On Jan 14, 2005, at 19:02, Glyph Lefkowitz wrote:
On Fri, 2005-01-14 at 10:07 -0500, Phillip J. Eby wrote:
Maybe I'm missing something, but for those interfaces, isn't it okay to
keep the state in the *adapted* object here? In other words, if PointPen
just added some private attributes to store the extra data?
Here's another example: I have a business logic class which lives in an object database, typically used for a web application. I convert this into a desktop application. Now, I want to adapt IBusinessThunk to IGtkUIPlug. In the process of doing so, I have to create a GTK widget, loaded out of some sort of resource file, and put it on the screen. I have to register event handlers which are associated with that adapter.
The IBusinessThunk interface doesn't specify a __dict__ attribute as
part of the interface, or the ability to set arbitrary attributes. And
nor should it! It is stored in an indexed database where every
attribute has to be declared, maybe, or perhaps it uses Pickle and
sticking a GTK widget into its representation would make it
un-pickleable. Maybe it's using an O/R mapper which loses state that is
not explicitly declared or explicitly touch()ed. There are a variety of
problems which using it in this unsupported way might create, but as the
implementor of a IGtkUIPlug, I should be concerned *only* with what
IBusinessThunk provides, which is .embezzle()
and .checkFundsAvailable(). I am not writing an adapter from
DBBusinessThunkImpl, after all, and perhaps I am receiving a test
implementation that works entirely differently.
This example gets to the heart of what makes interfaces useful to me -
model/view separation. Although one might be hard pressed to call some
of the things I use adaptation for "views", the idea of mediated access
from a user, or from network protocol, or from some internal code acting
on behalf of a user is the overwhelming majority of my use-cases.
I think the idea is that it's "better" to have an adapter from IBusinessThunk -> IGtkUIPlugFactory, which you can use to *create* a stateful object that complies with the IGtkUIPlug interface.
This way, you are explicitly creating something entirely new (derived from something else) with its own lifecycle and state and it should be managed accordingly. This is clearly not simply putting a shell around an IBusinessThunk that says "act like this right now".
-bob
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