class FormTestMachineNameValidationForm

class FormTestMachineNameValidationForm extends FormBase {
    
    /**
     * {@inheritdoc}
     */
    public function getFormId() {
        return 'form_test_machine_name_validation_form';
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildForm(array $form, FormStateInterface $form_state) {
        // Disable client-side validation so that we can test AJAX requests with
        // invalid input.
        $form['#attributes']['novalidate'] = 'novalidate';
        $form['name'] = [
            '#type' => 'textfield',
            '#default_value' => $form_state->getValue('name'),
            '#maxlength' => 50,
            '#required' => TRUE,
            '#title' => 'Name',
        ];
        // The default value simulates how an entity form works, which has default
        // values based on an entity, which is updated in an afterBuild callback.
        // During validation and after build, limit_validation_errors is not
        // in effect, which means that getValue('id') does return a value, while it
        // does not during the submit callback. Therefore, this test sets the value
        // in ::buildAjaxSnackConfigureFormValidate() and then uses that as the
        // default value, so that the default value and the value are identical.
        $form['id'] = [
            '#type' => 'machine_name',
            '#default_value' => $form_state->get('id'),
            '#maxlength' => 50,
            '#required' => TRUE,
            '#machine_name' => [
                'exists' => [
                    $this,
                    'load',
                ],
                'source' => [
                    'name',
                ],
            ],
        ];
        // Test support for multiple machine names on the form. Although this has
        // the default value duplicate it should not generate an error because it
        // is the default value.
        $form['id2'] = [
            '#type' => 'machine_name',
            '#default_value' => 'duplicate',
            '#maxlength' => 50,
            '#required' => TRUE,
            '#machine_name' => [
                'exists' => [
                    $this,
                    'load',
                ],
                'source' => [
                    'name',
                ],
            ],
        ];
        $form['snack'] = [
            '#type' => 'select',
            '#title' => $this->t('Select a snack'),
            '#options' => [
                'apple' => 'apple',
                'pear' => 'pear',
                'potato' => 'potato',
            ],
            '#required' => TRUE,
            '#ajax' => [
                'callback' => '::buildAjaxSnackConfigureForm',
                'wrapper' => 'snack-config-form',
                'method' => 'replace',
                'effect' => 'fade',
            ],
        ];
        $form['snack_configs'] = [
            '#type' => 'container',
            '#attributes' => [
                'id' => 'snack-config-form',
            ],
            '#tree' => TRUE,
        ];
        $form['submit'] = [
            '#type' => 'submit',
            '#value' => 'Save',
        ];
        return $form;
    }
    
    /**
     * Validate callback that forces a form rebuild.
     */
    public function buildAjaxSnackConfigureFormValidate(array $form, FormStateInterface $form_state) {
        $form_state->set('id', $form_state->getValue('id'));
    }
    
    /**
     * Submit callback that forces a form rebuild.
     */
    public function buildAjaxSnackConfigureFormSubmit(array $form, FormStateInterface $form_state) {
        $form_state->setRebuild();
    }
    
    /**
     * Handles changes to the selected snack configuration.
     */
    public function buildAjaxSnackConfigureForm(array $form, FormStateInterface $form_state) {
        return $form['snack_configs'];
    }
    
    /**
     * Loading stub for machine name
     *
     * @param $machine_name
     *
     * @return bool
     */
    public function load($machine_name) {
        if (strpos($machine_name, 'duplicate') !== FALSE) {
            return TRUE;
        }
        return FALSE;
    }
    
    /**
     * {@inheritdoc}
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
        $this->messenger()
            ->addStatus('The form_test_machine_name_validation_form form has been submitted successfully.');
    }

}