I have found a small design problem with the current event system code
and Jorrit recommended I ask the list for optionions on how to fix it.
So here it goes.
THE PROBLEM:
The csevSystemOpen (and csevSystemClose) events need to be delivered
in the same order that their plugins were loaded (reverse order for
csevSystemClose?). With the old system that was easy because event
handlers would just be put on the end of a list at subscription time
and would get handled in that order. With the new event system this
is no longer the case and event handlers are stored in a partial
ordering graph.
The plugin loading ordering dependencies are currently stored in the
.csplugin files while the event handler ordering constraints are
specified as part of member functions in the event handlers. These
two pieces of information have not been kept in sync. The current
partial ordering graph code just happens to order them in the correct
order, but as simple a change as the patch below [1] will cause the
event handlers to be in the wrong order. (I was writing some
optimizations for the partial order graph when I found this problem.
With the current code, patch [1] will result in a distorted view
frustum due to engine and render3d having their events in the wrong
order.)
Since we have not been able to keep the plugin loading and the event
system orderings in sync, it would be nice if we could derive the
event system ordering from the plugin loading order and avoid having
to specify the same information twice.
Anyone have any ideas for a solution?
OBSTACLES:
Two obstacles that you should be aware of are that plugin dependencies
may be specified as a wild card (e.g. "crystal.graphics3d." <- note
the dot) that loads a plugin (e.g. "crystal.graphics3d.opengl") with a
more specific name than the event handler name that the plugin uses
(e.g. "crystal.graphics3d"). The other is that the event handler may
be using a (false) embedded class such as
csGLGraphics3D::scfiEventHandler so it is not possible to just ask for
the event handler from the plugin and form their get it's event
handler name.
IDEAS:
Now I have a few ideas for working around this but other solutions are welcome:
(1) make event handler names be hierachrical (i.e. crystal.render3d as
a pre constraint means that crystal.render3d.opengl is also a
pre-constraint)
(2) whenever a wildcard plugin name is found (e.g. crystal.render3d.)
assume that it is a generic event name (e.g. crystal.render3d);
(3) actually put the event handler ordering constraints in csplugin
along side of and seperate from the plugin dependancy constraints;
(4) do special magic for csevSystemOpen that makes the partial order
behave like a FIFO for that particular event (may be important if an
event handler, a, requests plugins that have constraints requiring
it's event handler, b, to occur before an event handler, c, that has
already fired, with the current system, that new event handler, b,
will receive the csevSystemOpen message if c has already fired (the
user's fault but it might be hard to debug))
On IRC Jorrit noted that "(1) has the disadvantage that it ties a
meaning to a name and that it should be possible to make a renderer
event handler that is for example called
'cel.specialcelrenderer.eventhandler'. And if other applications
register their event handlers that implement some common interface
then they might want to use 'planeshift.render3d.' .... It might be
that the solutions are actually worse then the original problem
(duplication of information)."
Michael D. Adams
[1] This patch simply makes the partial order solution produces
results in a different though equally correct order.
Index: include/csutil/partialorder.h
===================================================================
--- include/csutil/partialorder.h (revision 25228)
+++ include/csutil/partialorder.h (working copy)
@@ -288,7 +288,8 @@
do
{
done = true;
- for (size_t iter=0 ; iter<Nodes.Length() ; iter++)
+ //for (size_t iter=0 ; iter<Nodes.Length() ; iter++)
+ for (size_t iter=Nodes.Length(); iter-- > 0; )
{
if (Nodes[iter].output == false)
{
@@ -391,7 +392,8 @@
*/
const T GetEnabled(T fail)
{
- for (size_t i=0 ; i<Nodes.Length() ; i++)
+ //for (size_t i=0 ; i<Nodes.Length() ; i++)
+ for (size_t i=Nodes.Length() ; i-- > 0; )
{
if (InternalIsEnabled(i))
return Nodes[i].self;
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