The Visitor Pattern – Part 3 – Page 2 – The Art of Delphi Programming

Now we can write a visitor that just counts the shapes visited:

unit uCounterVisitorIntf;

interface

uses
  uShapeBase;

type
  TShapeVisitorCounter = class(TShapeVisitor, IAbstractShapeVisitor)
  private
    FCounter: Integer;
  public
    procedure VisitAbstractShape(Instance: TAbstractShape);
    property Counter: Integer read FCounter write FCounter;
  end;

implementation

procedure TShapeVisitorCounter.VisitAbstractShape(Instance: TAbstractShape);
begin
  Inc(FCounter);
end;

end.

unit uCounterVisitorIntf;

interface

uses

uShapeBase;

type

TShapeVisitorCounter = class(TShapeVisitor, IAbstractShapeVisitor)

private

FCounter: Integer;

public

procedure VisitAbstractShape(Instance: TAbstractShape);

property Counter: Integer read FCounter write FCounter;

end;

implementation

procedure TShapeVisitorCounter.VisitAbstractShape(Instance: TAbstractShape);

begin

Inc(FCounter);

end;

end.

Pretty easy and effective, isn’t it?

But there is still some drawback, as the visitor is limited to visit TAbstractShape and its descendants. What if we have other classes to visit that don’t fit in this inheritance scheme? Can’t we find a more general approach? Here it is:

interface

uses
  Classes, SysUtils;

type
  IVisited = interface;

  IVisitor = interface
    ['{4B6D36B4-BF62-420E-A85E-F16BB135C6A4}']
    procedure Visit(Instance: IVisited);
  end;

  IVisited = interface
    ['{BCA80F82-4C43-4266-AB5D-C234668929A6}']
    procedure Accept(Visitor: IVisitor);
  end;

type
  TVisitor = class(TInterfacedPersistent, IVisitor)
  public
    procedure Visit(Instance: IVisited); virtual;
  end;

implementation

procedure TVisitor.Visit(Instance: IVisited);
begin
  Instance.Accept(Self);
end;

end.

interface

uses

Classes, SysUtils;

type

IVisited = interface;

IVisitor = interface

['{4B6D36B4-BF62-420E-A85E-F16BB135C6A4}']

procedure Visit(Instance: IVisited);

end;

IVisited = interface

['{BCA80F82-4C43-4266-AB5D-C234668929A6}']

procedure Accept(Visitor: IVisitor);

end;

type

TVisitor = class(TInterfacedPersistent, IVisitor)

public

procedure Visit(Instance: IVisited); virtual;

end;

implementation

procedure TVisitor.Visit(Instance: IVisited);

begin

Instance.Accept(Self);

end;

end.

With this TAbstractShape or any other class to be visited has to implement IVisited and we are done.

Author: Uwe Raabe

Addicted to Pascal/Delphi since the late 70's View all posts by Uwe Raabe