trait ComputedFieldItemTrait in Computed Field 3.x

trait ComputedFieldItemTrait {

  /**
   * {@inheritdoc}
   */
  public function isEmpty() {
    $value = $this
      ->getRawResult();
    if ($value === NULL || $value === '') {
      return TRUE;
    }
    return FALSE;
  }

  /**
   * Fetches the raw result of the computation.
   *
   * @return mixed
   */
  protected function getRawResult() {
    return $this
      ->executeCode();
  }

  /**
   * {@inheritdoc}
   */
  public function preSave() {
    $value = $this
      ->executeCode();
    $this
      ->setValue($value);
  }

  /**
   * Performs the field value computation.
   *
   * If this method is being overridden to return a typed result, the class must
   * use ComputedFieldStronglyTypedItemTrait to ensure access to raw results.
   *
   * @see ComputedFieldStronglyTypedItemTrait
   */
  public function executeCode() {
    $entity = $this
      ->getEntity();
    $delta = $this->name;
    $field_name = $this->definition
      ->getFieldDefinition()
      ->getName();
    $value = \Drupal::service('computed_field.helpers')
      ->executeCode($field_name, $entity, $delta);
    return $value;
  }

  /**
   * Default field settings form.
   */
  public function fieldSettingsForm(array $form, FormStateInterface $form_state) {
    $field_name = $this->definition
      ->getFieldDefinition()
      ->getName();
    $service = \Drupal::service('computed_field.helpers');
    $element = [];
    $element['hook_info'] = [
      '#markup' => t('
<p>The hook implementation function signature should be
  <strong>%function($entity_type_manager, $entity, $fields, $delta)</strong>,
  and the desired value should be returned.</em>
</p>
<p>The variables available to your code include:</p>
<ul>
  <li><code>$entity_type_manager</code>: The entity type manager.</li>
  <li><code>$entity</code>: The entity the field belongs to.</li>
  <li><code>$fields</code>: The list of fields available in this entity.</li>
  <li><code>$delta</code>: Current index of the field in case of multi-value computed fields (counting from 0).</li>
  <li><code>$value</code>: The resulting value to be set above, or returned in your hook implementation).</li>
</ul>
      ', [
        '%function' => $service
          ->getComputeFunctionName($field_name),
      ]),
    ];
    return $element;
  }

}