Several scripts calling one application...

Question

I'm setting up several stay-open applescripts. Each watches its own hotfolder for incoming PDFs, then tells Photoshop to process the files as needed, and ultimately saves each as a new file.
Sometimes, when two or more of these scripts call on Photoshop at the same time, it gets confused in various ways. One time it saved all the new files under an identical name. Another time, one of the files just stayed open and never saved. There's never a problem when only one script is working at a time, but I don't have the luxury of just avoiding simultaneous requests... in the intended environment, they're gonna happen.
Is there something I can do to make Photoshop deal with just one script at a time, and then move right on to the next?

Answer 1

There are several approaches to this.
One is to have a flag in your script that detects if an existing image is being processed - this could be as simple as touching a file on disk when you start and deleting it when you're finished. That way when your second script fires it checks for the file - if it's there it pauses until the file is deleted.
Another approach is to query the application to see what its doing - for example if Photoshop is normally running without any windows open, a quick check on the number of documents would tell you if Photoshop is busy.
A third approach (which may be the better one, but requires more knowledge of the script) is to properly target the document in Photoshop.
For example, if your script does something like:
open file
do action 1
do action 2
do action 3
close file
then there's nothing in those actions that tell Photoshop WHERE to perform the action. If you have multiple documents open it's bound to get confused at some point.
Instead you should consider something more like:
open file (and get its ID)
do action 1 on document ID
do action 2 on document ID
do action 3 on document ID
close file
In this way you're being specific about which document each action should be performed on. Since each open document would have a unique ID it's quite possible to have action 1 running on document 1 at the same time action 2 is running on document 2.
As I mentioned, this approach requires more scripting and requires support in Photoshop which may or may not be there (I don't run Photoshop to know how scriptable it is), but it's the most robust solution.