[M3devel] small design problem in ButtonVBT?

[Tony Hosking]

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.

Am I just being paranoid?

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
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