Stopping loops

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Program loops often may run for extended periods -  seconds, minutes or even hours.  In these cases, it is good coding practice to let the user terminate the loop early.    One way is described here: 

  1. Allocate a Stop button with its visible property set to false.  
  2. In the Onclick event exit for the Stop button set tag property to 1.  The tag property is a "spare" integer field in each control to be used as the programmer sees fit.   I usually use  the TForm1.tag field, but  one of the button tags would work as well.
  3. Before entering the loop:
    1. Set tag to  0
    2. Make the Stop button visible
    3. Disable the Start button (Enabled:=false) so the user cannot start a second copy of the loop while the first one is running.
    4. Either:
      1. Set screen.cursor to crHourglass or 
      2. Loop through all controls setting all to crHourGlass and then set the form cursor to crHourglass and the stop button cursor to crArrow.  This has the advantage of showing a normal arrow cursor when the user moves the mouse over the Stop  button and the hourglass cursor everywhere else.
  4. Make a test for tag<>0 one of the terminating  tests for the loop.
  5. Within the loop call application.processmessages periodically, probably once or twice per second is often enough.  Application.processmessages allows user actions, (such as  clicking the stop button), to be processed.  Each call takes a bit of CPU time so there is a trade off between responding to user clicks and loop speed.  I usually test for the loop counter being a multiple of 1024 or 4096 or some other power of 2  to trigger the test.   
  6. Before exiting the Onclick event, reverse the above above  steps.


Here's some sample code illustrating the technique.

procedure TForm1.StartBtnClick(Sender: TObject);
    tag:=0; {reset tag}

    {make all cursors  hourglass}
    for I:= 0 to controlcount-1 do controls[i].cursor:=crhourglass;
    StopBtn.cursor:=crArrow;  {except the stop button}

    {THE LOOP}
    while (tag=0) and (i<high(i)) do
        if i mod 1024=0 then application.processmessages;
        {do loop work here}

    {END LOOP}

    for I:= 0 to controlcount-1 do controls[i].cursor:=crdefault;

procedure TForm1.StopBtnClick(Sender: TObject);



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