[vwnc] denying unselection to a List widget

Hi all,

Is there a way to deny an attempt to take a single-selection List  
widget to an "unselected" state (where aList selection isNil)?

Thanks,
Ricky
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Richard Wettel wrote:
> Hi all,
>
> Is there a way to deny an attempt to take a single-selection List  
> widget to an "unselected" state (where aList selection isNil)?

AFAIK this is not readily available, you'll probably need to subclass
and specialize the Controller of your list widget to achieve this effect.

In practice we don't find this much of a problem because there are other
ways to enforce selection, for instance by disabling an 'accept' button
when there is no selection.

In several of our applications we have a status bar at the bottom of
their windows, part of this area is reserved for error messages ('No foo
selected') that explain why an 'accept' button is disabled. Our users
seem to like this arrangement.



R
-


>
> Thanks,
> Ricky
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> vwnc mailing list
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> http://lists.cs.uiuc.edu/mailman/listinfo/vwnc
>
>

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There is a way -- its not neat and clean.

First you need to know the current selection, so set up
a "Validation" method for "Entry".  When invoked, save
the current selection and return "true" (to let the select go ahead).
If you knew what the user was selecting here, you could probably
return "false" -- not sure that info is available though.

Then setup a "Notification" "Change" entry -- if the
when that method is invoked the selection is nil, just
re-select the saved item.

(Actually you may not need the "Validation" method, you could
    probably put it in "Notification Entry" -- which ignores what
    you return -- my pattern has been Validation-Entry + Notification-Change).

Reinout Heeck wrote:

Richard Wettel wrote:
  

Hi all,

Is there a way to deny an attempt to take a single-selection List  
widget to an "unselected" state (where aList selection isNil)?
    

AFAIK this is not readily available, you'll probably need to subclass 
and specialize the Controller of your list widget to achieve this effect.

In practice we don't find this much of a problem because there are other 
ways to enforce selection, for instance by disabling an 'accept' button 
when there is no selection.

In several of our applications we have a status bar at the bottom of 
their windows, part of this area is reserved for error messages ('No foo 
selected') that explain why an 'accept' button is disabled. Our users 
seem to like this arrangement.



R
-


  

Thanks,
Ricky
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On Jun 25, 2008, at 4:55 AM, Dennis Smith wrote:



There is a way -- it works pretty well I think.

For your list, add a callback for validation-->change. Call it  
something like #dontAllowUnselect:

Then add that method to your ApplicationModel, implemented as such:

dontAllowUnselect: aSequenceController
       
        ^aSequenceController view selectionIndex ~= aSequenceController view  
targetIndex


Promote the method up to ApplicationModel as a class extension and  
reuse at will.

The multi-selection variant is left to the student. :)

--
Travis Griggs
Objologist
"Every institution finally perishes by an excess of its own first  
principle." - Lord Acton



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Thanks for all your answers.

Cheers,
Ricky


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