class MigrateUpgradeFormBase

abstract class MigrateUpgradeFormBase extends FormBase {
    use MigrationConfigurationTrait;
    
    /**
     * Private temporary storage.
     *
     * @var \Drupal\Core\TempStore\PrivateTempStoreFactory
     */
    protected $store;
    
    /**
     * The destination site major version.
     *
     * @var string
     */
    protected $destinationSiteVersion;
    
    /**
     * Constructs the Migrate Upgrade Form Base.
     *
     * @param \Drupal\Core\Config\ConfigFactoryInterface $config_factory
     *   The config factory service.
     * @param \Drupal\migrate\Plugin\MigrationPluginManagerInterface $migration_plugin_manager
     *   The migration plugin manager service.
     * @param \Drupal\Core\State\StateInterface $state
     *   The state service.
     * @param \Drupal\Core\TempStore\PrivateTempStoreFactory $tempstore_private
     *   The private tempstore factory service.
     */
    public function __construct(ConfigFactoryInterface $config_factory, MigrationPluginManagerInterface $migration_plugin_manager, StateInterface $state, PrivateTempStoreFactory $tempstore_private) {
        $this->configFactory = $config_factory;
        $this->migrationPluginManager = $migration_plugin_manager;
        $this->state = $state;
        $this->store = $tempstore_private->get('migrate_drupal_ui');
    }
    
    /**
     * {@inheritdoc}
     */
    public static function create(ContainerInterface $container) {
        return new static($container->get('config.factory'), $container->get('plugin.manager.migration'), $container->get('state'), $container->get('tempstore.private'));
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildForm(array $form, FormStateInterface $form_state) {
        // Get the current major version.
        list($this->destinationSiteVersion) = explode('.', \Drupal::VERSION, 2);
        $form = [];
        $form['actions']['#type'] = 'actions';
        $form['actions']['submit'] = [
            '#type' => 'submit',
            '#value' => $this->getConfirmText(),
            '#button_type' => 'primary',
            '#weight' => 10,
        ];
        return $form;
    }
    
    /**
     * Helper to redirect to the Overview form.
     *
     * @return \Symfony\Component\HttpFoundation\RedirectResponse
     *   A redirect response object that may be returned by the controller.
     *
     * @throws \Drupal\Core\TempStore\TempStoreException
     *   Thrown when a lock for the backend storage could not be acquired.
     */
    protected function restartUpgradeForm() {
        $this->store
            ->set('step', 'overview');
        return $this->redirect('migrate_drupal_ui.upgrade');
    }
    
    /**
     * Returns a caption for the button that confirms the action.
     *
     * @return string
     *   The form confirmation text.
     */
    protected abstract function getConfirmText();

}