book_link_weight-1.0.0-beta1/book_link_weight.module

<?php

/**
 * @file
 * Hooks to enhance the book with a bool link weight interface.
 */

use Drupal\Core\Form\FormStateInterface;

/**
 * Implements hook_form_NODE_FORM_alter().
 */
function book_link_weight_form_node_form_alter(&$form, FormStateInterface $form_state, $form_id) {
  if (isset($form['book'])) {
    \Drupal::service('book_link_weight.form')->alterBookOutlineForm($form, $form_state);
  }
}

/**
 * Implements hook_form_alter().
 */
function book_link_weight_form_alter(&$form, FormStateInterface $form_state, $form_id) {
  // If this form is the book outline page then add the weight control
  // to the form.
  // Using this method because the book_outline_form does not provide
  // a base form id.
  if (substr($form_id, 0, 4) === 'node' && substr($form_id, -17) === 'book_outline_form') {
    if (isset($form['book'])) {
      \Drupal::service('book_link_weight.form')->alterBookOutlineForm($form, $form_state);
    }
  }
}