How can a control be notified when its parent receives and loses focus in Delphi?

Question

As the title says, I'd like a component (say, a label) to be notified when it's parent (say, a panel) receives and loses focus. I wandered a bit in Delphi source, in hope of using TControl.Notify, but it's only used to notify child controls of some property changes like font and color. Any suggestions?

Answer 1

Whenever the active control in an application changes, a CM_FOCUSCHANGED message is broadcast to all controls. Simply intercept it, and act accordingly.

Also, I assumed that by when it's parent (say, a panel) receives and loses focus you mean whenever a (nested) child control on that parent/panel receives or loses focus.

type
  TLabel = class(StdCtrls.TLabel)
  private
    function HasCommonParent(AControl: TWinControl): Boolean;
    procedure CMFocusChanged(var Message: TCMFocusChanged);
      message CM_FOCUSCHANGED;
  end;

procedure TLabel.CMFocusChanged(var Message: TCMFocusChanged);
const
  FontStyles: array[Boolean] of TFontStyles = ([], [fsBold]);
begin
  inherited;
  Font.Style := FontStyles[HasCommonParent(Message.Sender)];
end;

function TLabel.HasCommonParent(AControl: TWinControl): Boolean;
begin
  Result := False;
  while AControl <> nil do
  begin
    if AControl = Parent then
    begin
      Result := True;
      Break;
    end;
    AControl := AControl.Parent;
  end;
end;

If you don't like to subclass TJvGradientHeader, then it is possible to design this generically by the use of Screen.OnActiveControlChange:

  TForm1 = class(TForm)
    procedure FormCreate(Sender: TObject);
    procedure FormDestroy(Sender: TObject);
  private
    FHeaders: TList;
    procedure ActiveControlChanged(Sender: TObject);
  end;

procedure TForm1.FormCreate(Sender: TObject);
begin
  FHeaders := TList.Create;
  FHeaders.Add(Label1);
  FHeaders.Add(Label2);
  Screen.OnActiveControlChange := ActiveControlChanged;
end;

procedure TForm1.FormDestroy(Sender: TObject);
begin
  FHeaders.Free;
end;

function HasCommonParent(AControl: TWinControl; AMatch: TControl): Boolean;
begin
  Result := False;
  while AControl <> nil do
  begin
    if AControl = AMatch.Parent then
    begin
      Result := True;
      Break;
    end;
    AControl := AControl.Parent;
  end;
end;

procedure TForm1.ActiveControlChanged(Sender: TObject);
const
  FontStyles: array[Boolean] of TFontStyles = ([], [fsBold]);
var
  I: Integer;
begin
  for I := 0 to FHeaders.Count - 1 do
    TLabel(FHeaders[I]).Font.Style :=
      FontStyles[HasCommonParent(Screen.ActiveControl, TLabel(FHeaders[I]))];
end;

Note that I chose TLabel to demonstrate this works also for TControl derivatives.