Delphi Access Violation when moving button on form

Question

I am trying to move 2 buttons on the form using a while loop checking for the buttons left property, but I have an Access Violation. I am exeting the procedure with CreateThread()

The code:

procedure AnimButton1();
var ImageCount: integer;
var b1, b2: integer;
begin

  try

       while (b2 <> 187) do
       begin
           b2 := frmNotification.btnBuzina2.Left;
           frmNotification.btnBuzina2.Left := b2 - 1;

       end;

       while (b1 <> 256) do
       begin
           b1 := frmNotification.btnBuzina.Left;
           frmNotification.btnBuzina.Left := b1 - 1;

       end;

  except;
  end;

end;

BUT, when I use a Sleep() with at least 5 miliseconds, I dont have an access violation, like this:

procedure AnimButton1();
var ImageCount: integer;
var b1, b2: integer;
begin

  try

       while (b2 <> 187) do
       begin
           b2 := frmNotification.btnBuzina2.Left;
           frmNotification.btnBuzina2.Left := b2 - 1;
           Sleep(5);
       end;

       while (b1 <> 256) do
       begin
           b1 := frmNotification.btnBuzina.Left;
           frmNotification.btnBuzina.Left := b1 - 1;
           Sleep(5);
       end;

  except;
  end;

end;

Could someone help me to find out why without the sleep I get the access violation and with it I dont?

thx in advance!

Answer 1

I am executing the procedure with CreateThread().

That is your problem. VCL code must only be called from the main UI thread. Use TThread.Synchronize to invoke the VCL code on the main thread.

That said, a timer might be a more appropriate solution to you problem than a thread.