Question

With default Windows settings, accelerator keys are not meant to be shown on dialogs until the user presses the ALT key.

Delphi’s TLabel control does not obey this convention, as shown below:

Although both label and check box have an accelerator key specified, the check box correctly hides it, but the label does not. Of course, when ALT is pressed, the accelerator shows for the check box, but it’s the behaviour prior to that which is incorrect.

My understanding of why this happens is that the VCL code that implements this behaviour is contained in TWinControl, for example the UpdateUIState method, and relies on sending the underlying windowed control a WM_CHANGEUISTATE message. Since TLabel is not windowed, it misses out on this handling.

Can anybody suggest a way to achieve the desired behaviour for non-windowed controls?

Update 1

I’ve just discovered that group boxes and radio groups don’t respond to UI state either.

Update 2

QC#97044.

Answer 1

I think I’ve worked out a way of handling it.

function HideAccelFlag(Control: TControl): Integer;
begin
  //ask the top level window about its UI state
  while Assigned(Control.Parent) do begin
    Control := Control.Parent;
  end;
  if (Control.Perform(WM_QUERYUISTATE, 0, 0) and UISF_HIDEACCEL)=UISF_HIDEACCEL then begin
    Result := DT_HIDEPREFIX;
  end else begin
    Result := 0;
  end;
end;

type
  TUIStateAwareLabel = class(TLabel)
  protected
    procedure DoDrawText(var Rect: TRect; Flags: Longint); override;
  end;

procedure TUIStateAwareLabel.DoDrawText(var Rect: TRect; Flags: Integer);
begin
  if ShowAccelChar then begin
    Flags := Flags or HideAccelFlag(Self);
  end;
  inherited;
end;

I make sure that I always create TUIStateAwareLabel rather than TLabel by hooking the form streaming mechanism with TReader.OnFindComponentClass.

Dealing with TCustomGroupBox descendents is more messy. For them I resorted to copying the source code of TCustomGroupBox.Paint into my descendent and making use of HideAccelFlag again.

Next task is to write it up as a QC report.