class TableDragExampleNestedForm

class TableDragExampleNestedForm extends FormBase {
    
    /**
     * The database connection.
     *
     * @var \Drupal\Core\Database\Connection
     */
    protected $database;
    
    /**
     * The renderer.
     *
     * @var \Drupal\Core\Render\RendererInterface
     */
    protected $render;
    
    /**
     * {@inheritdoc}
     */
    public static function create(ContainerInterface $container) {
        return new static($container->get('database'), $container->get('renderer'));
    }
    
    /**
     * Construct a form.
     *
     * @param \Drupal\Core\Database\Connection $database
     *   The database connection.
     * @param \Drupal\Core\Render\RendererInterface $render
     *   The renderer.
     */
    public function __construct(Connection $database, RendererInterface $render) {
        $this->database = $database;
        $this->render = $render;
    }
    
    /**
     * {@inheritdoc}
     */
    public function getFormId() {
        return 'tabledrag_example_parent_form';
    }
    
    /**
     * Build the parent-child example form.
     *
     * Tabledrag will take care of updating the parent_id relationship on each
     * row of our table when we drag items around, but we need to build out the
     * initial tree structure ourselves. This means ordering our items such
     * that children items come directly after their parent items, and all items
     * are sorted by weight relative to their siblings.
     *
     * @param array $form
     *   Render array representing from.
     * @param \Drupal\Core\Form\FormStateInterface $form_state
     *   Current form state.
     *
     * @return array
     *   The render array defining the elements of the form.
     */
    public function buildForm(array $form, FormStateInterface $form_state) {
        $form['table-row'] = [
            '#type' => 'table',
            '#header' => [
                $this->t('Name'),
                $this->t('Description'),
                $this->t('Weight'),
                $this->t('Parent'),
            ],
            '#empty' => $this->t('Sorry, There are no items!'),
            // TableDrag: Each array value is a list of callback arguments for
            // drupal_add_tabledrag(). The #id of the table is automatically
            // prepended; if there is none, an HTML ID is auto-generated.
'#tabledrag' => [
                [
                    'action' => 'match',
                    'relationship' => 'parent',
                    'group' => 'row-pid',
                    'source' => 'row-id',
                    'hidden' => TRUE,
                    
                    /* hides the WEIGHT & PARENT tree columns below */
'limit' => FALSE,
                ],
                [
                    'action' => 'order',
                    'relationship' => 'sibling',
                    'group' => 'row-weight',
                ],
            ],
        ];
        // Build the table rows and columns.
        //
        // The first nested level in the render array forms the table row, on which
        // you likely want to set #attributes and #weight.
        // Each child element on the second level represents a table column cell in
        // the respective table row, which are render elements on their own. For
        // single output elements, use the table cell itself for the render element.
        // If a cell should contain multiple elements, simply use nested sub-keys to
        // build the render element structure for the renderer service as you would
        // everywhere else.
        $results = $this->getData();
        foreach ($results as $row) {
            // TableDrag: Mark the table row as draggable.
            $form['table-row'][$row->id]['#attributes']['class'][] = 'draggable';
            // Indent item on load.
            if (isset($row->depth) && $row->depth > 0) {
                $indentation = [
                    '#theme' => 'indentation',
                    '#size' => $row->depth,
                ];
            }
            // Some table columns containing raw markup.
            $form['table-row'][$row->id]['name'] = [
                '#markup' => $row->name,
                '#prefix' => !empty($indentation) ? $this->render
                    ->render($indentation) : '',
            ];
            $form['table-row'][$row->id]['description'] = [
                '#type' => 'textfield',
                '#required' => TRUE,
                '#default_value' => $row->description,
            ];
            // This is hidden from #tabledrag array (above).
            // TableDrag: Weight column element.
            $form['table-row'][$row->id]['weight'] = [
                '#type' => 'weight',
                '#title' => $this->t('Weight for ID @id', [
                    '@id' => $row->id,
                ]),
                '#title_display' => 'invisible',
                '#default_value' => $row->weight,
                // Classify the weight element for #tabledrag.
'#attributes' => [
                    'class' => [
                        'row-weight',
                    ],
                ],
            ];
            $form['table-row'][$row->id]['parent']['id'] = [
                '#parents' => [
                    'table-row',
                    $row->id,
                    'id',
                ],
                '#type' => 'hidden',
                '#value' => $row->id,
                '#attributes' => [
                    'class' => [
                        'row-id',
                    ],
                ],
            ];
            $form['table-row'][$row->id]['parent']['pid'] = [
                '#parents' => [
                    'table-row',
                    $row->id,
                    'pid',
                ],
                '#type' => 'number',
                '#size' => 3,
                '#min' => 0,
                '#title' => $this->t('Parent ID'),
                '#default_value' => $row->pid,
                '#attributes' => [
                    'class' => [
                        'row-pid',
                    ],
                ],
            ];
        }
        $form['actions'] = [
            '#type' => 'actions',
        ];
        $form['actions']['submit'] = [
            '#type' => 'submit',
            '#value' => $this->t('Save All Changes'),
        ];
        $form['actions']['cancel'] = [
            '#type' => 'submit',
            '#value' => 'Cancel',
            '#attributes' => [
                'title' => $this->t('Return to TableDrag Overview'),
            ],
            '#submit' => [
                '::cancel',
            ],
            '#limit_validation_errors' => [],
        ];
        return $form;
    }
    
