class TestItem

class TestItem extends FieldItemBase {
  
  /**
   * {@inheritdoc}
   */
  public static function defaultStorageSettings() {
    return [
      'test_field_storage_setting' => 'dummy test string',
      'changeable' => 'a changeable field storage setting',
      'unchangeable' => 'an unchangeable field storage setting',
      'translatable_storage_setting' => 'a translatable field storage setting',
    ] + parent::defaultStorageSettings();
  }
  
  /**
   * {@inheritdoc}
   */
  public static function defaultFieldSettings() {
    return [
      'test_field_setting' => 'dummy test string',
      'translatable_field_setting' => 'a translatable field setting',
    ] + parent::defaultFieldSettings();
  }
  
  /**
   * {@inheritdoc}
   */
  public static function propertyDefinitions(FieldStorageDefinitionInterface $field_definition) {
    $properties['value'] = DataDefinition::create('integer')->setLabel(new TranslatableMarkup('Test integer value'))
      ->setRequired(TRUE);
    return $properties;
  }
  
  /**
   * {@inheritdoc}
   */
  public static function schema(FieldStorageDefinitionInterface $field_definition) {
    return [
      'columns' => [
        'value' => [
          'type' => 'int',
          'size' => 'medium',
        ],
      ],
      'indexes' => [
        'value' => [
          'value',
        ],
      ],
    ];
  }
  
  /**
   * {@inheritdoc}
   */
  public function storageSettingsForm(array &$form, FormStateInterface $form_state, $has_data) {
    $form['cardinality_container'][] = [
      '#type' => 'html_tag',
      '#tag' => 'p',
      '#value' => 'Greetings from ' . __METHOD__,
    ];
    $element = [];
    $element['test_field_storage_setting'] = [
      '#type' => 'textfield',
      '#title' => $this->t('Field test field storage setting'),
      '#default_value' => $this->getSetting('test_field_storage_setting'),
      '#required' => FALSE,
      '#description' => $this->t('A dummy form element to simulate field storage setting.'),
    ];
    return $element;
  }
  
  /**
   * {@inheritdoc}
   */
  public function fieldSettingsForm(array $form, FormStateInterface $form_state) {
    $element = [];
    $element['test_field_setting'] = [
      '#type' => 'textfield',
      '#title' => $this->t('Field test field setting'),
      '#default_value' => $this->getSetting('test_field_setting'),
      '#required' => FALSE,
      '#description' => $this->t('A dummy form element to simulate field setting.'),
    ];
    return $element;
  }
  
  /**
   * {@inheritdoc}
   */
  public function delete() : void {
    // Reports that delete() method is executed for testing purposes.
    FieldTestHelper::memorize('field_test_field_delete', [
      $this->getEntity(),
    ]);
  }
  
  /**
   * {@inheritdoc}
   */
  public function getConstraints() {
    $constraint_manager = \Drupal::typedDataManager()->getValidationConstraintManager();
    $constraints = parent::getConstraints();
    $constraints[] = $constraint_manager->create('ComplexData', [
      'properties' => [
        'value' => [
          'TestField' => [
            'value' => -1,
            'message' => $this->t('%name does not accept the value @value.', [
              '%name' => $this->getFieldDefinition()
                ->getLabel(),
              '@value' => -1,
            ]),
          ],
        ],
      ],
    ]);
    return $constraints;
  }
  
  /**
   * {@inheritdoc}
   */
  public function isEmpty() {
    return empty($this->value);
  }
  
  /**
   * {@inheritdoc}
   */
  public static function storageSettingsToConfigData(array $settings) {
    $settings['config_data_from_storage_setting'] = 'TRUE';
    unset($settings['storage_setting_from_config_data']);
    return $settings;
  }
  
  /**
   * {@inheritdoc}
   */
  public static function storageSettingsFromConfigData(array $settings) {
    $settings['storage_setting_from_config_data'] = 'TRUE';
    unset($settings['config_data_from_storage_setting']);
    return $settings;
  }
  
  /**
   * {@inheritdoc}
   */
  public static function fieldSettingsToConfigData(array $settings) {
    $settings['config_data_from_field_setting'] = 'TRUE';
    unset($settings['field_setting_from_config_data']);
    return $settings;
  }
  
  /**
   * {@inheritdoc}
   */
  public static function fieldSettingsFromConfigData(array $settings) {
    $settings['field_setting_from_config_data'] = 'TRUE';
    unset($settings['config_data_from_field_setting']);
    return $settings;
  }

}