How to loop nav menu items in WordPress menu Walker?

I want to loop every single item in the menu with the walker class.

class Primary_Walker_Nav_Menu extends Walker_Nav_Menu {     public $  bg;      function __construct() {         $  this->bg = get_field('navigation_menu_background_image', 'option'); //acf array in function.php     }           } 

In $ this->bg are an array of images, I want to have $ depth less from 0, which means the only the first item in-depth, not sub-menu, nothing else.

My idea is to loop $ i item and get $ bg item on position $ i, how to do this?

Help, thanks!