NAME

       StateVar - state variable subject base class

SYNOPSIS

       #include <Unidraw/statevar.h>

DESCRIPTION

       StateVar  is  the  base  class  for  state  variable subjects.  State variables provide a standard way to
       represent and access state that can be modified by the user or through dataflow.  State variable subjects
       store the state, while state variable views provide a graphical interface for examining  and  potentially
       modifying  the  state  in  their  subject.   The  StateVar  class defines the protocol for state variable
       subjects.  It is an abstract class and thus cannot be instantiated.

PUBLIC OPERATIONS

       Connector* GetBinding()
              Return the connector to which the state variable is bound to support dataflow.  Data flows through
              connected connectors with bound state variables: the library automatically assigns one bound state
              variable's state to another's.  The direction of this  assignment  depends  on  the  participating
              connectors.  See Connector(3U) for details on how to bind state variables to connectors and how to
              specify the direction of flow.

       virtual void Attach(StateVarView*)
       virtual void Detach(StateVarView*)
              Attach associates the subject with a view, represented by an instance of a StateVarView  subclass.
              Detach  dissociates  a previously attached view.  These operations do not ensure that the argument
              is an appropriate view of the subject.

       virtual void Notify()
              Notify iterates through the views, calling each one's Update operation to notify the view that the
              subject has potentially changed.  Notify  is  usually  called  by  the  subject's  state-modifying
              operations.

       virtual StateVar& operator = (StateVar&)
              The  assignment operation should be redefined to carry out subclass-specific assignment semantics.
              Typically this operator calls the parent classes' assignment operator and then performs additional
              assignments of its own.  This operator is used during dataflow to transfer state variable values.

       virtual StateVar* Copy()
              Return a copy of the state variable.  Subclasses should  redefine  this  operation  to  return  an
              instance of their type.

       virtual void Read(istream&)
       virtual void Write(ostream&)
              Read  and write the state variable's contents to a stream to support catalog operations.  Read and
              write typically call first the corresponding operations defined by their parent  class,  and  then
              they  read  or  write  their class-specific state.  Note that you must ensure that the objects are
              read in the same order they are written.

       virtual ClassId GetClassId()
       virtual boolean IsA(ClassId)
              GetClassId returns the unique class identifier  for  the  StateVar  subclass,  while  IsA  returns
              whether  the  instance  is  of  a  class  or  subclass corresponding to the given identifier.  IsA
              typically checks the given identifier against the instance's own (as  defined  by  its  GetClassId
              operation)  and,  failing  that,  calls  its  parent  classes' IsA operation.  All subclasses must
              redefine GetClassId and IsA to ensure that their identifiers are unique  and  that  instances  are
              written and read properly.

PROTECTED OPERATIONS

       StateVar()
              The constructor is protected to prevent instantiation.

SEE ALSO

       Catalog(3U), Connector(3U), Creator(3U), StateVarView(3U), classes(3U), istream(3C++), ostream(3C++)

Unidraw                                           6 August 1990                                     StateVar(3U)