When creating components you may want to know when the user changed the font or text. This can be quite easily done using the CM_FONTCHANGED and CM_TEXTCHANGED messages.

type
  ...
  TComponent1 = class(TComponent)
  private
    ...
    procedure CMFontChanged(var Message: TMessage); message CM_FONTCHANGED;
    procedure CMTextChanged(var Message: TMessage); message CM_TEXTCHANGED;
    ...
  end;

procedure TComponent1.CMTextChanged(var Message:TMessage);
begin
  ...
end;
procedure TComponent1.CMFontChanged(var Message:TMessage);
begin
  ...
end;