AppleScript: The Definitive Guide, 2nd Edition

Event Handlers

Handlers that you define freely in code are user handlers. But there are also handlers whose definition comes from the dictionary of some scripting addition or application. These are event handlers (a command defined in a dictionary is technically called an event, because it is actually an Apple event, as discussed in Chapter 3).

An event handler 's parameter syntax is a little different from that of a user handler :

No parameters

The handler takes no parentheses.

First parameter

The first parameter (the direct object) simply follows the handler name directly, without parentheses.

Subsequent parameters

Subsequent parameters are labeled. Labeled parameters are the same as prepositional parameters, but the labels are not limited to the list in Table 9-1; basically, the dictionary can define any labels it likes.

Also, the name of the command, and any of the labels, can consist of multiple words .

An event handler is often used in connection with an automatic location (Chapter 2). Some application is going to send a message to your script, and it has defined what that message will be. To receive the message, your script must contain an event handler with a matching definition. The event handler is an entry point to your script.

Take, for example, folder actions (Chapter 26). You can attach a script to a folder as a folder action, so your script is called (by the System Events application) when certain things happen in the Finder. Let's say you want your script to be ...