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 con-
              nected  connectors  with bound state variables: the
              library automatically assigns one bound state vari-
              able'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, repre-
              sented 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 dur-
              ing dataflow to transfer state variable values.

       virtual StateVar* Copy()
              Return a copy of the  state  variable.   Subclasses

       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 opera-
              tions  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  correspond-
              ing  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 instantia-
              tion.

SEE ALSO

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