prepopulate-8.x-2.3/prepopulate.module

<?php

/**
 * @file
 * Fill form elements with data from GET or POST values.
 */

use Drupal\Core\Form\FormStateInterface;

/**
 * Implements hook_form_alter().
 */
function prepopulate_form_alter(&$form, FormStateInterface $form_state, $form_id) {
  // If this is a subsequent step of a multi-step form, the prepopulate values
  // have done their work, and the user may have modified them: bail.
  if ($form_state->isRebuilding()) {
    return;
  }
  if (\Drupal::request()->query->has('edit')) {
    $form['#after_build'][] = 'prepopulate_after_build';
  }
}

/**
 * An #after_build function to set the values prepopulated in the request.
 */
function prepopulate_after_build($form) {
  /** @var \Drupal\prepopulate\Populate $populate */
  $populate = \Drupal::service('prepopulate.populator');
  $populate->populateForm($form);
  return $form;
}