filter default query to show just selected level of child pages in wordpress

Question

I'm working on a site that has a fairly large page structure a few levels deep - in some sections there are a lot of pages.

So I want to have functionality if user choose on select box level 1 then only pages in level 1 will show in below list of pages.
if he choose level 2 then only level 2 pages, same go to level 3 level 4 level 5 level 6.

it's working for top level parent pages when I set query_vars['post_parent'] = 0; and I want to have same functionality to show list of level 1 child page,level 2 child pages and so on...

I am stuck on it. please I will be great full if anyone can help me for it. Thanks
see screenshot link
https://i.sstatic.net/EKpy6.png

function level_page_admin_posts_filter( $query ) {
  global $pagenow;
  if ( is_admin() && $pagenow == 'edit.php' && !empty($_GET['lavel_pages'])) {
      $level = $_GET['lavel_pages'];
      if($level == 'parent'){
        $query->query_vars['post_parent'] = 0;
      }else 
      if($level == 1){

      }else
      if($level == 2){

      }
  }
}
add_filter( 'parse_query', 'level_page_admin_posts_filter' );

function admin_page_filter_level_pages() {
  global $wpdb;
  if (isset($_GET['post_type']) && $_GET['post_type'] == 'page') {
  
  $select = '
    <select name="lavel_pages">
      <option value="parent">Parent Pages</option>
      <option value="1">Level 1</option>
      <option value="2">Level 2</option>
      <option value="3">Level 3</option>
      <option value="4">Level 4</option>
      ';
  
  $select .= ' </select>';
  echo $select;
} else {
  return;
}
}
add_action( 'restrict_manage_posts', 'admin_page_filter_level_pages' );

Answer 1

It is possible if we get all the pages and work out what level each page is on and then filter the query using "post__in". Try the following:

//recursive function to get page level, function returns when parent page is zero
function level_page_admin_get_level($parentIds, $pageId, $level)
{
  $parentId = $parentIds[$pageId];
  if($parentId>0)
  {
    $level++;
    return level_page_admin_get_level($parentIds, $parentId, $level);
  }
  return $level;
}

function level_page_admin_posts_filter($query)
{
  global $pagenow;
  if(is_admin() && $pagenow == 'edit.php' && !empty($_GET['lavel_pages']))
  {
    $level = $_GET['lavel_pages'];
    if($level == 'parent')
    {
      $query->query_vars['post_parent'] = 0;
    }
    else
    {
      //get all the pages
      $pages = get_pages();

      //loop all the pages and store the parent id
      $parentIds = array();
      foreach($pages as $page)
      {
        $parentIds[$page->ID] = $page->post_parent;
      }

      //second time loop through all the pages this time call recursive function to get level
      foreach($pages as $page)
      {
        if($page->post_parent>0)
        {
          $pageLevels[$page->ID] = level_page_admin_get_level($parentIds, $page->ID, 0);
        }
      }

      //build array of only page ids on level
      $pageIds = array();
      foreach($pageLevels as $pageId => $pageLevel)
      {
        if($pageLevel==$level)
        {
          $pageIds[] = $pageId;
        }
      }

      if(!empty($pageIds))
      {
        //limit query to those page ids
        $query->query_vars['post__in'] = $pageIds;
      }
    }
  }
}