delphi - TLabel and TGroupbox Captions Flicker on Resize

Question

• So, I have an application that loads different plugins and creates a new tab on a TPageControl for each one.
• Each DLL has a TForm associated with it.
• The forms are created with their parent hWnd as the new TTabSheet.

• Since the TTabSheets aren't a parent of the form as far as VCL is concerned (didn't want to use dynamic RTL, and plugins made in other languages) I have to handle resizes manually. I do this like below:

  var
    ChildHandle : DWORD;
  begin
    If Assigned(pcMain.ActivePage) Then
      begin
      ChildHandle := FindWindowEx(pcMain.ActivePage.Handle, 0, 'TfrmPluginForm', nil);
      If ChildHandle > 0 Then
        begin
        SetWindowPos(ChildHandle, 0, 0, 0, pcMain.ActivePage.Width, pcMain.ActivePage.Height, SWP_NOZORDER + SWP_NOACTIVATE + SWP_NOCOPYBITS);
      end;
    end;
  

Now, my problem is that when the application is resized, all the TGroupBoxes and the TLabels inside the TGroupBoxes flicker. The TLabels that are not inside TGroupboxes are fine and don't flicker.

Things I've tried:

• WM_SETREDRAW followed by a RedrawWindow
• ParentBackground on the TGroupBoxes and TLabels set to False
• DoubleBuffer := True
• LockWindowUpdate (Yes, even though I know it's very very wrong)
• Transparent := False (even overriding create to edit ControlState)

Any ideas?

Answer 1

The only thing I have found to work well is to use the WS_EX_COMPOSITED window style. This is a performance hog so I only enable it when in a sizing loop. It is my experience that, with the built-in controls, in my app, flickering only occurs when resizing forms.

You should first perform a quick test to see if this approach will help you by simply adding the WS_EX_COMPOSITED window style to all your windowed controls. If that works you can consider the more advanced approach below:

Quick hack

procedure EnableComposited(WinControl: TWinControl);
var
  i: Integer;
  NewExStyle: DWORD;
begin
  NewExStyle := GetWindowLong(WinControl.Handle, GWL_EXSTYLE) or WS_EX_COMPOSITED;
  SetWindowLong(WinControl.Handle, GWL_EXSTYLE, NewExStyle);

  for i := 0 to WinControl.ControlCount-1 do
    if WinControl.Controls[i] is TWinControl then
      EnableComposited(TWinControl(WinControl.Controls[i]));
end;

Call this, for example, in the OnShow for your TForm, passing the form instance. If that helps then you really should implement it more discerningly. I give you the relevant extracts from my code to illustrate how I did that.

Full code

procedure TMyForm.WMEnterSizeMove(var Message: TMessage);
begin
  inherited;
  BeginSizing;
end;

procedure TMyForm.WMExitSizeMove(var Message: TMessage);
begin
  EndSizing;
  inherited;
end;

procedure SetComposited(WinControl: TWinControl; Value: Boolean);
var
  ExStyle, NewExStyle: DWORD;
begin
  ExStyle := GetWindowLong(WinControl.Handle, GWL_EXSTYLE);
  if Value then begin
    NewExStyle := ExStyle or WS_EX_COMPOSITED;
  end else begin
    NewExStyle := ExStyle and not WS_EX_COMPOSITED;
  end;
  if NewExStyle<>ExStyle then begin
    SetWindowLong(WinControl.Handle, GWL_EXSTYLE, NewExStyle);
  end;
end;

function TMyForm.SizingCompositionIsPerformed: Boolean;
begin
  //see The Old New Thing, Taxes: Remote Desktop Connection and painting
  Result := not InRemoteSession;
end;
procedure TMyForm.BeginSizing;
var
  UseCompositedWindowStyleExclusively: Boolean;
  Control: TControl;
  WinControl: TWinControl;
begin
  if SizingCompositionIsPerformed then begin
    UseCompositedWindowStyleExclusively := Win32MajorVersion>=6;//XP can't handle too many windows with WS_EX_COMPOSITED
    for Control in ControlEnumerator(TWinControl) do begin
      WinControl := TWinControl(Control);
      if UseCompositedWindowStyleExclusively then begin
        SetComposited(WinControl, True);
      end else begin
        if WinControl is TPanel then begin
          TPanel(WinControl).FullRepaint := False;
        end;
        if (WinControl is TCustomGroupBox) or (WinControl is TCustomRadioGroup) or (WinControl is TCustomGrid) then begin
          //can't find another way to make these awkward customers stop flickering
          SetComposited(WinControl, True);
        end else if ControlSupportsDoubleBuffered(WinControl) then begin
          WinControl.DoubleBuffered := True;
        end;
      end;
    end;
  end;
end;

procedure TMyForm.EndSizing;
var
  Control: TControl;
  WinControl: TWinControl;
begin
  if SizingCompositionIsPerformed then begin
    for Control in ControlEnumerator(TWinControl) do begin
      WinControl := TWinControl(Control);
      if WinControl is TPanel then begin
        TPanel(WinControl).FullRepaint := True;
      end;
      UpdateDoubleBuffered(WinControl);
      SetComposited(WinControl, False);
    end;
  end;
end;

function TMyForm.ControlSupportsDoubleBuffered(Control: TWinControl): Boolean;
const
  NotSupportedClasses: array [0..1] of TControlClass = (
    TCustomForm,//general policy is not to double buffer forms
    TCustomRichEdit//simply fails to draw if double buffered
  );
var
  i: Integer;
begin
  for i := low(NotSupportedClasses) to high(NotSupportedClasses) do begin
    if Control is NotSupportedClasses[i] then begin
      Result := False;
      exit;
    end;
  end;
  Result := True;
end;

procedure TMyForm.UpdateDoubleBuffered(Control: TWinControl);

  function ControlIsDoubleBuffered: Boolean;
  const
    DoubleBufferedClasses: array [0..2] of TControlClass = (
      TMyCustomGrid,//flickers when updating
      TCustomListView,//flickers when updating
      TCustomStatusBar//drawing infidelities , e.g. my main form status bar during file loading
    );
  var
    i: Integer;
  begin
    if not InRemoteSession then begin
      //see The Old New Thing, Taxes: Remote Desktop Connection and painting
      for i := low(DoubleBufferedClasses) to high(DoubleBufferedClasses) do begin
        if Control is DoubleBufferedClasses[i] then begin
          Result := True;
          exit;
        end;
      end;
    end;
    Result := False;
  end;

var
  DoubleBuffered: Boolean;

begin
  if ControlSupportsDoubleBuffered(Control) then begin
    DoubleBuffered := ControlIsDoubleBuffered;
  end else begin
    DoubleBuffered := False;
  end;
  Control.DoubleBuffered := DoubleBuffered;
end;

procedure TMyForm.UpdateDoubleBuffered;
var
  Control: TControl;
begin
  for Control in ControlEnumerator(TWinControl) do begin
    UpdateDoubleBuffered(TWinControl(Control));
  end;
end;

This won't compile for you, but it should contain some useful ideas. ControlEnumerator is my utility to turn a recursive walk of the child controls into a flat for loop. Note that I also use a custom splitter that calls BeginSizing/EndSizing when it is active.

Another useful trick is to use TStaticText instead of TLabel which you occasionally need to do when you have deep nesting of page controls and panels.

I've used this code to make my app 100% flicker free but it took me ages and ages of experimenting to get it all in place. Hopefully others can find something of use in here.