class Simplest

class Simplest extends FormBase {
  
  /**
   * {@inheritdoc}
   */
  public function getFormId() {
    return 'ajax_example_simplest';
  }
  
  /**
   * {@inheritdoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state) {
    $form['changethis'] = [
      '#title' => $this->t("Choose something and explain why"),
      '#type' => 'select',
      '#options' => [
        'one' => 'one',
        'two' => 'two',
        'three' => 'three',
      ],
      '#ajax' => [
        // #ajax has two required keys: callback and wrapper.
        // 'callback' is a function that will be called when this element
        // changes.
'callback' => '::promptCallback',
        // 'wrapper' is the HTML id of the page element that will be replaced.
'wrapper' => 'replace-textfield-container',
      ],
    ];
    // The 'replace-textfield-container' container will be replaced whenever
    // 'changethis' is updated.
    $form['replace_textfield_container'] = [
      '#type' => 'container',
      '#attributes' => [
        'id' => 'replace-textfield-container',
      ],
    ];
    $form['replace_textfield_container']['replace_textfield'] = [
      '#type' => 'textfield',
      '#title' => $this->t("Why"),
    ];
    // An AJAX request calls the form builder function for every change.
    // We can change how we build the form based on $form_state.
    $value = $form_state->getValue('changethis');
    // The getValue() method returns NULL by default if the form element does
    // not exist. It won't exist yet if we're building it for the first time.
    if ($value !== NULL) {
      $form['replace_textfield_container']['replace_textfield']['#description'] = $this->t("Say why you chose '@value'", [
        '@value' => $value,
      ]);
    }
    return $form;
  }
  
  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {
    // No-op. Our form doesn't need a submit handler, because the form is never
    // submitted. We add the method here so we fulfill FormInterface.
  }
  
  /**
   * Handles switching the available regions based on the selected theme.
   */
  public function promptCallback($form, FormStateInterface $form_state) {
    return $form['replace_textfield_container'];
  }

}