I think there's something to be said for exploring a somewhat generic
preferences architecture, or at the very least a set of conventions -
duck typing for preferences if you will (if it looks like a preference,
and acts like a preference, its probably a preference)
In the mozilla project (full disclosure: I was the owner of the
preferences backend) we developed a system where all prefs are stored in
a central place, and each pref has its own name within the a private
namespace, such as "browser.cache.memory.enable". 3rd party
plugins/extensions can "register" new prefs. This allows for a
particularly useful feature in mozilla, 'about:config' - go ahead - type
this into the URL bar in FireFox. What you'll get is a UI to edit all
global prefs across the system. Neat, huh?
However, I personally don't think that kind of system is quite
appropriate for a project like ours, especially given the dynamic nature
of both python and the repository.. but there are a few things about the
mozilla prefs system that were useful specifically for preferences:
1) a global list of 'all preferences' so that they could be reflected
into the UI dynamically like about:config.
Of course, all of our repository can be reflected into the UI, but I
think there is value in distinguishing those values in the repository
that give some obvious, useful, and predictable change in the behavior
of the application.
2) the ability to register for changes to a preference, or a set of
preferences. This is useful because any one of a number of actions could
change the value of a preference, not just a preferences dialog box.
For instance, you can register a callback for when the value of
"network.protocol-handler.external.mailto" changed, or for
"network.protocol-handler.external.*" to capture all sub-changes.
Both of these features are almost trivial given the repository.
I like philip's PrefsForMyParcel, and in fact I think we could
accomplish most of 1) and 2) by making a base Kind/class, ParcelPrefs
(or something..maybe just 'Preferences') that each parcel could derive
from, and declare new schema attributes to store individual preferences.
The KindCollection for ParcelPrefs would then be the 'registry' of all
prefs in the system.
the only trick at that point is the notification when preferences
change... but we have a number of systems for notifications, so I'm
willing to bet that would be fairly easy and we can address it when
there is actually a need.
Alec
Phillip J. Eby wrote:
At 08:57 PM 11/2/2005 -0800, Jeffrey Harris wrote:
Hi Folks,
What's our current thinking on how a developer should go about
establishing and using what I'll call a preference, essentially a single
persistent value with a well-known name (in this case, I'm wanting to
persist the last directories chosen for import and export)?
We've got lots of well-known collections living in
parcels/osaf/app/__init__.py, perhaps that's the appropriate place for
preferences? It doesn't feel quite right to me...
I'm sending this question to the list instead of asking one person or
another because
A) I think there might be different opinions, and
B) I'm hoping someone will write up a detailed example so that knowledge
can live on in the list, not just my brain :)
The simplest thing that would work:
class PrefsForMyParcel(schema.Item):
some_pref = schema.One(sometype, defaultValue=something)
# ... other preferences
def installParcel(parcel, oldVersion=None):
PrefsForMyParcel.update(parcel, "prefs")
Accessing preferences can then be done via:
schema.ns("my.parcel", view).prefs.some_pref
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