Copyright (C) 1992, Digital Equipment Corporation                         
 All rights reserved.                                                      
 See the file COPYRIGHT for a full description.                            
 Last modified on Mon Jun 14 20:56:38 PDT 1993 by meehan 
      modified on Thu Feb  4 17:23:31 PST 1993 by mhb    
      modified on Tue Jun 16 13:09:00 PDT 1992 by muller 
      modified on Thu Apr  2 21:31:51     1992 by steveg 
A ChoiceVBT multi-filter behaves in concert with other ChoiceVBTs to implement {\em radio buttons}. Abstractly, a ChoiceVBT v consists of

      state(v)       TRUE or FALSE
      group(v)       a set of ChoiceVBTs (the {\em radio group})
A group g consist of

      selection(g)   the one member of g whose state is TRUE,
                     or NIL if there is no such member.
state(v) is defined as v = selection (group (v)).

Structurally, a ChoiceVBT is identical to a BooleanVBT: it is a multi-filter that maintains a Boolean state for its VBT-child. All events are forwarded to the VBT-child.

When the action procedure of the button would normally be invoked, the value of the state of the ChoiceVBT is toggled and the callback method on the ChoiceVBT is invoked.

The multi-child of a ChoiceVBT is defined to be the multi-child of the ButtonVBT.


IMPORT BooleanVBT, ButtonVBT;

  <* SUBTYPE T <: MultiFilter.T *>
  T <: Public;
  Public = BooleanVBT.T OBJECT
             <* LL <= VBT.mu *>
             init (button: ButtonVBT.T; group: Group): T;
The call v.init(...) initializes v as a ChoiceVBT with an initial state of FALSE. It is added to the radio group group.

TYPE Group <: ROOT;
A Group is a set of ChoiceVBTs.

A ChoiceVBT v is added to a group when v is initialized. When v is discarded, it is removed from its group.

PROCEDURE Get (v: T): T;
<* LL.sup = VBT.mu *>
Return selection(group(v))

<* LL.sup = VBT.mu *>
Equivalent to selection(group(v)) := v

PROCEDURE Clear (v: T);
<* LL.sup = VBT.mu *>
Equivalent to selection(group(v)) := NIL

PROCEDURE Selection (group: Group): T;
<* LL.sup = VBT.mu *>
Return selection(group)

END ChoiceVBT.