[Opengc-devel] status report

[Damion S. <da...@op...>]

Hi folks,

A quick update as to what I've been working on in the past month. I 
_finally_ solved the problem of how to maintain an authoritative list 
of objects within the OpenGC world for the purposes of passing messages 
around (for instance, between the zoom in/out buttons on the nav 
display and the nav map). My first thought was to try to adapt the 
Object class from the Insight Toolkit project, but after several days 
of experimentation this proved to be much harder than I had hoped and 
required a great deal of code which would have no real function in 
OpenGC.

The solution I ended up implementing is to write a simple ogcObject 
class that handles all of the message passing tasks, and then derive 
the RenderObject (and therefore all gauges) from this. Eventually, data 
sources and render windows will also be ogcObjects. The new ogcObject 
contains a static linked list of pointers to ogcObjects. The 
constructor of an ogcObject adds the "this" pointer to this list and 
the destructor searches the list and removes it. The net result is that 
there is a statically accessible list of all ogcObjects; message 
passing occurs by iterating through this list and posting the message 
to each object.

In retrospect this is a fairly obvious solution, and it seems to work 
as expected. The code that was "broken" in the new OpenGC repository 
(the map zoom, keypad demo, etc.) is now working again. I've updated 
the opengcnew CVS repository if you want to give this a try.

The other model for message passing that I plan on adding is a 
transmitter/receiver model. Right now, messages are dispatched to _all_ 
OpenGC objects even if they're only appropriate for one. Ambiguity 
could result if there were two nav displays, for example, because the 
zoom controls for one should not affect the other (but would, at the 
moment). This should be easy to avoid by providing a means to set 
certain objects as receivers of the messages of a particular 
"transmitter", and providing a second dispatch function that iterates 
only over a given object's receivers rather than every OpenGC object.

Issues with my current design MAY include:

1) Thread safety
2) The inconvenience of having to initialize a static member (the 
object list) from outside the class

I have not done much programming with static members, so if anyone can 
comment on the initialization process I'd appreciate it.

Cheers,
-Damion-