Update screen during lengthy process

[Richard Gaskin]

tkuypers wrote:

 > I've created a hot-folder based app, that executes a few scripts when
 > a file if dropped into a specific folder.
 > But these scripts can take up to 4 minutes to end and during that
 > period LiveCode is locked.
 >
 > I'm using a send command to watch the folder every few seconds, is a
 > file is found then it is processed, but I want to show some kind of
 > progress on screen to show the app is working.
 >
 > I've tried firing off a second event, to show a counter, but that
 > doesn't do anything.
 > And of course clicking on the "Stop Processing" button doesn't work
 > as well :-(
 >
 > Any suggestions on how to show a progress, and how to make the "Stop"
 > button to work during the execution of an applescript?

My WebMerge product has a similar challenge: for most folks it runs in 
just a few seconds, but it's sometimes used to process very large 
collections of product lists (300,000 or more) which can take a few minutes.

So I needed both a way to indicate progress, and a way to cancel the 
task, just as you do.

Here's what I did:

In the main repeat loop I have a call to a CheckCancel handler, e.g.:

    ...
    global gProcessActive
    put true into gProcessActive
    show button "Cancel"
    --
    repeat for each line tLine in tData
       CheckCancel
       --
       Processs tLine
    end repeat
    ...

Here's the CheckCancel handler:

on CheckCancel
    global gProcessActive
    wait 0 with messages
    if gProcessActive <> true then
      DoCleanUpAndExit
    else
      ShowProgress
    end if
end CheckCancel

Then in a button labeled "Cancel" I have this script:

on mouseUp
    global gProcessActive
    put false into gProcessActive
end mouseUp


So first the gProcessActive flag is set before entering the loop, and 
within the loop CheckCancel is called to do two things:

1. Allow messages to be sent ("wait 0 with messages")
2. Check if the flag has been set to false

Because messages are processed thanks to the "with messages" modifier, 
if the user clicks on the "Cancel" button that button's script sets the 
flag to false, and the next call to CheckCancel will notice that and 
call DoCleanUpAndExit if it needs to.

What happens in DoCleanUpAndExit will of course depend on what your 
program is doing (though you should include a "exit to top" statement to 
exit the loop from outside of it), and the same is true for 
ShowProgress.   Because WebMerge's process is of a knowable length (the 
number of lines of the data it's working on) I use a progress bar, but 
you could use a spinning wheel or indeterminate progress bar or even 
just updating text in a field, whichever works for your UI.

I've experimented with various methods of canceling long processes, and 
thus far this solution based around "wait 0 with messages" has shown 
itself to require the least overhead of the options I've tried.

HTH -

--
  Richard Gaskin
  Fourth World
  LiveCode training and consulting: http://www.fourthworld.com
  Webzine for LiveCode developers: http://www.LiveCodeJournal.com
  LiveCode Journal blog: http://LiveCodejournal.com/blog.irv