Eelco Hillenius wrote:
Hi,
One /very/ important reason not to use interfaces in this case is that:
interfaces are hard to evolve. Interfaces have to be pretty darn stable
before even considering throwing them out in the public, as there is no
way (not without a tough fight at least) you can alter it later on -
even adding a new method will break all clients.
Hi
Agree completely here. I still have on my ming DirectShow interfaces
like IFilterGraph, IFilterGraph2, IMediaEvent, IMediaEventEx. Interfaces
and backward compatibility, that's not a good match.
Although I do use interfaces for my facade/dao. Spring like pattern. But
adding one method means changing facade interface, facade
implementation, dao interface and dao implementation. That can be a lot
of work. But for that you get declarative transaction management,
logging, etc.
But wicket pages? IMHO injecting pages doesn't make much sense, because
they are often being serialized and serializing facade/dao/ (The usual
injected things) is often impossible.
AOP could be nice, although I can't really think of anything useful that
wouldn't be possible with subclassing. And for that you would have to
handle spring creating pages and give up passing arguments through page
contructor.
Just my two cents...
-Matej
Another reason not to use interfaces here is that there is so much
behaviour in here (in the pages and components), that it is very hard to
extract something usefull out of it. What would the interface of a Page
be? One of the good practices when making interfaces is that they should
be small. Having an interface like
http://jakarta.apache.org/tapestry/tapestry/apidocs/org/apache/tapestry/IPage.html
doesn't really display that. They can be large, like the collection
interfaces are, but they have to be really, really stable and consistent.
Just to be sure, and as I didn't make this decision for Wicket I asked
Jonathan Locke (who is doing movie stuff now, and left the mailing lists
for some time) why he choose for interfaces. Here's a partial transcript
of our chat:
[22:31] jonathan: i don't believe in big interfaces
[22:31] jonathan: no interface should be as large as Page
[22:32] jonathan: it's too fragile
[22:35] jonathan: abstract classes are evolvable
[22:36] jonathan: interfaces should always be a set of methods that you
cannot ever imagine extending
[22:36] jonathan: it encapsulates a very specific set of behaviors
[22:37] jonathan: everyone should feel free to create/extract his own
IPage interface for use with proxies
[22:38] jonathan: there's something fundamentally wrong with the way
that current OO languages try to wire things together with static
interfaces.
[22:39] jonathan: unless i'm missing something, this is really not a
problem with wicket
[22:40] jonathan: in fact i think it's a feature
[22:40] jonathan: interfaces are really most useful for things that are
truly pluggable
[22:43] jonathan: my ideal number of methods in an interface is 1
[22:43] jonathan: ;-)
[22:44] jonathan: 2 is okay in some cases
[22:44] jonathan: 3 often should be re-thought
[22:44] jonathan: the more methods you have, the more likely it is that
you've got a bad design
So, it is a design decission not to work with interfaces. However, as
Jonathan stated, you could extract an interface, let your page class
implement it and off you go. Furthermore, I wonder what it is you want
to do with a proxy that you can't do by simply subclassing? Or AOP?
As a final statement I'd like to add that a framework like Wicket is
very different from a business system, which could have tons of business
methods that must be pluggable. Though even in that case, I think too
that interfaces are overused/ get too much focus with some of the
current popular frameworks. Seems to be getting a knee-jerk reaction
instead of a design decision. But maybe that's just me.
Regards,
Eelco
Peter Veentjer - Anchor Men wrote:
I`m trying to add Spring annotations to the Page, but because there is
no page interface and only a Page class (with final methods) it is not
possible to create a proxy and therefor it is impossible to enhance
objects. This is very inconvenient..
Why are the components not designed from interfaces? Designing from
interfaces is common oo practice.
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