private void OnTriggerEnter2D(Collider2D collision)
 {
     if (collision.GetComponent <DustDropper>() != null)
     {
         _activeDropper = collision.GetComponent <DustDropper>();
     }
 }
 private void OnTriggerExit2D(Collider2D collision)
 {
     if (collision.GetComponent <DustDropper>() == _activeDropper)
     {
         _activeDropper = null;
     }
 }