TrackBar OnChange event

[Davo]

I've noticed that when a TrackBar button is moved the TrackBar's OnChange event is triggered three times in succession. For example if a TrackBar has min and max settings of 0 and 10 respectively and a breakpoint is set on the following single line of code in its OnChange event handler, when the button is just nudged so that its position changes by just 1 the breakpoint is triggered three times.

procedure TForm1.TrackBar1Change(Sender: TObject);
begin
  label1.caption := IntToStr(TrackBar1.position);
end;

Is this normal behaviour? Is there a setting that I've missed that ensures that the OnChange event handler is triggered only once? Or do I have to resort to using boolean flags in the code to skip the action in the OnChange event handler for the second and third visits there?

[Mike.Cornflake]

I'm on Windows - I'll confirm tonight, but if you're correct, this would explain an oddity I've been observing in one of my apps.   

What OS and widgetset are you using?

[Davo]

Win7 Home Premium

Lazarus version 1.0.10
FPC version 2.6.2
SVN Revision 41613
i386-win32-win32/win64

Running on a 64 bit computer

If I knew how to determine the widgetset I'd let you know.

[Mike.Cornflake]

Ah, you'll be using win32/win64 widgetset then, same as me.  Right, I'll definitely check this out tonight...

FYI: You set the Widgetset for your project in Project Options.  You set the Widgetset for Lazarus in Tools - Configure "Build Lazarus".  I *think* the two are unrelated, haven't done much work switching between Widgetsets...

Update:  I also think the string "i386-win32-win32/win64" is TargetCPU-Widgetset :-)

[Blaazen]

I can confirm with Wine + Laz. 1.2.0.
One OnChange is triggered when knob moves and two additional OnChange on MouseUp.

Is this normal behaviour? Is there a setting that I've missed that ensures that the OnChange event handler is triggered only once? Or do I have to resort to using boolean flags in the code to skip the action in the OnChange event handler for the second and third visits there?

You should open a bugreport, category Widgetset-Win32.

[Davo]

Project Options | Inherited | LCL 1.0.10 | LCL Widget Type = “win/32”
Tools | Configure “Build Lazarus” | LCL widget type | win32/win64

Both settings are the default values obtained on downloading Lazarus.

Exploring further and a]ltering the code to :

Code: [Select]

procedure TForm1.TrackBar1Change(Sender: TObject);
begin
  showmessage('Change');
  label1.caption := inttostr(TrackBar1.position);
end;

procedure TForm1.TrackBar1MouseUp(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
  showmessage('Mouseup');
end;
gives the following result :

If the trackbar button is clicked without releasing the mouse button nothing happens. Which is logical.

If the trackbar button is clicked without moving it along the bar the message “Change” is displayed three times in succession and then the message “Mouseup” is shown (it is necessary to click the message box's OK button each time).

If the trackbar button is moved along the bar the message “Change” is displayed three times in succession but not the “Mouseup”message even 'though a MouseUp event must have occurred because the mouse was moved from the trackbar to click the OK button in the message box.

[Mike.Cornflake]

I confirmed this on my system as well, but this wasn't the source of my oddity.

For now, I've implemented the following in my onchange

Code: [Select]

If Trackbar1.Position<>FLastPosition Then
Begin
  Try 
     ... Code ...
  Finally
    FLastPosition := Trackbar1.Position;
  End; 
End;
and in my OnCreate I initialise FLastPosition to -1.

Could you please raise a bug report on this item?   If you don't get a chance today, I'll do one tonight...

[Davo]

Mike. Thanks for your confirmations. Your code fix works well in the small application that I'm currently building – thanks again for that. I won't get a chance to submit a bug report within the next 24 hours so, if you don't mind, can you submit it? And anyway I'm sure that you will do a better job of it than I can.

[Mike.Cornflake]

Your code fix works well in the small application that I'm currently building

Sweet :-)

if you don't mind, can you submit it?

No problems - I'll get onto it tonight :-)

[Mike.Cornflake]

Reported.
http://bugs.freepascal.org/view.php?id=26358

[Mike.Cornflake]

Patch applied.  Will be 1.2.6 & is currently in Trunk.