Drupal 8 Form API

form-validation-tree.php

  <?php

  /**
   * {@inheritdoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state) {
    // todo: add to schema.
    $form['sync'] = [
      '#type' => 'details',
      '#title' => $this->t('Synchronization settings'),
      '#open' => TRUE,
    ];
    $form['sync']['rules'] = [
      '#type' => 'details',
      '#title' => $this->t('Rules'),
      '#required' => TRUE,
      '#open' => TRUE,
      '#tree' => TRUE,
    ];
    $form['sync']['rules']['import'] = [
      '#type' => 'textarea',
      '#title' => $this->t('Import rule'),
      '#default_value' => $this->config('pim.sync')->get('rules.import'),
      '#rows' => 10,
    ];
    $form['sync']['rules']['publication'] = [
      '#type' => 'textarea',
      '#title' => $this->t('Publication rule'),
      '#default_value' => $this->config('pim.sync')->get('rules.publication'),
      '#rows' => 10,
    ];

    return parent::buildForm($form, $form_state);
  }
  
  /**
   * {@inheritdoc}
   */
  public function validateForm(array &$form, FormStateInterface $form_state) {

    
    $expressionLanguage = new ExpressionLanguage();
    $import = $form_state->getValue(['rules', 'import']);
    if ($expressionLanguage->evaluate($import)) {
      $form_state->setErrorByName('rules][import', $this->t('The phone number is too short. Please enter a full phone number.'));
    }
  }