StateVar(3U)						    InterViews Reference Manual 					      StateVar(3U)

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  inter-
       face  for  examining and potentially modifying the state in their subject.  The StateVar class defines the protocol for state variable sub-
       jects.  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 spec-
	      ify 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)