class FieldTestItem

class FieldTestItem extends FieldItemBase {
  
  /**
   * Counts how many times all items of this type are saved.
   *
   * @var int
   */
  protected static $counter = [];
  
  /**
   * {@inheritdoc}
   */
  public static function propertyDefinitions(FieldStorageDefinitionInterface $field_definition) {
    $properties['value'] = DataDefinition::create('string')->setLabel(new TranslatableMarkup('Test value'))
      ->setRequired(TRUE);
    return $properties;
  }
  
  /**
   * {@inheritdoc}
   */
  public static function schema(FieldStorageDefinitionInterface $field_definition) {
    return [
      'columns' => [
        'value' => [
          'type' => 'varchar',
          'length' => 255,
        ],
      ],
    ];
  }
  
  /**
   * {@inheritdoc}
   */
  public function __construct(DataDefinitionInterface $definition, $name = NULL, TypedDataInterface $parent = NULL) {
    parent::__construct($definition, $name, $parent);
    $name = $this->getFieldDefinition()
      ->getName();
    static::$counter[$name] = 0;
  }
  
  /**
   * {@inheritdoc}
   */
  public function preSave() {
    $name = $this->getFieldDefinition()
      ->getName();
    static::$counter[$name]++;
    // Overwrite the field value unless it is going to be overridden, in which
    // case its final value will already be different from the current one.
    if (!$this->getEntity()
      ->isNew() && !$this->mustResave()) {
      $this->setValue('overwritten');
    }
  }
  
  /**
   * {@inheritdoc}
   */
  public function postSave($update) {
    // Determine whether the field value should be rewritten to the storage. We
    // always rewrite on create as we need to store a value including the entity
    // id.
    $resave = !$update || $this->mustResave();
    if ($resave) {
      $entity = $this->getEntity();
      $definition = $this->getFieldDefinition();
      $name = $definition->getName();
      $value = 'field_test:' . $name . ':' . $entity->id() . ':' . static::$counter[$name];
      $this->setValue($value);
    }
    return $resave;
  }
  
  /**
   * Checks whether the field item value should be resaved.
   *
   * @return bool
   *   TRUE if the item should be resaved, FALSE otherwise.
   */
  protected function mustResave() {
    return $this->getValue()['value'] == 'resave';
  }
  
  /**
   * {@inheritdoc}
   */
  public function delete() {
    parent::delete();
    $deleted_languages = \Drupal::state()->get('entity_test.delete.' . $this->getFieldDefinition()
      ->getName(), []);
    $deleted_languages[] = $this->getLangcode();
    \Drupal::state()->set('entity_test.delete.' . $this->getFieldDefinition()
      ->getName(), $deleted_languages);
  }

}