layout switcher for Drupal

layouts.module.php

<?php
function lib_layouts_entity_view_mode_alter(&$view_mode, EntityInterface $entity, $context) {
  if ($entity->getEntityTypeId() == 'node' && $entity->bundle() == 'landing_page' && $view_mode == 'full') {

    if ($entity->hasField('field_layout')) {
      $field = $entity->get('field_layout');
      if (!$field->isEmpty()) {
        $layout = $entity->get('field_layout')->getString();
        $available_modes = \Drupal::service('entity_display.repository')->getViewModes('node');
        if (array_key_exists($layout, $available_modes)) {
          $view_mode = $layout;
        }
      }
    }
  }
}