[Lazarus] Working with threads and GUI

[ik]

Hello,

I'm writing an application that need to perform a counter on big
numbers (0..n).
I wish to display the counter on screen, but also allow to stop the
process itself.

At the moment I did something like:

for i := 0 to Number do
  begin
    edtValue.text := IntToStr(Number);
    application.processmessages;
    sleep(200);
  end;

How can I use it under a thread that the view will be updated in the
form, but I could also cancel the action on a very long action ?

Thanks,
Ido