needs-update

Called by the snip to request that the snip's display needs to be updated. The administrator determines when to actually update the snip; the snip's drawmethod is eventually called.

No update occurs if the given snip is not managed by this administrator.

• (send a-snip-admin needs-update snip localx localy w h) -> void
      snip : snip% object
      localx : real number
      localy : real number
      w : non-negative real number
      h : non-negative real number

  The localx, localy, w, and h arguments specify a region of the snip to be refreshed (in snip coordinates).