ExternView,  PreorderView,  InorderView,  PostorderView  -
       base class for external representation  objects  and  sub-
       classes for common traversals

SYNOPSIS

       #include <Unidraw/Components/externview.h>

DESCRIPTION

       ExternView  is the abstract base class for external repre-
       sentation objects.  Generally speaking, external represen-
       tation  objects convey domain-specific information outside
       the editor.  They are simply component views that  extract
       information  from  their  subject  and externalize it as a
       stream of bytes; thus the ExternView base class is derived
       from ComponentView.

       There  are  three  predefined  ExternView subclasses: Pre-
       orderView, InorderView,  and  PostorderView.   These  sub-
       classes  manage  subviews and support three common traver-
       sals of the external view hierarchy.

EXTERNVIEW PUBLIC OPERATIONS

       virtual boolean Emit(ostream&)
       virtual boolean Definition(ostream&)
              Emit and Definition make up  the  basic  ExternView
              protocol.   Emit  initiates external representation
              generation and calls  Definition  on  each  of  its
              children.   Emit normally calls the external view's
              own Definition operation first.  Then if the exter-
              nal  view contains subviews, Emit invokes the chil-
              dren's Definition operations in the proper order to
              ensure a syntactically correct external representa-
              tion.   Both  of  these  operation  do  nothing  by
              default,  and  both return a boolean that indicates
              whether the stream of bytes was generated  success-
              fully.

       ExternView* GetView(Iterator)
       void SetView(ExternView*, Iterator&)
              These operations do nothing by default.  Subclasses
              that contain children should redefine them as  fol-
              lows: GetView should return the ExternView to which
              an iterator points.  SetView should initialize  the
              iterator to point to a particular ExternView in the
              list of children; it should initialize the iterator
              to  point to a nil instance if the given ExternView
              is not a child.

EXTERNVIEW PROTECTED OPERATIONS

       ExternView(Component* = nil)
              The abstract base class constructor is protected to
              guard  against instantiation.  It takes the subject

       PreorderView,  InorderView,  and  PostorderView   redefine
       their  Definition operations to perform preorder, inorder,
       and postorder traversals of their children,  respectively.
       Each  child's  Definition  operation  is called during the
       traversal.  Note that every child must be an  instance  of
       the  same  ExternView  subclass  to  ensure  a  consistent
       traversal order.

SEE ALSO

       Component(3U),      ComponentView(3U),       Iterator(3U),
       ostream(3C++)