为什么我不能声明通用形式

Question

what im trying to do is basically declare a form like this:

TFormCRUDBase<T : class> = class(TFormLayoutBase, MVP.Intf.CRUD.ICRUDView<T>)
{ ... }
end;

the compiler doesn't accuse any error or warning messages if this feature is allowed or not, anyway but when i try to add some controls in the designer i get this message

Declaration of class TFormCRUDBase is missing or incorrect.

so the question is why the syntax allow me to use and the IDE simply kicks me away.

thx in advance

Answer 1

Simply put, the form designer does not support generic types.

The compiler is fine with generic types. You can build the UI, add controls etc. with run time code. But the form designer does not support generic types.

Answer 2

You asked for a work arround. The solurion are simple, yet not elegant. You have to make at strong type class, and make you form inhirited from that.

First I make some dummy types:

type
  ICRUDView<T> = Interface
   function GetObject : T;
  end;

  TMyObject = class
    function Time: String;
  end;

and a Simple implementation:

{ TMyObject }

function TMyObject.Time: String;
begin
  Result := TimeToStr(now);
end;

procedure TForm5.FormCreate(Sender: TObject);
begin
  Caption := Self.GetObject.Time;
end;

Then the strong type from which you have to inherited from:

type
  TMyClassForm = class(TForm, ICRUDView<TMyObject>)
  strict private
    InternalObject: TMyObject;
  public
    function GetObject: TMyObject;
  end;

and its implementation:

{ TMyClassForm }

function TMyClassForm.GetObject: TMyObject;
begin
  if (InternalObject = nil) then
    InternalObject := TMyObject.Create;

  Result := InternalObject;
end;

At last you make your form inhirited from your newly created class:

  TForm5 = class(TMyClassForm)
    procedure FormCreate(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

and use it in your form

procedure TForm5.FormCreate(Sender: TObject);
begin
  Caption := Self.GetObject.Time;
end;

It's working but its not elegant !