Automator: Wait for file

Question

I'm trying to write an automator workflow, that i can remotely trigger using something like dropbox. What I want is to have it watch a folder, and when it finds a file with a specific file, it moves on in the workflow. I haven't been able to find anything that would wait like this. Does it exist? Thanks

Answer 1

There are a few solutions to this, you'll need to decide which one meets your needs best.
First off, your script on Computer B could poll the size of the file, waiting until the file stops increasing in size for a sufficient length of time - long enough for you to assume it's finished copying, at which point you can invoke the rest of your script.
The other options depends on the ability of Computer A and there are two classic solutions.
One is to have Computer A write a tempoary lock file (or, conversely, a done file). Your script looks for this file to determine when the file is ready - in the case of a lock file, you create the lock file to indicate that the file is not ready, then write the file, then delete the lock file when it's done. The script on Computer B notices the lock file being deleted and knows the correspoinding real file is ready.
The variation on this is to have a done file - you write the file to the disk and when its finished you create/update the done file to indicate the new file. Your script feeds off this done file to know when a file is ready.
Both these options involve the use of temporary semaphore files that signal to computer B when to proceed.
The other option that doesn't use semaphores is for Computer A to write the file to a temporary subdirectory, rather than the drop folder. It then moves the file to the drop folder when it's ready. In this way the script on computer B doesn't need to worry about how long it takes to generate the file.
All the latter options involve additional smarts on the source machine A since that's the only one that really knows the state of the file.