AppleScript: The Definitive Guide, 2nd Edition

The Open Scripting Architecture

When System 7 was being created, along with Apple events and many other new technologies, it was already Apple's plan to create a language, AppleScript, that would give end users access to the power of Apple events. But, much to the disappointment of users and developers, there wasn't time to create AppleScript before the release of System 7 in mid-1991, and the bulk of the work was postponed until 1992-1993.

One of the conundrums facing the founders of AppleScript at this time was the architectural question of where the language should live. They could have made AppleScript the internal scripting language of a single application, like HyperCard's HyperTalk , but this would mean that the user would run AppleScript code entirely from within this one application, which was unacceptable. AppleScript needed to be available everywhere, and thus would somehow have to be part of the system. But what part? There was no good place, so a new one was created: the resulting structure is the Open Scripting Architecture (OSA).

Components

Under the OSA, a scripting language is implemented by a something called a component . (Components were not invented specially for the OSA; they existed already in connection with QuickTime.) Think of a component as a piece of self-contained functionality made available at system level so that any program can hook up to it and use it. One thing that's special about components is that they can be installed and uninstalled dynamically. ...