class LayoutTestPlugin

class LayoutTestPlugin extends LayoutDefault implements PluginFormInterface {
  
  /**
   * {@inheritdoc}
   */
  public function defaultConfiguration() {
    return [
      'setting_1' => 'Default',
    ];
  }
  
  /**
   * {@inheritdoc}
   */
  public function buildConfigurationForm(array $form, FormStateInterface $form_state) {
    $form['setting_1'] = [
      '#type' => 'textfield',
      '#title' => 'Blah',
      '#default_value' => $this->configuration['setting_1'],
    ];
    return $form;
  }
  
  /**
   * {@inheritdoc}
   */
  public function validateConfigurationForm(array &$form, FormStateInterface $form_state) {
    if ($form_state->getValue('setting_1') === 'Test Validation Error Message') {
      $form_state->setErrorByName('setting_1', 'Validation Error Message');
    }
  }
  
  /**
   * {@inheritdoc}
   */
  public function submitConfigurationForm(array &$form, FormStateInterface $form_state) {
    $this->configuration['setting_1'] = $form_state->getValue('setting_1');
  }

}