Delphi – TMainMenu Not Shown When VCL Styles is Removed from NC Area

delphidelphi-xe2vcl-styles

I'm using this code to remove the vcl styles from the non client area of a form.

type
  TFormStyleHookNC= class(TMouseTrackControlStyleHook)
  protected
    procedure PaintBackground(Canvas: TCanvas); override;
    constructor Create(AControl: TWinControl); override;
  end;

constructor TFormStyleHookNC.Create(AControl: TWinControl);
begin
  inherited;
  OverrideEraseBkgnd := True;
end;

procedure TFormStyleHookNC.PaintBackground(Canvas: TCanvas);
var
  Details: TThemedElementDetails;
  R: TRect;
begin
  if StyleServices.Available then
  begin
    Details.Element := teWindow;
    Details.Part := 0;
    R := Rect(0, 0, Control.ClientWidth, Control.ClientHeight);
    StyleServices.DrawElement(Canvas.Handle, Details, R);
  end;
end;


initialization
 TStyleManager.Engine.RegisterStyleHook(TForm3, TFormStyleHookNC);

Before to apply this style hook the form looks like

enter image description here

and after

enter image description here

As you can see the menu disappears, The question is : how I can fix this? I mean how i can remove the vcl styles from the non client area of a form without remove the TMainMenu?

Best Answer

When you uses the vcl styles, the TMain menu is drawn by the TMainMenuBarStyleHook vcl style hook, which is defined inside of the TFormStyleHook (the hook of the forms), in this case because you are not using this hook there is not code to draw the TMainMenu.

Two possible solutions are

1) Implement the vcl style hook for the TMainMenu inside of the TFormStyleHookNC , just like the TFormStyleHook does.

2)or even better use a TActionMainMenuBar component instead of a TMainMenu, this component is very well integrated with the vcl styles (check the next sample image).

enter image description here

Related Solutions

Delphi VCL Style – DLL Affecting TMenuItem in Application

This is a fundamental problem with VCL styles and the way that they style menus. The styling is implemented with a process wide hook. Specifically a CBT hook installed by a call to SetWindowsHookEx from TCustomStyleEngine.CreateSysHook in the Vcl.Themes unit. In fact, the hook applies just to the GUI thread, but that is process wide in the sense that there is exactly one GUI thread in the process.

Since you have multiple instances of the VCL in your application (one in the DLL and one in the application), two hooks are installed. That is one too many. The hook installed most recently (the DLL as it happens) wins, and that's why the DLL menu styling infects your executable. And why you encounter an access violation. The DLL is trying to operate on a menu that belongs to the executable. And so, in spite of your best efforts, you've ended up with the DLL code accessing VCL objects from the host executable.

There's no simple way to work around this and support styles fully in both modules. What we have here is a fundamental consequence of the design. The system was not designed to support multiple VCL instances. If you wish to use VCL styles in multiple modules, then the designers expect you to use runtime packages.

I suppose that you might be able to get some traction by operating the DLL out of a completely different thread. That would involve loading the DLL from that different thread so that the VCL is initialized in the thread. And all calls to the DLL would have to be from that thread. And you'd need to run a message loop in that thread. It's possible that you might be able to make that work, but I doubt it. Even with all the provisos mentioned you still have to handle the fact that you have two GUI threads which presents all sorts of issues with the input queue handling.

Perhaps another approach would be to uninstall the hook from the DLL. So long as your DLL is not showing menus then you may well be able to get away with uninstalling that hook. It would disable styling for menus shown by the DLL, but perhaps that's acceptable.

This version of your DLL (after I simplified it somewhat also) uninstalls the hook.

library VCLStyleDLL;

{$R 'Style.res' 'Style.rc'}

uses
  VCL.Styles,
  VCL.Themes,
  VCL.SysStyles; // to gain access to TSysPopupStyleHook

{$R *.res}

begin
  TStyleManager.TrySetStyle('Glossy', false);
  TCustomStyleEngine.UnRegisterSysStyleHook('#32768', TSysPopupStyleHook);
end.

With this version of the DLL, the host executable does not suffer the problems your describe in your question.

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