德尔福企业版 : how can I apply the Visitor Pattern without circular references?

使用 Delphi 2009 Enterprise，我在模型 View 中为 GoF 访问者模式创建了代码，并将代码分成两个单元:一个用于域模型类，一个用于访问者(因为我可能需要其他单元来实现不同的访问者实现，所有在一个单位？前面的'Big ball of mud'!)。

unit VisitorUnit;

interface

uses
  ConcreteElementUnit;

type
  IVisitor = interface;

  IElement = interface
  procedure Accept(AVisitor :IVisitor);
  end;

  IVisitor = interface
  procedure VisitTConcreteElement(AElement :TConcreteElement);
  end;

  TConcreteVisitor = class(TInterfacedObject, IVisitor)
  public
    procedure VisitTConcreteElement(AElement :TConcreteElement);
  end;

implementation

procedure TConcreteVisitor.VisitTConcreteElement(AElement :TConcreteElement);
begin
  { provide implementation here }
end;

end.

以及商业模式类(class)的第二单元

unit ConcreteElementUnit;

interface

uses
  VisitorUnit;

type
  TConcreteElement = class(TInterfacedObject, IElement)
  public
    procedure Accept(AVisitor :IVisitor); virtual;
  end;

  Class1 = class(TConcreteElement)
  public
    procedure Accept(AVisitor :IVisitor);
  end;

implementation

{ Class1 }

procedure Class1.Accept(AVisitor: IVisitor);
begin
  AVisitor.VisitTConcreteElement(Self);
end;

end.

看到问题了吗？循环单位引用。有没有优雅的解决方案？我猜它需要“n+1”个带有基接口(interface)/基类定义的附加单元来避免CR问题，以及像硬转换这样的技巧？

最佳答案

我使用以下方案来实现灵活的访问者模式:

基本访问者类型的声明

unit uVisitorTypes;
type
  IVisited = interface
  { GUID }
    procedure Accept(Visitor: IInterface);
  end;

  IVisitor = interface
  { GUID }
    procedure Visit(Instance: IInterface);
  end;

  TVisitor = class(..., IVisitor)
    procedure Visit(Instance: IInterface);
  end;

procedure TVisitor.Visit(Instance: IInterface);
var
  visited: IVisited;
begin
  if Supports(Instance, IVisited, visited) then
    visited.Accept(Self)
  else
    // raise exception or handle error elsewise
end;

元素类的单位

unit uElement;

type
  TElement = class(..., IVisited)
    procedure Accept(Visitor: IInterface);
  end;

  // declare the visitor interface next to the class-to-be-visited declaration
  IElementVisitor = interface
  { GUID }
    procedure VisitElement(Instance: TElement);
  end;

procedure TElement.Accept(Visitor: IInterface);
var
  elementVisitor: IElementVisitor;
begin
  if Supports(Visitor, IElementVisitor, elementVisitor) then
    elementVisitor.VisitElement(Self)
  else
    // if override call inherited, handle error or simply ignore
end;

实际访问者实现

unit MyVisitorImpl;

uses
  uVisitorTypes, uElement;

type
  TMyVisitor = class(TVisitor, IElementVisitor)
    procedure VisitElement(Instance: TElement);
  end;

procedure TMyVisitor.VisitElement(Instance: TElement);
begin
  // Do whatever you want with Instance
end;

给访客打电话

uses
  uElement, uMyElementVisitor;

var
  visitor: TMyVisitor;
  element: TElement;
begin
  // get hands on some element

  visitor := TMyVisitor.Create;
  try
    visitor.Visit(element);
  finally
    visitor.Free;
  end;
end;