AppleScript is a scripting language devised by Apple Computer, and built into Mac OS. More generally, AppleScript is the word used to designate the Mac OS scripting interface, which is meant to operate in parallel with the graphical user interface.
History
The AppleScript project was an outgrowth of the HyperCard project. HyperCard had an English language based scripting language called HyperTalk which could be used for embedding logic and behavior into a HyperCard stack. Apple engineers recognized that a similar type of scripting language could be designed so as to be used with any application, and the AppleScript project was born.
The Natural Language metaphor
In keeping with the Mac OS tradition of ease-of-use, the AppleScript language is designed on the natural language metaphor, just as the graphical user interface is designed on the desktop metaphor. Thus, the concept of an object hierarchy is expressed using nested prepositional phrases:
pixel 7 of row 3 of TIFF image "my bitmap"
which in another programming language might be expressed as sequential function calls:
getTIFF("my bitmap")->getRow(3)->getPixel(7)
Interapplication communication
AppleScript was designed to be primarily used as a scripting language to control other applications. As such, it depends on the Mac OS interapplication communication protocol called AppleEvents. AppleEvents are essentially a byte-code representation of a message sent from one application to another. AppleScript uses an application dictionary to associate the bytecodes with human readable terminology, thus allowing the translation back and forth between human readable AppleScript and bytecode AppleEvents.
To designate which application is meant to be the target of such a message, AppleScript uses a 'tell' construct:
tell application "Microsoft Word" to quit
AppleScript on its own
AppleScript need not depend on other applications. For very simple tasks, AppleScript can be used for self-contained applets. For instance, the code:
set pix to 72
set answer to text returned of (display dialog "Enter in the number of inches" default answer "1")
display dialog answer & "in = " & (answer * pix) & "px"
Brings up a dialog box requesting a number of inches from the user. This number is then converted to pixels on a system that uses 72 pixels per inch. A second dialog box is brought up displaying the result.
AppleScript Studio
With Mac OS X, AppleScript has grown well beyond its humble beginnings. AppleScript Studio is a development environment which is free with Mac OS X which uses AppleScript as the primary programming language, in conjunction with the Cocoa-based ProjectBuilder framework used to construct graphical user interfaces.
With Mac OS X 10.3 (aka Panther) AppleScript Studio and ProjectBuilder have been rolled into the XCode integrated development environment. AppleScript Studio lets you build a User Interface in a drag and drop fashion (similar to Visual Basic) and then 'run' the User Interface to see what the forms and menus looks like.
Panther also comes with Script Editor, which is a minimalist editor for compiling and running AppleScripts. A nice feature of this editor is that if you right click (command click) on the editing area you get a pop up menu with a large range of options for script fragments to paste into your code. This is an excellent feature for people learning to write AppleScript. You can also from that menu open up the directory where these scripts are kept, and have a look at them. You can also add your own Scripts (though you need to restart Script Editor for these changes to show up in the pop up menu. Using these features, you can turn Script Editor into a minimalist, but extensible, Java IDE for example.
AppleScript dialects
For a short time, AppleScript supported the idea of multiple dialects, which included English, French, Japanese, Japanese (romaji), and Italian. Terminology was made available for each dialect, such that the AppleScript compiler could compile and decompile scripts written in any dialect to any other dialect. While the project was a technical success, few application developers provided terminology in multiple languages, and technical support and testing proved to be far too much effort for the very little return on investment. Support for multiple dialects was dropped by Apple in Mac OS 8.5.
AppleScript language essentials
- basic data types are string, integer, real, list, record and object
- different types can coexist in a list, including nested lists
- records are lists of key-value pairs
- standard control flow with if / else constructions and repeat loops
- variables are instantiated when used, and are not strictly typed
- script objects can encapsulate methods and data
- script objects can inherit behavior from a parent script
- 'tell' contruct used to identify target of message
- applications can define terminology at runtime
- runtime compiling possible using 'run script' construct
- persistence possible using 'store script' and 'load script'