    /**
     * Form submission handler for the 'Return to' action.
     *
     * @param array $form
     *   An associative array containing the structure of the form.
     * @param \Drupal\Core\Form\FormStateInterface $form_state
     *   The current state of the form.
     */
    public function cancel(array &$form, FormStateInterface $form_state) {
        $form_state->setRedirect('tabledrag_example.description');
    }
    
    /**
     * Submit handler for the form.
     *
     * Updates the 'weight' column for each element in our table, taking into
     * account that item's new order after the drag and drop actions have been
     * performed.
     *
     * @param array $form
     *   An associative array containing the structure of the form.
     * @param \Drupal\Core\Form\FormStateInterface $form_state
     *   The current state of the form.
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
        // Because the form elements were keyed with the item ids from the database,
        // we can simply iterate through the submitted values.
        $submissions = $form_state->getValue('table-row');
        foreach ($submissions as $id => $item) {
            $this->database
                ->update('tabledrag_example')
                ->fields([
                'weight' => $item['weight'],
                'pid' => $item['pid'],
                'description' => $item['description'],
            ])
                ->condition('id', $id, '=')
                ->execute();
        }
    }
    
    /**
     * Retrieves the tree structure from db and sorts by parent/child/weight.
     *
     * The sorting should result in children items immediately following their
     * parent items, with items at the same level of the hierarchy sorted by
     * weight.
     *
     * The approach used here may be considered too database-intensive.
     * Optimization of the approach is left as an exercise for the reader. :)
     *
     * @return array
     *   An associative array storing our ordered tree structure.
     */
    public function getData() {
        // Get all 'root node' items (items with no parents), sorted by weight.
        $root_items = $this->database
            ->select('tabledrag_example', 't')
            ->fields('t')
            ->condition('pid', '0', '=')
            ->condition('id', 11, '<')
            ->orderBy('weight')
            ->execute()
            ->fetchAll();
        // Initialize a variable to store our ordered tree structure.
        $tree = [];
        // Depth will be incremented in our getTree()
        // function for the first parent item, so we start it at -1.
        $depth = -1;
        // Loop through the root item, and add their trees to the array.
        foreach ($root_items as $root_item) {
            $this->getTree($root_item, $tree, $depth);
        }
        return $tree;
    }
    
    /**
     * Recursively adds $item to $item_tree, ordered by parent/child/weight.
     *
     * @param mixed $item
     *   The item.
     * @param array $tree
     *   The item tree.
     * @param int $depth
     *   The depth of the item.
     */
    public function getTree($item, array &$tree = [], &$depth = 0) {
        // Increase our $depth value by one.
        $depth++;
        // Set the current tree 'depth' for this item, used to calculate
        // indentation.
        $item->depth = $depth;
        // Add the item to the tree.
        $tree[$item->id] = $item;
        // Retrieve each of the children belonging to this nested demo.
        $children = $this->database
            ->select('tabledrag_example', 't')
            ->fields('t')
            ->condition('pid', $item->id, '=')
            ->condition('id', 11, '<')
            ->orderBy('weight')
            ->execute()
            ->fetchAll();
        foreach ($children as $child) {
            // Make sure this child does not already exist in the tree, to
            // avoid loops.
            if (!in_array($child->id, array_keys($tree))) {
                // Add this child's tree to the $itemtree array.
                $this->getTree($child, $tree, $depth);
            }
        }
        // Finished processing this tree branch.  Decrease our $depth value by one
        // to represent moving to the next branch.
        $depth--;
    }

}