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) {
        // This is called very early by the user entity roles field. Prevent
        // early t() calls by using the TranslatableMarkup.
        $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);
    }

}