notify-on-change

Attaches a callback to the style list. The callback is invoked whenever a style is modified.

Often, a change in one style will trigger a change in several other derived styles; to allow clients to handle all the changes in a batch, #f is passed in as the changing style after a set of styles has been processed.

The return value from notify-on-change is an opaque key to be used with forget-notification.

• (send a-style-list notify-on-change f) -> value
      f : procedure of one argument: a style<%> object or #f