
    }
    
    /**
     * {@inheritdoc}
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
        // Nothing to submit.
    }

}