How to set CreateParams after the constructor has run?

Question

  TForm1 = class(TForm)
    procedure FormCreate(Sender: TObject);
  end;

  TForm2 = class(TForm)
  private
    FAppWindow: Boolean;
  protected
    procedure CreateParams(var Params: TCreateParams); override;
  public
    property AppWindow: Boolean read FAppWindow write FAppWindow;
  end;

procedure TForm1.FormCreate(Sender: TObject);
begin
  Form2 := TForm2.Create(Self);
  Form2.AppWindow := True;
  Form2.Show;
end;

procedure TForm2.CreateParams(var Params: TCreateParams);
begin
  inherited;
  if FAppWindow then begin
    Params.Style := Params.Style or WS_EX_APPWINDOW;
    Params.WndParent := 0;
  end;
end;

This doesn't work, because the window handle is created during the constructor of TForm, so CreateParams is run too early and FAppWindow is always False.

Writing a custom constructor also doesn't work since you have to eventually call the inherited constructor which creates the handle before you can save any data to the instance:

constructor TForm2.CreateAppWindow(AOwner: TComponent);
begin
  inherited Create(AOwner);
  FAppWindow := True;
end;

Is there a way to:

• Delay the creation of the window handle?
• Alter the window style after creation of the window handle?
• Recreate the window handle after the constructor has run?
• Some other option I haven't thought of, yet?

How can I change the style of a form from the "outside" of the class?

Answer 1

The simplest solution is to pass the parameter to the form in its constructor, rather than wait until it has finished being created.

That means you need to introduce a constructor for TForm2 that accepts as parameters whatever information you need to pass on in CreateParams.

Make a note of any state before you call the inherited constructor. Also, there's no need to set WS_EX_APPWINDOW when you are setting the owner to be zero.