[udig-devel] Catalog / Layer contract, was UDig on Windows XP

[Jody Garnett <jgarnett@xxxxxxxxxxxxxxx>]

Morning Moovida; this email is a replacement for your Thursday IRC 
meeting ;-)

It seems that there has been a break down of trust between the project 
and the catalog layers of our application. I was not aware of this 
change when it was introduced, it seems that the project plugin is using 
both:
- "layer id" to look up an entry in the catalog; and
- it was also using the connection parameters

There is a relationship between layer id, and connection parameters. 
They need to be exactly in sync or the catalog starts getting confused.

I was surprised that this was even possible - here is how it was done:
- IServiceFactory.acquire( id, params )

The javadocs for this method say: "is intended to be used when replacing 
an IService entry in a catalog". This method is used by the catalog 
framework when setting up for a replacing a moved service, ie it is a 
step that is needed when we are about to fire a REPLACE event.

So what has happened is when lazy loading of a map was introduced; maps 
were no longer around to listen to REPLACE events and thus could not be 
kept in sync with the catalog. The above acquire method was used to 
allow these out of sync map layers to fumble along using an ID that did 
not exactly match the intrinsic identity of the resource on disk.

This amounts to a breakdown of trust between the two systems :-) The 
concept of an ID is no longer trusted by layer to a be a good handle for 
resource lookup; this is something we can fix.

I would like to introduce a "IForward" class into the catalog; something 
that can be left behind in the catalog when a resource is moved. That 
way projects that were closed still have a way of noticing and fixing 
themselves when they are loaded.  These handles can be used just alike a 
IService returned from a search; they can act as a wrapper for the 
actual service (so simple code can stay simple), although I will make 
them printin warning messages if uDig is being run in -debug mode.

It is my hope that with this addition trust can be established again and 
the project layer implementation can be simplified, and made much less 
insane ;-)

Jody