class ExportForm

class ExportForm extends FormBase {
    
    /**
     * The language manager.
     *
     * @var \Drupal\Core\Language\LanguageManagerInterface
     */
    protected $languageManager;
    
    /**
     * The file system service.
     *
     * @var \Drupal\Core\File\FileSystemInterface
     */
    protected $fileSystem;
    
    /**
     * Constructs a new ExportForm.
     *
     * @param \Drupal\Core\Language\LanguageManagerInterface $language_manager
     *   The language manager.
     * @param \Drupal\Core\File\FileSystemInterface $file_system
     *   The file system service.
     */
    public function __construct(LanguageManagerInterface $language_manager, FileSystemInterface $file_system) {
        $this->languageManager = $language_manager;
        $this->fileSystem = $file_system;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function create(ContainerInterface $container) {
        return new static($container->get('language_manager'), $container->get('file_system'));
    }
    
    /**
     * {@inheritdoc}
     */
    public function getFormId() {
        return 'locale_translate_export_form';
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildForm(array $form, FormStateInterface $form_state) {
        $languages = $this->languageManager
            ->getLanguages();
        $language_options = [];
        foreach ($languages as $langcode => $language) {
            if (locale_is_translatable($langcode)) {
                $language_options[$langcode] = $language->getName();
            }
        }
        $language_default = $this->languageManager
            ->getDefaultLanguage();
        if (empty($language_options)) {
            $form['langcode'] = [
                '#type' => 'value',
                '#value' => LanguageInterface::LANGCODE_SYSTEM,
            ];
            $form['langcode_text'] = [
                '#type' => 'item',
                '#title' => $this->t('Language'),
                '#markup' => $this->t('No language available. The export will only contain source strings.'),
            ];
        }
        else {
            $form['langcode'] = [
                '#type' => 'select',
                '#title' => $this->t('Language'),
                '#options' => $language_options,
                '#default_value' => $language_default->getId(),
                '#empty_option' => $this->t('Source text only, no translations'),
                '#empty_value' => LanguageInterface::LANGCODE_SYSTEM,
            ];
            $form['content_options'] = [
                '#type' => 'details',
                '#title' => $this->t('Export options'),
                '#tree' => TRUE,
                '#states' => [
                    'invisible' => [
                        ':input[name="langcode"]' => [
                            'value' => LanguageInterface::LANGCODE_SYSTEM,
                        ],
                    ],
                ],
            ];
            $form['content_options']['not_customized'] = [
                '#type' => 'checkbox',
                '#title' => $this->t('Include non-customized translations'),
                '#default_value' => TRUE,
            ];
            $form['content_options']['customized'] = [
                '#type' => 'checkbox',
                '#title' => $this->t('Include customized translations'),
                '#default_value' => TRUE,
            ];
            $form['content_options']['not_translated'] = [
                '#type' => 'checkbox',
                '#title' => $this->t('Include untranslated text'),
                '#default_value' => TRUE,
            ];
        }
        $form['actions'] = [
            '#type' => 'actions',
        ];
        $form['actions']['submit'] = [
            '#type' => 'submit',
            '#value' => $this->t('Export'),
        ];
        return $form;
    }
    
    /**
     * {@inheritdoc}
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
        // If template is required, language code is not given.
        if ($form_state->getValue('langcode') != LanguageInterface::LANGCODE_SYSTEM) {
            $language = $this->languageManager
                ->getLanguage($form_state->getValue('langcode'));
        }
        else {
            $language = NULL;
        }
        $content_options = $form_state->getValue('content_options', []);
        $reader = new PoDatabaseReader();
        $language_name = '';
        if ($language != NULL) {
            $reader->setLangcode($language->getId());
            $reader->setOptions($content_options);
            $languages = $this->languageManager
                ->getLanguages();
            $language_name = isset($languages[$language->getId()]) ? $languages[$language->getId()]
                ->getName() : '';
            $filename = $language->getId() . '.po';
        }
        else {
            // Template required.
            $filename = 'drupal.pot';
        }
        $item = $reader->readItem();
        if (!empty($item)) {
            $uri = $this->fileSystem
                ->tempnam('temporary://', 'po_');
            $header = $reader->getHeader();
            $header->setProjectName($this->config('system.site')
                ->get('name'));
            $header->setLanguageName($language_name);
            $writer = new PoStreamWriter();
            $writer->setURI($uri);
            $writer->setHeader($header);
            $writer->open();
            $writer->writeItem($item);
            $writer->writeItems($reader);
            $writer->close();
            $response = new BinaryFileResponse($uri);
            $response->setContentDisposition('attachment', $filename);
            $form_state->setResponse($response);
        }
        else {
            $this->messenger()
                ->addStatus($this->t('Nothing to export.'));
        }
    }

}