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'];
    }

}