Language Guide

CHAPTER 8
Handlers
250 Command Handlers for Script Applications
Interrupting a Script Application’s Handlers 8
A stay-open script application handles incoming commands even if it is
already running a handler in response to a previous command. This means that
execution of a handler can be interrupted while another handler is run. Because
script applications are not multitasking, execution of the first handler halts
until the second one finishes.
This can cause problems if both handlers modify the same script property or
global variable or if both attempt to modify an application’s data. For example,
suppose that running a script application named Increment causes it to
increment the property p for several minutes:
property p : 0
on close
set temp to p
set p to 0
return temp
end close
set p to 0
repeat 1000000 times
set p to p + 1
end repeat
If while this script application is running it receives a Close command, the
property p is reset to 0 and the script application begins incrementing p all
over again:
tell application "Increment" to close
AppleScript can’t deal with such interruptions automatically.