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++)