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Snapshot of Cocoa Scripting

The work done by your scriptable application is divided between the application and Cocoa scripting. Your application implements classes and methods that perform its scriptable operations; it also provides information that describes its scriptability. Cocoa receives Apple events and uses your scriptability information to interpret them and perform the specified operations, calling on your code to do the actual work.

Here is a summary of how this process works:

1. The application defines scriptability information (in an sdef file, or in the older style script suite and script terminology files) that includes both the terms a scripter can use and the application information for supporting those terms. This information typically includes the Standard suite (implemented by Cocoa scripting), which supports standard AppleScript commands and classes.

   The application defines classes for the scriptable objects it supports and provides keys for their scriptable properties and elements. It also defines additional script command classes if it has scriptable operations that can't be performed by one of the standard command classes provided by Cocoa.

   Scriptability is generally provided through the application's model objects (in terms of the Model-View-Controller paradigm).

2. The application is key-value coding (KVC) compliant in naming the instance variables or accessor methods for the scriptable properties and elements of its scriptable classes.

3. For each scriptable class, the application implements an object specifier method, which locates a scriptable object of that type within the application’s containment hierarchy.

4. The application’s Info.plist file has entries that activate Cocoa scripting and specify an sdef file, as shown in “Turn On Scripting Support in Your Application.”

5. When it is first needed, Cocoa loads the application's scriptability information and automatically registers event handlers for the supported commands.

6. When the application receives an Apple event for a registered command, Cocoa instantiates a script command object containing all the information needed to identify the application objects on which the command should operate. All command objects use KVC to locate the specified scriptable objects to operate on.

   Cocoa then executes the command, which sends messages to the appropriate application objects to perform the work. For many commands, Cocoa uses KVC to get or set values of the specified objects.

7. When a command needs to return a value, Cocoa scripting packages the information in a reply Apple event and returns it.

   If an error occurs while executing the command, Cocoa returns the error information (including any information added by the application) in the reply Apple event. For details, see “Error Handling.”

8. If a command requires asynchronous processing (such as the gathering of information through a sheet), the application can suspend it, so that the application doesn't receive additional Apple events during processing. For details, see “Suspending and Resuming Apple Events and Script Commands.”

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