[Deskbar] Anticipating future backwards incompatibility

["Nigel Tao" <nigel tao gnome gmail com>]

Just a thought I had when playing around with the 2.19 series.  It's
had a good scrubbing in the refactoring effort, but e.g. extensions
written for the 2.18 series no longer work.

It might be possible to write an adapter class to convert a 2.18 style
Handler to a 2.20 style Module (e.g. wrap or dynamically mix-in or
some other Python magic) an old-style Handler to add a get_actions()
method to a 2.18 Match object, for example.  For that to be even
feasible, we would have to be able to tell the old and new style
apart.

Fortunately, we can distinguish between 2.18 (and 2.16 and 2.14) style
Handlers and 2.20 style Modules because the former live in
~/.gnome2/deskbar-applet/handlers and the latter in
~/.gnome2/deskbar-applet/modules.  Which is all well and good.  But
suppose that we do another refactoring in 2.26 that breaks API
compatibility again.  We might not find a suitable name that isn't
"Handler" or "Module".

Thus, it might be better to rename ~/.gnome2/deskbar-applet/modules to
~/.gnome2/deskbar-applet/modules-2.20-compatible

Similarly, we might want to rename history_new.pickle to
history-2.20-compatible.pickle, or in general, stamp a version on
anything persistent, in anticipation of future backwards
incompatibility.

Also, we might require Module writers to implement an
is_compatible_with(deskbar_version) method or maybe a
get_module_for_deskbar_version(deskbar_version_string) method.  For
example, [1] presents a Tracker Handler/Module that attempts to be
both 2.14 and 2.20 compatible.  I'm not sure if it's best if the core
deskbar code does this sort of conditional loading, or whether the
extension writers do this.  It would be nice if, supposing we broke
the Module API in 2.32, that my old 2.20 Module still worked with
Deskbar 2.32 since Deskbar 2.32 first asked for a 2.32-compatible
version and, failing that, adapted a 2.20-compatible version.

Something to think about, perhaps, before we actually mark our
upcoming 2.19 / 2.20 Modules as stable.


[1] http://mail.gnome.org/archives/tracker-list/2007-August/msg00118.html