book_form_update($form, FormStateInterface$form_state)
Renders a new parent page select element when the book selection changes.
This function is called via Ajax when the selected book is changed on a node or book outline form.
The rendered parent page select element.
function book_form_update($form, FormStateInterface $form_state) { return $form['book']['pid']; }
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https://api.drupal.org/api/drupal/core!modules!book!book.module/function/book_form_update/8.1.x