Why do I get multiple windows messages of same kind? - windows

Question

I'm trying to respond to some windows and application messages, but I get them multiple times.
For example, I write the following code to show a message box when the date of system is changed using WM_TIMECHANGE. WMTimeChange is executed more than once, and I see multiples (most times two or three) messageboxes one behind another. Maybe, I am missing something?
unit Unit1;
interface
uses
Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
Dialogs;
type
TForm1 = class(TForm)
protected
procedure WMTimeChange(var Msg: TMessage) ; message WM_TIMECHANGE;
end;
var
Form1: TForm1;
implementation
{$R *.dfm}
{ TForm1 }
procedure TForm1.WMTimeChange(var Msg: TMessage);
begin
showmessage('Date/Time has changed!');
end;
end.
Testing in Windows XP.
EDIT: Just to clarify, my intention is understand WHY that happen and not how to get around the multiple calls. Anyway, if an answer to that is not possible, I will probably accept one answer to the later.
EDIT2: Removed Delphi Tag as it seems not a Delphi issue.

Answer 1

Your code is correct. It sounds like that Windows is sending the WM_TIMECHANGE message several times.
So you can just add a small time-hysteresis comparison to let your message be triggered only once per a 1% of day, i.e. more or less 15 minutes:
type
TForm1 = class(TForm)
protected
FWMTimeChangeTimeStamp: TDateTime;
procedure WMTimeChange(var Msg: TMessage) ; message WM_TIMECHANGE;
end;
var
Form1: TForm1;
implementation
{$R *.dfm}
{ TForm1 }
procedure TForm1.WMTimeChange(var Msg: TMessage);
begin
if Now-FWMTimeChangeTimeStamp>0.01 then
begin
showmessage('Date/Time has changed!');
FWMTimeChangeTimeStamp := Now;
end;
end;

Answer 2

This is kind of what I've used in my case to be resilient to this behavior. But as said in comments, will only work if the user take time to answer the application. So, Arnaud Bounchez is a better approach to general use. Only don't forget to initialize the FWMTimeChangeStamp to something that is different from current computer clock.
unit Unit1;
interface
uses
Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
Dialogs;
type
TForm1 = class(TForm)
procedure FormCreate(Sender: TObject);
protected
procedure WMTimeChange(var Msg: TMessage) ; message WM_TIMECHANGE;
private
isTimeChangeEventShowing: Boolean;
end;
var
Form1: TForm1;
implementation
{$R *.dfm}
{ TForm1 }
procedure TForm1.FormCreate(Sender: TObject);
begin
isTimeChangeEventShowing := false
end;
procedure TForm1.WMTimeChange(var Msg: TMessage);
begin
if not isTimeChangeEventShowing then
begin
isTimeChangeEventShowing := true;
showmessage('Date/Time has changed!');
isTimeChangeEventShowing := false;
end;
end;
end.