Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!usc!snorkelwacker!bloom-beacon!attunix.att.COM!alecci
From: alecci@attunix.att.COM
Newsgroups: comp.windows.x
Subject: Re: OpenLook: drawing in bulletin boards
Message-ID: <9007241400.AA13424@expo.lcs.mit.edu>
Date: 24 Jul 90 13:57:00 GMT
Sender: daemon@athena.mit.edu (Mr Background)
Organization: The Internet
Lines: 64

> Using the bulletin board widget of Open Look I represent some widgets at
> particular coordinates. I want to draw lines or arrows between some of
> these widgets. But somehow what is drawn to the bulletin board never
> appears at the screen, not even after the bulletin board has been realised.
> 
> Investigation shows that for the bulletin board widget no expose procedure
> is defined, which gives an indication why nothing appears.
> 
> Now I have two questions:
> 
> 1. is my line of reasoning okay ?
> 
> 2. maybe somebody knows a workaround ? like a specialization of the
>    bulletin board that handles expose events ? please comment.
> 
> --------------------------------------------------------------------------------
> Finus van Cadsand, ORIGIN-MW, Eindhoven, the Netherlands
> email: finus@ait.philips.nl (will be changed to: finus@ait.origin.nl)

right, the bulletin board does not express interest in expose events.
The easiest and most straight-forward solution to your problem is to add an
event handler (using XtAddEventHandler) to the bulletin board widget that
expresses interest in exposures (event_mask set to ExposureMask).

Once this is done, your registered routine is called whenever the graphics
in the bulletin board needs refreshing.

For example:

/* ARGSUSED */
static void
DrawLines(w, client_data, xevent, continue_to_dispatch)
	Widget    w;                      /* the bulletin Board's id */
	XtPointer client_data;            /* your data               */
	XEvent *  xevent;                 /* real XEvent             */
	Boolean * continue_to_dispatch;   /* ignored                 */
{
	.... /* your interesting code */
} /* end of DrawLines() */


.... somewhere in your creation code ....

	bbw = XtCreateManagedWidget(name, bulletinBoardWidgetClass,
					parent, args, num_args);

	XtAddEventHandler(bbw, ExposureMask, False, DrawLines, NULL);


Note:
	If you create and destroy the bulletin board widget often, you
may want to remove your added event handler to free any associated memory.

For Example:

	XtRemoveEventHandler(bbw, ExposureMask, False, DrawLines, NULL);
	XtDestroyWidget(bbw);

-o Don

P.S. If you were simply doing graphics without adding children to the
bulletin board widget, I would have suggested using the Stub Widget instead
since it allows a programmer to specify (via a resource) a drawing routine as
well as provide exposure compression.