RE: Re: Class Inheritance

[Kevin Hawkins]

Sorry guys,

I have a domestic issue that has cropped up that is taking my time -
back very shortly - keep up the discussions - and please raise any of the
other topics too - hopefully it will be all sorted in a couple of days
time.

K

> -----Original Message-----
> From: Stuart Booth [mailto:<a
href="/group/xAP_developer/post?postID=CyKfO7Eri8Kh8iFBl36LjYLLHE0IxYnPfZiLYbLdP2hOt2cvqZIjxcp5csZ2JgcQzZUrGu_WARGsUZi0fk0oDw">lists@s...</a>]
> Sent: 25 May 2003 11:31
> To: <a
href="/group/xAP_developer/post?postID=8013Ws0uZmosZZV8MMY_ftRfienXVnaQSd7NYhvtBzJJ_xqgECwy-u9GGJv-LvY0AmYbET683w3y7t_aleyJ3lv_kDtNJg">xAP_developer@xxxxxxx</a>
> Subject: Re: [xAP_developer] Re: Class Inheritance
>
> On Sun, 25 May 2003 09:50:58 -0000, "Patrick Lidstone"
> <<a
href="/group/xAP_developer/post?postID=8SKya5TyhowcQoEC0GgoafPIv4rtsidYVZDRrGDIlLWNmtTdsCSp0GvCtjoBp7_glOr6_4k9wGTXoEtahgs">patrick@l...</a>>
wrote:
>
> >> But here I'm thinking that the derived class schema alters
the
> >> formatting/meaning of the value entirely.
> >
> >Not sure I've understood you PoV here.
>
> Ahh, I'm just mentioning the various ways I was interpreting your own
> thoughts. Reinterpreting vs Redefining were the two ways I thought of
> based on what you'd suggested.
>
> > We cannot allow a derived
> >class to redefine a base class element, only to extend the class.
> >Otherwise no device that understands just the base class can make
use
> >of it, because the meaning of a particular value may have changed!
>
> Great! I didn't much like that either.
>
> >There's an argument to be made for calling the block
> >audio.transport.meridian since the base class can be derived
> >implicitly, and this adds a clear indication of the expected block
> >content when it is broken apart from the header.
>
> So here then, a Class of xAP-Audio.Transport.Meridian demands a block
> name of Audio.Transport.Meridian.
>
> But an app listening for xAP-Audio.Transport messages could still pick
> up the xAP-Audio.Transport components and ignore anything it doesn't
> recognise.
>
> It could still read the Audio.Transport.Meridian block, but only deal
> with Audio.Transport items/values, ignoring the rest. This passes:
>
> Audio.Transport.Meridian
> {
> Command=scan
> }
>
> And if the block was named Audio.Transport, anything not part of the
> Audio.Transport block would instead be flagged as invalid. This fails:
>
> Audio.Transport
> {
> Command=scan
> }
>
> Just a thought on message validation.
>
> >I have thought of one problem with the MI arrangement. What
happens
> >if you want to control more than one "thing" in one
message (ie
> >repeat the same class more than once, in a different control
context
> >each time - perhaps to set a lighting scene) - you can't identify
the
> >significance of each block because it doesn't have a name. You
could
> >solve this easily enough by changing the format of the block name
to
> >be say <class>:<label> where :<label> is
optional. It's a big deal to
> >change the naming scheme, but it's also a big deal to limit
messages
> >to only controlling a single thing...
> >
> >Splitting two pieces of control information that belong together
into
> >separate messages is a really bad idea. It means that the receiver
> >has to be stateful (difficult on say a PIC), and it means that you
> >can have all sorts of unintended behaviour if messages arrive out
of
> >order or if message gets lost.
>
> Oh, yes, of course. That makes perfect sense.
>
> S
> --
> Stuart Booth
> xAPFramework.net - a reusable xAP framework for .net
>
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