Delphi FormKeyDown ALT Key Stuck

Question

I have a form with 2 CheckBoxes (1 is for holding CTRL - 1 is for holding ALT). Both CheckBoxes are disabled so the KeyDown Event of the form works properly. There is also a TTimer that synchronizes every 10ms if the ALT/CTRL key is pressed.

My timer:

procedure TForm1.Timer1Timer(Sender: TObject);
begin
 CheckBox1.Checked := ALTDOWN;  // ALTDOWN IS GLOBAL
 CheckBox2.Checked := CTRLDOWN; // CTRLDOWN IS GLOBAL
end;

My KeyDown Event:

procedure TForm1.FormKeyDown(Sender: TObject; var Key: Word;
Shift: TShiftState);
begin
 if Key = VK_MENU then begin
  ALTDOWN := TRUE;
  exit;
 end;
 if Key = VK_CONTROL then begin
  CTRLDOWN := TRUE;
  exit;
 end;
end;

My KeyUP Event:

procedure TForm1.FormKeyUp(Sender: TObject; var Key: Word;
Shift: TShiftState);
begin
 if Key = VK_MENU then begin
  ALTDOWN := FALSE;
  exit;
 end;
 if Key = VK_CONTROL then begin
  CTRLDOWN := FALSE;
  exit;
 end;
end;

This works without any problems with the CTRL key. But the ALT key get's stuck sometimes or does not even show up at all. This happens when I press ONLY the ALT key (without any other keys in combination).

Why is that and how can I fix this?

Answer 1

If you are going to run a timer in that way, then you may as well just call GetAsyncKeyState

procedure TForm1.Timer1Timer(Sender: TObject);
begin
 CheckBox1.Checked := GetAsyncKeyState(VK_MENU)<0;
 CheckBox2.Checked := GetAsyncKeyState(VK_CONTROL)<0;
end;

And just get rid of your OnKeyDown and OnKeyUp event handlers. There's really no point in you trying to keep track of whether the key is up or down when the system already does so.

Your timer interval is quite short. The system won't fire them that frequently anyway. If I recall correctly the timer resolution is typically around 50ms.

Answer 2

Read microsoft documentation when you want to get inside its internal ways (and KeyUp is that) http://msdn.microsoft.com/en-us/library/windows/desktop/ms646281.aspx

You can see that releasing Alt key is system event and would not be expected in WM_KeyUp. Add monitoring of WM_SYSCOMMAND, WM_SYSKEYUP and WM_SYSKEYDOWN as well.