Delphi passing Types in parameters

Question

an example, I need to do this to solve a problem in a framework I am developing :

//Unit2 :
    procedure A(aForm : TForm; AClassType: TFormClass); 
    begin 
       ShowMessage (AClassType(aForm).edtUser.text);
    end; 

    ...

//Unit1 :

    Uses Unit2;

    begin
       form1 := TForm1.Create;
       Try  
          A(form1, TForm1); 
       Finally
          form1.Free;
       End;
    end;

The compiler does not accept this line:

AClassType (aform).edtUser.text

One solution would be to use:

Uses
    UnitofTForm1;

The procedure (aform: TForm; AClassType: TForm1);
begin
    ShowMessage (AClassType (aform).edtUser.text);
end;

But I can not do so because they are giving circular reference and I need some decoupling in my framework

Is there any way to make typecast passing as parameter the type of form or is there another way to do this ?

Answer 1

What you are asking for can be done by either:

1. deriving the various Form classes from a common base class that exposes the field you want to access:

   procedure A(aForm : TBaseForm); 
   begin 
     ShowMessage(aForm.edtUser.Text);
   end; 
   

   type
     TBaseForm = class(TForm)
       edtUser: TEdit;
       ...
     end;
   
     TDerivedForm = class(TBaseForm)
       ...
     end;
   
   ...
   
   frm := TDerivedForm.Create;
   try  
     A(frm); 
   finally
     frm.Free;
   end;
   

2. or, use Generics:

   type
     TBaseForm = class(TForm)
       edtUser: TEdit;
       ...
     end;
   
     TMyClass = class
       class procedure A<T: TBaseForm>(aForm : T); 
     end;
   
   class procedure TMyClass.A<T>(aForm : T); 
   begin 
      ShowMessage(aForm.edtUser.Text);
   end; 
   

   frm := TDerivedForm.Create;
   try  
     A<TDerivedForm>(frm); 
   finally
     frm.Free;
   end;
   

3. if a common base class is not possible, use an interface instead:

   type
     IMyFormAccess = interface
       function GetUserText: string;
     end;
   
   procedure A(aForm : IMyFormAccess); 
   begin 
     ShowMessage(aForm.GetUserText);
   end; 
   

   type
     TForm1 = class(TForm, IMyFormAccess)
       ...
       function GetUserText: string;
       ...
     end;
   
   function TForm1.GetUserText: string;
   begin
     Result := edtUser.Text;
   end;
   
   ...
   
   frm := TForm1.Create;
   try  
     A(frm as IMyFormAccess);
   finally
     frm.Free;
   end;
   

4. or, use legacy RTTI (only works with published properties):

   uses
     TypInfo;
   
   procedure A(aForm : TForm); 
   var
     Edt: TEdit;
   begin 
     Edt := TEdit(GetObjectProp(aForm, 'edtUser', TEdit));
     if Edt <> nil then
       ShowMessage(Edt.Text);
   end; 
   

   frm := TForm1.Create;
   try  
     A(frm);
   finally
     frm.Free;
   end;
   

5. or, use extended RTTI (works with all fields and visibility):

   uses
     System.Rtti;
   
   procedure A(aForm : TForm); 
   var
     Ctx: TRttiContext;
     Fld: TRttiField;
     Value: TValue;
     Edt: TEdit;
   begin 
     Fld := Ctx.GetType(aForm.ClassType).GetField('edtUser');
     if Fld <> nil then
     begin
       Value := Fld.GetValue(aForm);
       if (not Value.IsEmpty) and Value.IsObject then
       begin
         Edt := Value.AsObject as TEdit;
         ShowMessage(Edt.Text);
       end;
     end;
   end;
   

   frm := TForm1.Create;
   try  
     A(frm);
   finally
     frm.Free;
   end;