class BooleanItem

class BooleanItem extends FieldItemBase implements OptionsProviderInterface {
  
  /**
   * {@inheritdoc}
   */
  public static function defaultFieldSettings() {
    return [
      'on_label' => new TranslatableMarkup('On'),
      'off_label' => new TranslatableMarkup('Off'),
    ] + parent::defaultFieldSettings();
  }
  
  /**
   * {@inheritdoc}
   */
  public static function propertyDefinitions(FieldStorageDefinitionInterface $field_definition) {
    $properties['value'] = DataDefinition::create('boolean')->setLabel(t('Boolean value'))
      ->setRequired(TRUE);
    return $properties;
  }
  
  /**
   * {@inheritdoc}
   */
  public static function schema(FieldStorageDefinitionInterface $field_definition) {
    return [
      'columns' => [
        'value' => [
          'type' => 'int',
          'size' => 'tiny',
        ],
      ],
    ];
  }
  
  /**
   * {@inheritdoc}
   */
  public function fieldSettingsForm(array $form, FormStateInterface $form_state) {
    $element = [];
    $element['on_label'] = [
      '#type' => 'textfield',
      '#title' => $this->t('"On" label'),
      '#default_value' => $this->getSetting('on_label'),
      '#required' => TRUE,
    ];
    $element['off_label'] = [
      '#type' => 'textfield',
      '#title' => $this->t('"Off" label'),
      '#default_value' => $this->getSetting('off_label'),
      '#required' => TRUE,
    ];
    return $element;
  }
  
  /**
   * {@inheritdoc}
   */
  public function getPossibleValues(AccountInterface $account = NULL) {
    return [
      0,
      1,
    ];
  }
  
  /**
   * {@inheritdoc}
   */
  public function getPossibleOptions(AccountInterface $account = NULL) {
    return [
      0 => $this->getSetting('off_label'),
      1 => $this->getSetting('on_label'),
    ];
  }
  
  /**
   * {@inheritdoc}
   */
  public function getSettableValues(AccountInterface $account = NULL) {
    return [
      0,
      1,
    ];
  }
  
  /**
   * {@inheritdoc}
   */
  public function getSettableOptions(AccountInterface $account = NULL) {
    return $this->getPossibleOptions($account);
  }
  
  /**
   * {@inheritdoc}
   */
  public static function generateSampleValue(FieldDefinitionInterface $field_definition) {
    $values['value'] = mt_rand(0, 1);
    return $values;
  }

}