delphi - 如何链接 "parallel"类层次结构？

Question

我有一个小的类层次结构，其中每个类对应于某个 TComponent 后代(例如基类 TDefaultFrobber 及其后代 TActionFrobber 和 TMenuItemFrobber，分别对应于 TComponent、TCustomAction 和 TMenuItem)。现在我想要一个像这样的工厂(？)函数:

function CreateFrobber(AComponent: TComponent): IFrobber;
begin
  if AComponent is TCustomAction then
    Result := TActionFrobber.Create(TCustomAction(AComponent))
  else if AComponent is TMenuItem then
    Result := TMenuItemFrobber.Create(TMenuItem(AComponent))
  else
    Result := TDefaultFrobber.Create(AComponent);
end;

我可以以某种方式重构它以使用虚拟函数或类似的东西来代替 if-else 级联或 RTTI 吗？

编辑:我现在的解决方案:

unit Frobbers;

interface

uses
  Classes;

type
  IComponentFrobber = interface
  end;

  TComponentFrobberClass = class of TComponentFrobber;

  TComponentFrobber = class(TInterfacedObject, IComponentFrobber)
  strict private
    FComponent: TComponent;
  protected
    constructor Create(AComponent: TComponent);
    property Component: TComponent read FComponent;
  public
    class function FindFrobberClass(AComponentClass: TComponentClass): TComponentFrobberClass; overload; static;
    class function FindFrobberClass(AComponent: TComponent): TComponentFrobberClass; overload; static;
    class procedure RegisterFrobber(AComponentClass: TComponentClass; AFrobberClass: TComponentFrobberClass); static;
  end;

implementation

uses
  ActnList,
  Menus;

type
  TComponentFrobberRegistryItem = record
    ComponentClass: TComponentClass;
    FrobberClass: TComponentFrobberClass;
  end;

var
  FComponentFrobberRegistry: array of TComponentFrobberRegistryItem;

class function TComponentFrobber.FindFrobberClass(AComponentClass: TComponentClass): TComponentFrobberClass;
var
  i: Integer;
begin
  // Search backwards, so that more specialized frobbers are found first:
  for i := High(FComponentFrobberRegistry) downto Low(FComponentFrobberRegistry) do
    if FComponentFrobberRegistry[i].ComponentClass = AComponentClass then
    begin
      Result := FComponentFrobberRegistry[i].FrobberClass;
      Exit;
    end;
  Result := nil;
end;

constructor TComponentFrobber.Create(AComponent: TComponent);
begin
  inherited Create;
  FComponent := AComponent;
end;

class function TComponentFrobber.FindFrobberClass(AComponent: TComponent): TComponentFrobberClass;
var
  i: Integer;
begin
  // Search backwards, so that more specialized frobbers are found first:
  for i := High(FComponentFrobberRegistry) downto Low(FComponentFrobberRegistry) do
    if AComponent is FComponentFrobberRegistry[i].ComponentClass then
    begin
      Result := FComponentFrobberRegistry[i].FrobberClass;
      Exit;
    end;
  Result := nil;
end;

class procedure TComponentFrobber.RegisterFrobber(AComponentClass: TComponentClass;
  AFrobberClass: TComponentFrobberClass);
var
  i: Integer;
begin
  Assert(FindFrobberClass(AComponentClass) = nil, 'Duplicate Frobber class');
  i := Length(FComponentFrobberRegistry);
  SetLength(FComponentFrobberRegistry, Succ(i));
  FComponentFrobberRegistry[i].ComponentClass := AComponentClass;
  FComponentFrobberRegistry[i].FrobberClass := AFrobberClass;
end;

function CreateComponentFrobber(AComponent: TComponent): IComponentFrobber;
var
  FrobberClass: TComponentFrobberClass;
begin
  FrobberClass := TComponentFrobber.FindFrobberClass(AComponent);
  Assert(FrobberClass <> nil);
  Result := FrobberClass.Create(AComponent);
end;

type
  TActionFrobber = class(TComponentFrobber);
  TMenuItemFrobber = class(TComponentFrobber);

initialization
  TComponentFrobber.RegisterFrobber(TCustomAction, TActionFrobber);
  TComponentFrobber.RegisterFrobber(TMenuItem, TMenuItemFrobber);
end.

感谢 Cesar、Gamecat 和 mghie。

Answer 1

如果您使用虚拟构造函数创建一个类并为该类创建一个类类型。您可以根据组件类名称创建查找列表。

示例:

type
  TFrobber = class 
  public
    constructor Create; virtual;

    class function CreateFrobber(const AComponent: TComponent): TFrobber;
  end;
  TFrobberClass = class of TFrobber;

  type 
    TFrobberRec = record 
      ClassName: ShortString;
      ClassType: TFrobberClass;
    end;

  const
    cFrobberCount = 3;
    cFrobberList : array[1..cFrobberCount] of TFrobberRec = (
      (ClassName : 'TAction'; ClassType: TActionFrobber),
      (ClassName : 'TButton'; ClassType: TButtonFrobber),
      (ClassName : 'TMenuItem'; ClassType: TMenuItemFrobber)
    );

  class function TFrobber.CreateFrobber(const AComponent: TComponent): TFrobber;
  var
    i : Integer;
  begin
    Result := nil;
    for i := 1 to cFrobberCount do begin
      if AComponent.ClassName = cFrobberList[i].ClassName then begin
        Result := cFrobberList[i].ClassType.Create();
        Exit;
      end;
    end;
  end;

您当然也可以使用动态列表(字典)，但是您必须以某种方式注册每个组合。

更新

对Mghie的言论进行评论。

你说得完全正确。但这不可能没有真正丑陋的伎俩。 现在，您必须使用单元的初始化/终结部分来注册类。但是向类添加初始化/终结类方法会很酷。这些必须与单元的初始化(和终结)一起调用。像这样:

class 
  TFrobber = class
  private
    initialization Init; // Called at program start just after unit initialization
    finalization Exit;  // called at program end just before unit finalization.
  end;