The IMContext.RetrieveSurrounding event utilizes this delegate:

Event data is passed via the Gtk.RetrieveSurroundingArgs parameter.

To attach a Gtk.RetrieveSurroundingHandler to an event, add the RetrieveSurroundingHandler instance to the event. The methods referenced by the RetrieveSurroundingHandler instance are invoked whenever the event is raised, until the RetrieveSurroundingHandler is removed from the event.

Handlers of this class should set the input method surrounding context by calling Gtk.IMContext.SetSurrounding. The method returns %TRUE if the signal was handled by the callback.