code: def show_focus_indicator(viewer, tf, color='white'):
      
  
      tag = '_$focus_indicator'
      canvas = viewer.get_private_canvas()
      try:
          fcsi = canvas.get_object_by_tag(tag)
          if not tf:
              canvas.delete_object_by_tag(tag)
          else:
              fcsi.color = color
  
      except KeyError:
          if tf:
              Fcsi = canvas.get_draw_class('focusindicator')
              fcsi = Fcsi(color=color)
              canvas.add(fcsi, tag=tag, redraw=False)
              viewer.add_callback('focus', fcsi.focus_cb)
  
      canvas.update_canvas(whence=3)
docs: Show a focus indicator in the window.
  
      Parameters
      ----------
      viewer : an ImageView subclass instance
          If True, show the color bar; else remove it if present.
  
      tf : bool
          If True, show the color bar; else remove it if present.
  
      color : str
          Color for the focus indicator.