// The context needed to add root,child elements to a Tree. There is only one alternative
// (i.e., a list of children). We subclass to specialize. MakeGrammar.addElementToCurrentAlt
// will work correctly now for either a block of alts or a Tree child list.
//
// The first time addAlternativeElement is called, it sets the root element
// rather than adding it to one of the alternative lists.  Rather than have
// the grammar duplicate the rules for grammar atoms etc... we use the same
// grammar and same refToken behavior etc...  We have to special case somewhere
// and here is where we do it.

unit dpglib.TreeBlockContext;

interface
uses
   dpglib.types,
   dpglib.BlockContext;

type
   TTreeBlockContext = class( TBlockContext)
      protected
         fNextElementIsRoot: boolean;

      public
         procedure   AddAlternativeElem( pElem: IAlternativeElem); override;

      public
         procedure   AfterConstruction; override;
   end;

implementation

{ TTreeBlockContext }

// ================================================================================================
// After Construction
// ================================================================================================
procedure TTreeBlockContext.AfterConstruction;
begin
   inherited;
   fNextElementIsRoot := true;
end;

// ================================================================================================
// Add Alternative Elem
// ================================================================================================
procedure TTreeBlockContext.AddAlternativeElem(pElem: IAlternativeElem);
var
   tree: ITreeElem;

begin
   Block.QueryInterface( ITreeElem, tree);

   if fNextElementIsRoot then
   begin
      tree.Root := pElem as IGrammarAtom;
      fNextElementIsRoot := false
   end

   else
      inherited
end;

end.