AppleScript: The Definitive Guide, 2nd Edition

Scripting Additions

One of the main reasons for using scripting additions is speed. For repeated trigonometric calculations, for example, it is certainly going to be a lot faster to use a scripting addition, such as the Satimage osax, than to roll your own calculation (as disingenuously suggested at Apple's web site). Similarly, a scripting addition that implements transformations to a list, such as returning a list with a particular element deleted, is going to be faster than coding the same operation in AppleScript (see "LISP-likeness" in Chapter 4). Just how quickly a scripting addition is called, however, depends on how you call it.

The osax architecture is such that a scripting addition appears to be present "inside" whatever application is being targeted when the scripting addition command is called. This behavior is noticeable, and useful, when a scripting addition puts up some user interface. For example, if the display dialog command is called from within a tell block targeting the Finder, the dialog appears within the Finder; it's as if you'd given the Finder a new dialog.

Behind the scenes, though, this architecture involves a serious inefficiency. The application itself is sent the Apple event denoting a scripting addition command. Obviously the application can't deal with this Apple event, so at that point the message is sent on up to the realm of scripting additions as a kind of fallback. This means that when you use a scripting addition command while targeting an application, ...