class DateTimeFormatterBase

abstract class DateTimeFormatterBase extends FormatterBase {
  
  /**
   * The date formatter service.
   *
   * @var \Drupal\Core\Datetime\DateFormatterInterface
   */
  protected $dateFormatter;
  
  /**
   * The date format entity storage.
   *
   * @var \Drupal\Core\Entity\EntityStorageInterface
   */
  protected $dateFormatStorage;
  
  /**
   * Constructs a new DateTimeDefaultFormatter.
   *
   * @param string $plugin_id
   *   The plugin ID for the formatter.
   * @param mixed $plugin_definition
   *   The plugin implementation definition.
   * @param \Drupal\Core\Field\FieldDefinitionInterface $field_definition
   *   The definition of the field to which the formatter is associated.
   * @param array $settings
   *   The formatter settings.
   * @param string $label
   *   The formatter label display setting.
   * @param string $view_mode
   *   The view mode.
   * @param array $third_party_settings
   *   Third party settings.
   * @param \Drupal\Core\Datetime\DateFormatterInterface $date_formatter
   *   The date formatter service.
   * @param \Drupal\Core\Entity\EntityStorageInterface $date_format_storage
   *   The date format entity storage.
   */
  public function __construct($plugin_id, $plugin_definition, FieldDefinitionInterface $field_definition, array $settings, $label, $view_mode, array $third_party_settings, DateFormatterInterface $date_formatter, EntityStorageInterface $date_format_storage) {
    parent::__construct($plugin_id, $plugin_definition, $field_definition, $settings, $label, $view_mode, $third_party_settings);
    $this->dateFormatter = $date_formatter;
    $this->dateFormatStorage = $date_format_storage;
  }
  
  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
    return new static($plugin_id, $plugin_definition, $configuration['field_definition'], $configuration['settings'], $configuration['label'], $configuration['view_mode'], $configuration['third_party_settings'], $container->get('date.formatter'), $container->get('entity_type.manager')
      ->getStorage('date_format'));
  }
  
  /**
   * {@inheritdoc}
   */
  public static function defaultSettings() {
    return [
      'timezone_override' => '',
    ] + parent::defaultSettings();
  }
  
  /**
   * {@inheritdoc}
   */
  public function settingsForm(array $form, FormStateInterface $form_state) {
    $form = parent::settingsForm($form, $form_state);
    $form['timezone_override'] = [
      '#type' => 'select',
      '#title' => $this->t('Time zone override'),
      '#description' => $this->t('The time zone selected here will always be used'),
      '#options' => TimeZoneFormHelper::getOptionsListByRegion(TRUE),
      '#default_value' => $this->getSetting('timezone_override'),
    ];
    return $form;
  }
  
  /**
   * {@inheritdoc}
   */
  public function settingsSummary() {
    $summary = parent::settingsSummary();
    if ($override = $this->getSetting('timezone_override')) {
      $summary[] = $this->t('Time zone: @timezone', [
        '@timezone' => $override,
      ]);
    }
    return $summary;
  }
  
  /**
   * {@inheritdoc}
   */
  public function viewElements(FieldItemListInterface $items, $langcode) {
    $elements = [];
    foreach ($items as $delta => $item) {
      if ($item->date) {
        /** @var \Drupal\Core\Datetime\DrupalDateTime $date */
        $date = $item->date;
        $elements[$delta] = $this->buildDateWithIsoAttribute($date);
        if (!empty($item->_attributes)) {
          $elements[$delta]['#attributes'] += $item->_attributes;
          // Unset field item attributes since they have been included in the
          // formatter output and should not be rendered in the field template.
          unset($item->_attributes);
        }
      }
    }
    return $elements;
  }
  
  /**
   * Creates a formatted date value as a string.
   *
   * @param object $date
   *   A date object.
   *
   * @return string
   *   A formatted date string using the chosen format.
   */
  abstract protected function formatDate($date);
  
  /**
   * Sets the proper time zone on a DrupalDateTime object for the current user.
   *
   * A DrupalDateTime object loaded from the database will have the UTC time
   * zone applied to it.  This method will apply the time zone for the current
   * user, based on system and user settings.
   *
   * @param \Drupal\Core\Datetime\DrupalDateTime $date
   *   A DrupalDateTime object.
   */
  protected function setTimeZone(DrupalDateTime $date) {
    if ($this->getFieldSetting('datetime_type') === DateTimeItem::DATETIME_TYPE_DATE) {
      // A date without time has no timezone conversion.
      $timezone = DateTimeItemInterface::STORAGE_TIMEZONE;
    }
    else {
      $timezone = date_default_timezone_get();
    }
    $date->setTimeZone(timezone_open($timezone));
  }
  
  /**
   * Gets a settings array suitable for DrupalDateTime::format().
   *
   * @return array
   *   The settings array that can be passed to DrupalDateTime::format().
   */
  protected function getFormatSettings() {
    $settings = [];
    if ($this->getSetting('timezone_override') != '') {
      $settings['timezone'] = $this->getSetting('timezone_override');
    }
    return $settings;
  }
  
  /**
   * Creates a render array from a date object.
   *
   * @param \Drupal\Core\Datetime\DrupalDateTime $date
   *   A date object.
   *
   * @return array
   *   A render array.
   */
  protected function buildDate(DrupalDateTime $date) {
    $this->setTimeZone($date);
    $build = [
      '#markup' => $this->formatDate($date),
      '#cache' => [
        'contexts' => [
          'timezone',
        ],
      ],
    ];
    return $build;
  }
  
  /**
   * Creates a render array from a date object with ISO date attribute.
   *
   * @param \Drupal\Core\Datetime\DrupalDateTime $date
   *   A date object.
   *
   * @return array
   *   A render array.
   */
  protected function buildDateWithIsoAttribute(DrupalDateTime $date) {
    // Create the ISO date in Universal Time.
    $iso_date = $date->format("Y-m-d\\TH:i:s") . 'Z';
    $this->setTimeZone($date);
    $build = [
      '#theme' => 'time',
      '#text' => $this->formatDate($date),
      '#attributes' => [
        'datetime' => $iso_date,
      ],
      '#cache' => [
        'contexts' => [
          'timezone',
        ],
      ],
    ];
    return $build;
  }

}