class AjaxColorForm

class AjaxColorForm extends DemoBase {
    
    /**
     * {@inheritdoc}
     */
    public function getFormId() {
        return 'form_api_example_ajax_color_demo';
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildForm(array $form, FormStateInterface $form_state) {
        $form['description'] = [
            '#type' => 'item',
            '#markup' => $this->t('This form example demonstrates functioning of an AJAX callback.'),
        ];
        // The #ajax attribute used in the temperature input element defines an ajax
        // callback that will invoke the 'updateColor' method on this form object.
        // Whenever the temperature element changes, it will invoke this callback
        // and replace the contents of the 'color_wrapper' container with the
        // results of this method call.
        $form['temperature'] = [
            '#title' => $this->t('Temperature'),
            '#type' => 'select',
            '#options' => $this->getColorTemperatures(),
            '#empty_option' => $this->t('- Select a color temperature -'),
            '#ajax' => [
                // Could also use [get_class($this), 'updateColor'].
'callback' => '::updateColor',
                'wrapper' => 'color-wrapper',
            ],
        ];
        // Add a wrapper that can be replaced with new HTML by the ajax callback.
        // This is given the ID that was passed to the ajax callback in the '#ajax'
        // element above.
        $form['color_wrapper'] = [
            '#type' => 'container',
            '#attributes' => [
                'id' => 'color-wrapper',
            ],
        ];
        // Add a color element to the color_wrapper container using the value
        // from temperature to determine which colors to include in the select
        // element.
        $temperature = $form_state->getValue('temperature');
        if (!empty($temperature)) {
            $form['color_wrapper']['color'] = [
                '#type' => 'select',
                '#title' => $this->t('Color'),
                '#options' => $this->getColorsByTemperature($temperature),
            ];
        }
        // Add a submit button that handles the submission of the form.
        $form['actions'] = [
            '#type' => 'actions',
            'submit' => [
                '#type' => 'submit',
                '#value' => $this->t('Submit'),
            ],
        ];
        return $form;
    }
    
    /**
     * Ajax callback for the color dropdown.
     */
    public function updateColor(array $form, FormStateInterface $form_state) {
        return $form['color_wrapper'];
    }
    
    /**
     * Returns colors that correspond with the given temperature.
     *
     * @param string $temperature
     *   The color temperature for which to return a list of colors. Can be either
     *   'warm' or 'cool'.
     *
     * @return array
     *   An associative array of colors that correspond to the given color
     *   temperature, suitable to use as form options.
     */
    protected function getColorsByTemperature($temperature) {
        return $this->getColors()[$temperature]['colors'];
    }
    
    /**
     * Returns a list of color temperatures.
     *
     * @return array
     *   An associative array of color temperatures, suitable to use as form
     *   options.
     */
    protected function getColorTemperatures() {
        return array_map(function ($color_data) {
            return $color_data['name'];
        }, $this->getColors());
    }
    
    /**
     * Returns an array of colors grouped by color temperature.
     *
     * @return array
     *   An associative array of color data, keyed by color temperature.
     */
    protected function getColors() {
        return [
            'warm' => [
                'name' => $this->t('Warm'),
                'colors' => [
                    'red' => $this->t('Red'),
                    'orange' => $this->t('Orange'),
                    'yellow' => $this->t('Yellow'),
                ],
            ],
            'cool' => [
                'name' => $this->t('Cool'),
                'colors' => [
                    'blue' => $this->t('Blue'),
                    'purple' => $this->t('Purple'),
                    'green' => $this->t('Green'),
                ],
            ],
        ];
    }

}