[M3devel] small design problem in ButtonVBT?
hendrik at topoi.pooq.com
hendrik at topoi.pooq.com
Fri Aug 28 05:17:23 CEST 2009
On Thu, Aug 27, 2009 at 03:15:41PM -0400, Tony Hosking wrote:
> I am nervous about having the two alternative entries to the action
> procedure that other code might not be aware of. Yes, you've modified
> the default mouse method to call the action method instead of the
> procedure, but what about other places where the mouse method has been
> overridden.
>
> If you want to be smart about it in your own code you can simply
> invert your suggestion and do the following:
>
> TYPE T = ButtonVBT.T OBJECT
> METHODS action(cd: VBT.MouseRec) := Action;
>
> PROCEDURE Action(v: T; cd: VBT.MouseRec) = ...
>
> PROCEDURE ActionProc(v: ButtonVBT.T; cd: VBT.MouseRec) =
> BEGIN
> NARROW(v, T).action(cd);
> END ActionProc;
>
> and initialize your T thingy with ButtonVBT.New(..., action :=
> ActionProc; ...)
>
> Then in your code you can go ahead and override the action method to
> your hearts content.
This kind of thing would work. But, like the existing semantics of
ButtonVBT, it seems to be fighting the language instead of working with
it. I'd be happiest to get rid of the action variable altogether, but
that would be a massively incompatible change, and would require change
to a lot of existing code. What I proposed is compatible with existing
code, and also has the advantage that someone who subclasses ButtonVBT.T
and overrides act can completely ignore the action variable altogether.
You say
> and initialize your T thingy with ButtonVBT.New(..., action :=
> ActionProc; ...)
But that requires every object I make of type T or any sibclass to be
explicitly initialised with ActionProc, certainly a nuisance, and
certainly error-prone.
The inelegance I see with my approach is that there are two mechanisms
for specifying the action to be taken. Either will work without the
programmer paying any attention to the other, but if you use both, one
will override the other. This is not a problem with legacy software.
I'll have to come up with some documentation that makes this clear.
It's the inelegance that comes from backward compatibility, and I think
my solution is less inelegant, and easier to use, than yours. Yours,
however, involves no change in existing code.
>
> Am I just being paranoid?
No. Perhaps just paranoid about different things.
I had hoped for someone to come up with a solid technical reason for the
action variable. The original designers were clever people, and I'd
*still* like to know why they thought it was a good idea. It's awkward
and error-prone to use, and they had to use the PutProp/GetProp kludge
just to approximate what the language provided for free with inheritance.
I'm tempted to go over the entire code-base and change the buttons to
use act instead of action, but I have to admit I'm not tempted enough to
actually do it...
-- hendrik
> On 27 Aug 2009, at 08:17, Olaf Wagner wrote:
>
> >Though I promised to do it, I have lost sight of this task while
> >trying to build some RC3 archives on Windows. As I'm not at home this
> >evening, can somebody else please commit Hendrik's extension?
> >
> >Is this something we should should have in the release branch, too?
> >Probably not necessary?
> >
> >Olaf
> >
> >Quoting hendrik at topoi.pooq.com:
> >
> >>On Wed, Apr 01, 2009 at 12:03:47PM +0200, Olaf Wagner wrote:
> >>>Quoting hendrik at topoi.pooq.com:
> >>>
> >>>>The BottonVBT contains an action, which is a procedure rather
> >>>than a
> >>>>method. b This makes it awkward to subclass ButtonVBT becaue the
> >>>action
> >>>>will still expect its first parameter to be a ButtonVBT instead of
> >>>>belonging to the subclass, and an explicit downcast of some sort
> >>>will be
> >>>>needed to acess the subclass's members.
> >>>>
> >>>>The Trestle manual says this was a deliberate decision:
> >>>>
> >>>>: The action procedure is a field rather than a method in order
> >>>to allow
> >>>>: buttons with different action procedures to share their method
> >>>suites.
> >>>>
> >>>>I, however, find it massively inconvenient because it mans I have
> >>>to
> >>>>resort to downcasting or the very kludgey VBT.GetProp to access
> >>>what
> >>>>should be just there. I'd rather the action procedure was a
> >>>method.
> >>>>
> >>>>But it should be possible to have the best of both worlds. Have an
> >>>>action2 (better name wanted here) method that is available for
> >>>>overriding, and by default calls the procedure in the existing
> >>>action.
> >>>>field. That action2 method wounl then be called by Trestle
> >>>wherever
> >>>>action is now called.
> >>>>
> >>>>Does anyone see problems with this?
> >>>
> >>>Sounds fine to me offhand. You should test your extensions with
> >>>some of the existing larger applications, like mentor and Juno-2,
> >>>of course.
> >>
> >>OK. Here's the patch. Would it be possible for someone to check it
> >>in
> >>for me?
> >>
> >>mentor runs fine with this change; I watched it animate a heap-sort.
> >>
> >>I have no idea what Juno is supposed to do, but it did respond to
> >>button-pushes.
> >>
> >>
> >>hendrik at lovesong:~/cm3/love-sq/cm3/m3-ui/ui/src/split$ cvs diff
> >>cvs diff: Diffing .
> >>Index: ButtonVBT.i3
> >>===================================================================
> >>RCS file: /usr/cvs/cm3/m3-ui/ui/src/split/ButtonVBT.i3,v
> >>retrieving revision 1.2
> >>diff -r1.2 ButtonVBT.i3
> >>39a40
> >>> act(READONLY cd: VBT.MouseRec);
> >>Index: ButtonVBT.m3
> >>===================================================================
> >>RCS file: /usr/cvs/cm3/m3-ui/ui/src/split/ButtonVBT.m3,v
> >>retrieving revision 1.2
> >>diff -r1.2 ButtonVBT.m3
> >>28c28,29
> >>< init := Be
> >>---
> >>> init := Be;
> >>> act := act
> >>46a48,52
> >>>PROCEDURE act(v : T; READONLY cd: VBT.MouseRec) =
> >>> BEGIN
> >>> v.action(v, cd);
> >>> END act;
> >>>
> >>59c65
> >>< v.action(v, cd);
> >>---
> >>> v.act(cd);
> >>hendrik at lovesong:~/cm3/love-sq/cm3/m3-ui/ui/src/split$
> >>
> >>-- hendrik
> >>
> >
> >
> >
> >--
> >Olaf Wagner -- elego Software Solutions GmbH
> > Gustav-Meyer-Allee 25 / Gebäude 12, 13355 Berlin,
> >Germany
> >phone: +49 30 23 45 86 96 mobile: +49 177 2345 869 fax: +49 30 23
> >45 86 95
> > http://www.elegosoft.com | Geschäftsführer: Olaf Wagner | Sitz:
> >Berlin
> >Handelregister: Amtsgericht Charlottenburg HRB 77719 | USt-IdNr:
> >DE163214194
> >
>
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