Drupal 8 tabledrag form example

<?php

/**
 * {@inheritdoc}
 */
public function buildForm(array $form, FormStateInterface $form_state) {
  $group_class = 'group-order-weight';
  $items = [
    1 => [
      'id' => 2,
      'label' => $this->t('Label 1'),
      'weight' => 1,
    ],
    2 => [
      'id' => 5,
      'label' => $this->t('Label 2'),
      'weight' => 2,
    ],
    3 => [
      'id' => 56,
      'label' => $this->t('Label 3'),
      'weight' => 3,
    ],
  ];

  // Build table.
  $form['items'] = [
    '#type' => 'table',
    '#caption' => $this->t('Items'),
    '#header' => [
      $this->t('Label'),
      $this->t('ID'),
      $this->t('Weight'),
    ],
    '#empty' => $this->t('No items.'),
    '#tableselect' => FALSE,
    '#tabledrag' => [
      [
        'action' => 'order',
        'relationship' => 'sibling',
        'group' => $group_class,
      ]
    ]
  ];

  // Build rows.
  foreach ($items as $key => $value) {
    $form['items'][$key]['#attributes']['class'][] = 'draggable';
    $form['items'][$key]['#weight'] = $value['weight'];

    // Label col.
    $form['items'][$key]['label'] = [
      '#plain_text' => $value['label'],
    ];

    // ID col.
    $form['items'][$key]['id'] = [
      '#plain_text' => $value['id'],
    ];

    // Weight col.
    $form['items'][$key]['weight'] = [
      '#type' => 'weight',
      '#title' => $this->t('Weight for @title', ['@title' => $value['label']]),
      '#title_display' => 'invisible',
      '#default_value' => $value['weight'],
      '#attributes' => ['class' => [$group_class]],
    ];
  }

  // Form action buttons.
  $form['actions'] = ['#type' => 'actions'];
  $form['actions']['submit'] = [
    '#type' => 'submit',
    '#value' => $this->t('Save'),
  ];

  return $form;
}