> from: David Megginson <[EMAIL PROTECTED]>
> date: Tue, 18 Jun 2002 14:53:00
> to: [EMAIL PROTECTED]
> subject: Re: [Flightgear-devel] static variables
>
> Curt and I have discussed this before (either online or offline). We
> could certainly have per-vehicle subtrees, i.e.
>
...
>
> and so on, and that would let us share some common, sim-wide
> information at the top level, and would make all program-wide
> information available at a single glance. The alternative is to use
> the multi-process model rather than a multi-thread model, and to swap
> entire property trees in and out like memory pages; we'll certainly
> avoid a lot of bugs that way (because the wrong properties simply
> won't be available).
>
> What does everyone else think?
I think it will become clear straight away that the vehicle-specific properties will
have to be organizationally separated from the base simulator properties. The initial
profusion of bugs will be due to treating a property as global when it should really
be vehicle-specific, and vice versa. I don't think there that one vehicle will
accidentally access another vehicle's properties.
You can't swap multiple copies of the _whole_ property tree, because base properties
would get in a complete mess. You could swap the major branch that represents the
vehicle, but you still have to re-organise so that all (and only) relevant properties
are in that branch.
At that point, the difference between swapping among copies of a branch and indexing
that branch is fundamentally very little. The indexing could and should be made
almost transparent within a given module, say the FDM, by having FDM property accessor
functions that automatically supply the appropriate index or node pointer for that
instance of the FDM. E.g. fdmGetInt("/rudder-pedal-pos") calls
fgGetInt("/vehicle[n]/fdm/rudder-pedal-pos") [that is just an example of the concept,
not a suggestion of actual names or style].
Although the reorganisation will be a lot of work, it will improve the modularity and
clarity of the program design even when it is only running one vehicle.
- Julian
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