CLOSE button is ignored whilst in a WHILE LOOP

Question

I have a few simple while loops in my program, such as a Countdown program. I also have a bitmap button for close (Doesn't require any code right?). When The countdown is in progress(while), everything (All buttons and edits) are ignored/unable to use until the requirements are met with the loop.

Is there a way to stop the loop before the requirements are met?

Here's my code:

while iCountDown > 0 do
    begin
      application.ProcessMessages;
      dec(iCountDown)
      lblOutput.Caption := IntToStr(iCountDown);
      sleep(1000);
    end;
    ShowMessage ('Countdown has finished!');

Thanks.

Answer 1

Declare a private field of your form like this:

FCancelLoop: Boolean;

Set the variable to False before you start your loop. In the OnClick event handler of your close button, set FCancelLoop to True. Check the value of FCancelLoop inside the loop and break out of the loop if you find it is True.

FCancelLoop := False;
while iCountDown > 0 do
begin
  Application.ProcessMessages;
  if FCancelLoop then
    exit; // or perhaps break, but you get the idea
  dec(iCountDown);
  lblOutput.Caption := IntToStr(iCountDown);
  Sleep(1000);
end;

However, this code has other problems. It uses the notorious Application.ProcessMessages which has a tendency to make your life more complicated than you want it to be. Are your event handlers protected against re-entrancy? And the call to Sleep on your main thread blocks that thread and makes it unresponsive. You really don't want that.

A better solution to your problem is to run your countdown loop with a timer.

Answer 2

A different solution could be a thread. Its advantages:

• it does not block the main thread when calling Sleep
• Application.ProcessMessages is not needed
• it does not interfere with the Close button

When used with synchronize, it also can update the Label caption and other UI elements.

The code for the count-down message ('Countdown has finished!') can be executed in the OnTerminate event handler - without synchronize as it will executed in the main thread context.