class ConfigTestForm

class ConfigTestForm extends EntityForm {
    
    /**
     * {@inheritdoc}
     */
    public function form(array $form, FormStateInterface $form_state) {
        $form = parent::form($form, $form_state);
        $entity = $this->entity;
        $form['label'] = [
            '#type' => 'textfield',
            '#title' => 'Label',
            '#default_value' => $entity->label(),
            '#required' => TRUE,
        ];
        $form['id'] = [
            '#type' => 'machine_name',
            '#default_value' => $entity->id(),
            '#required' => TRUE,
            '#machine_name' => [
                'exists' => [
                    $this,
                    'exists',
                ],
                'replace_pattern' => '[^a-z0-9_.]+',
            ],
        ];
        $form['weight'] = [
            '#type' => 'weight',
            '#title' => 'Weight',
            '#default_value' => $entity->get('weight'),
        ];
        $form['style'] = [
            '#type' => 'select',
            '#title' => 'Image style',
            '#options' => [],
            '#default_value' => $entity->get('style'),
            '#access' => FALSE,
        ];
        if ($this->moduleHandler
            ->moduleExists('image')) {
            $form['style']['#access'] = TRUE;
            $form['style']['#options'] = image_style_options();
        }
        // The main premise of entity forms is that we get to work with an entity
        // object at all times instead of checking submitted values from the form
        // state.
        $size = $entity->get('size');
        $form['size_wrapper'] = [
            '#type' => 'container',
            '#attributes' => [
                'id' => 'size-wrapper',
            ],
        ];
        $form['size_wrapper']['size'] = [
            '#type' => 'select',
            '#title' => 'Size',
            '#options' => [
                'custom' => 'Custom',
            ],
            '#empty_option' => '- None -',
            '#default_value' => $size,
            '#ajax' => [
                'callback' => '::updateSize',
                'wrapper' => 'size-wrapper',
            ],
        ];
        $form['size_wrapper']['size_submit'] = [
            '#type' => 'submit',
            '#value' => t('Change size'),
            '#attributes' => [
                'class' => [
                    'js-hide',
                ],
            ],
            '#submit' => [
                [
                    static::class,
                    'changeSize',
                ],
            ],
        ];
        $form['size_wrapper']['size_value'] = [
            '#type' => 'select',
            '#title' => 'Custom size value',
            '#options' => [
                'small' => 'Small',
                'medium' => 'Medium',
                'large' => 'Large',
            ],
            '#default_value' => $entity->get('size_value'),
            '#access' => !empty($size),
        ];
        $form['langcode'] = [
            '#type' => 'language_select',
            '#title' => t('Language'),
            '#languages' => LanguageInterface::STATE_ALL,
            '#default_value' => $entity->language()
                ->getId(),
        ];
        $form['actions'] = [
            '#type' => 'actions',
        ];
        $form['actions']['submit'] = [
            '#type' => 'submit',
            '#value' => 'Save',
        ];
        $form['actions']['delete'] = [
            '#type' => 'submit',
            '#value' => 'Delete',
        ];
        return $form;
    }
    
    /**
     * Ajax callback for the size selection element.
     */
    public static function updateSize(array $form, FormStateInterface $form_state) {
        return $form['size_wrapper'];
    }
    
    /**
     * Element submit handler for non-JS testing.
     */
    public static function changeSize(array $form, FormStateInterface $form_state) {
        $form_state->setRebuild();
    }
    
    /**
     * {@inheritdoc}
     */
    public function save(array $form, FormStateInterface $form_state) {
        $entity = $this->entity;
        $status = $entity->save();
        if ($status === SAVED_UPDATED) {
            $this->messenger()
                ->addStatus(new FormattableMarkup('%label configuration has been updated.', [
                '%label' => $entity->label(),
            ]));
        }
        else {
            $this->messenger()
                ->addStatus(new FormattableMarkup('%label configuration has been created.', [
                '%label' => $entity->label(),
            ]));
        }
        $form_state->setRedirectUrl($this->entity
            ->toUrl('collection'));
        return $status;
    }
    
    /**
     * Determines if the entity already exists.
     *
     * @param string|int $entity_id
     *   The entity ID.
     * @param array $element
     *   The form element.
     * @param \Drupal\Core\Form\FormStateInterface $form_state
     *   The current state of the form.
     *
     * @return bool
     *   TRUE if the entity exists, FALSE otherwise.
     */
    public function exists($entity_id, array $element, FormStateInterface $form_state) {
        
        /** @var \Drupal\Core\Config\Entity\ConfigEntityInterface $entity */
        $entity = $form_state->getFormObject()
            ->getEntity();
        
        /** @var \Drupal\Core\Config\Entity\ConfigEntityStorageInterface $storage */
        $storage = $this->entityTypeManager
            ->getStorage($entity->getEntityTypeId());
        return (bool) $storage->getQuery()
            ->condition($entity->getEntityType()
            ->getKey('id'), $entity_id)
            ->execute();
    }

}