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From: kit@ATHENA.MIT.EDU (Chris D. Peterson)
Newsgroups: comp.windows.x
Subject: Re: Widgets and Xlib
Message-ID: <8901262004.AA03005@DORA.MIT.EDU>
Date: 26 Jan 89 20:04:33 GMT
References: <33500005@iuvax>
Sender: daemon@bloom-beacon.MIT.EDU
Organization: The Internet
Lines: 75


> From: beckman@iuvax.cs.indiana.edu

> I am using the HP Widget set.  How do I use the Xlib graphic primitives 
> within a widget to draw lines, boxes, etc.  I haven't had any success. 
> I have tried lots of different things, the most recent being this:
> 
> Display *dpy;
> Drawable win;
> GC gc;
> 
> dpy = XtDisplay(widget)
> win = XtWindow(widget);
> gc = XtGetGC(widget);
> 
> XDrawLine(dpy,win,gc,0,0,100,100);

     This is relatively common problem that many people run into when
beginning to program in X because X is an asynchronous environment.
This is necessary to achieve network transparency and not be horribly
slow.  Here are some concepts that you need to be aware of when
programming in X.

1) X is a client server model, all application programs are clients
   that because X has a standard protocol are able to talk to any X server.

2) Because X is an asynchronous environment you are never sure when
   your X functions will actually be processed by the X server.

3) X does not remember what is drawn into the window, and it is the
   client's responsibility to repair any damage done to his window.

4) Since window managers can intercept mapping events you are never
   sure when you client's windows will actually get mapped, unless 
   you are looking for the correct events.

Thus the program above may end up drawing the line before the window is 
visible on the screen.

    All good X clients are responsible for repainting their contents
when these contents become damaged.  The server will notify the client
that some part of his window is damaged and that it needs to be
repainted by issuing an Expose event on that window for each
rectangular region that has been damaged.  Clever clients will only
repaint the particular region of the screen that has been damaged.
Clients that are not quite this clever can repaint the entire window
when an expose event is received for a particular window.  It is
important to check the exposure count field (count == 0) to make sure
that you do not repaint the window more often than necessary.

     If you are using the X toolkit the easiest way to receive expose
events is either:

1) Add an entry in the translation table and register an action
   that should be performed on that entry.

2) Us XtAddEventHandler to register a function that should be called
   when an exposure event is received.

References:  Xlib Manual Chapter 8  : Events and Event Handling.

             Xtk  Manual Chapter 7  : Event Management.
	                 Chapter 10 : Translation Management.
	                 Appendix B : Translation Table Syntax.


						Chris D. Peterson     
						MIT X Consortium /
						Project Athena 

Net:	kit@athena.mit.edu		
Phone: (617) 253 - 1326			
USMail: MIT - Room E40-321
	77 Massachusetts Ave.		
	Cambridge, MA 02139