class NestedConfigTargetForm

class NestedConfigTargetForm extends TreeConfigTargetForm {
  
  /**
   * {@inheritdoc}
   */
  protected function getEditableConfigNames() {
    return [
      'form_test.object',
    ];
  }
  
  /**
   * {@inheritdoc}
   */
  public function getFormId() {
    return 'form_test_nested_config_target_form';
  }
  
  /**
   * {@inheritdoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state) {
    $form['favorites'] = [
      '#type' => 'details',
      '#open' => TRUE,
      '#tree' => TRUE,
      '#title' => 'Favorite fruits',
    ];
    $form['favorites']['first'] = [
      '#type' => 'textfield',
      '#title' => 'First choice',
      '#config_target' => new ConfigTarget('form_test.object', 'favorite_fruits', fromConfig: fn(?array $favorite_fruits): string => $favorite_fruits[0] ?? 'Mango', toConfig: fn(string $first, FormStateInterface $form_state): array => [
        0 => $first,
        1 => $form_state->getValue([
          'favorites',
          'second',
        ]),
      ]),
    ];
    $form['favorites']['second'] = [
      '#type' => 'textfield',
      '#title' => 'Second choice',
      '#config_target' => new ConfigTarget('form_test.object', 'favorite_fruits.1', fn(?string $second_favorite_fruit): string => $second_favorite_fruit ?? 'Orange', fn() => ToConfig::NoOp),
    ];
    $form['could_not_live_without'] = [
      '#weight' => 10,
      '#type' => 'textfield',
      '#title' => 'I could not live without',
      '#placeholder' => 'vegetables',
      '#wrapper_attributes' => [
        'class' => [
          'container-inline',
        ],
      ],
      '#config_target' => new ConfigTarget('form_test.object', 'could_not_live_without', toConfig: function (string $could_not_live_without, FormStateInterface $form_state) : ToConfig|string {
        if (empty($form_state->getValue([
          'favorites',
          'first',
        ]))) {
          return ToConfig::DeleteKey;
        }
        if (empty($form_state->getValue([
          'favorites',
          'second',
        ]))) {
          return ToConfig::DeleteKey;
        }
        if (empty($form_state->getValue([
          'vegetables',
          'favorite',
        ]))) {
          return ToConfig::DeleteKey;
        }
        if (empty($form_state->getValue([
          'vegetables',
          'nemesis',
        ]))) {
          return ToConfig::DeleteKey;
        }
        return $could_not_live_without;
      }),
      // Only if everything else is answered will this be asked.
'#states' => [
        'visible' => [
          // 2 favorite fruits.
':input[name="favorites[first]"]' => [
            'empty' => FALSE,
          ],
          ':input[name="favorites[second]"]' => [
            'empty' => FALSE,
          ],
          // Favorite & nemesis vegetable.
':input[name="vegetables[favorite]"]' => [
            'empty' => FALSE,
          ],
          ':input[name="vegetables[nemesis]"]' => [
            'empty' => FALSE,
          ],
        ],
      ],
    ];
    return parent::buildForm($form, $form_state);
  }

}