How to have Autohotkey 'listen' for a change to a file?

Question

I have a text file, lets call it C:\to_run.txt. I'd like my Autohotkey script to 'listen' to this file in such a way that when it detects a change, it performs an action immediately based on the contents of the file, and then make the file blank again.

I can handle the last parts, so really I'm asking for an efficient way to detect file changes? I say efficient because my Autohotkey script is getting rather long and I don't want this listening function to hang up the rest of the script in any way.

Answer 1

Assuming we are really talking of only one file to check on: Surely not as beautiful as Sidola's answer, but without the need for external libraries:

#persistent

lastFileContent := ""
setTimer, checkFile, 20
return

checkFile:
fileread newFileContent, changingDocument.txt
if(newFileContent != lastFileContent) {
    lastFileContent := newFileContent
    msgbox, content changed to: %newFileContent%
}
return

In this case, for checking on larger files, it might be better to compare MD5-checksums instead of the whole file content.

Note: I have not tested the performance implications on this. This script opens up the file 50 times per second, could be pretty hard drive consuming.