Inside Macintosh: AppleScript Language Guide / Part 2 - AppleScript Language Reference

Chapter 9 - Script Objects

Initializing Script Objects

When you define a script object, you define a collection of handlers and properties. When you run a script containing a script object definition, AppleScript creates a script object with the properties and handlers listed
in the definition. This is called initializing a script object. A script object
must be initialized before it can respond to commands.

If you include a script object definition at the top level of a script--that is, as part of the script's Run handler--AppleScript initializes the script object each time the script's Run handler is executed. (For more information about Run handlers, see page 243.)

Similarly, if you include a script definition in another handler within a script, AppleScript initializes a script object each time the handler is called. The parameter variables in the handler definition become local variables of the script object. For example, the makePoint handler in the following script contains a script object definition for the script object point:

on makePoint(x, y)
   script point
      property xCoordinate:x
      property yCoordinate:y
   end script
   return point
end makePoint

set myPoint to makePoint(10,20)
get xCoordinate of myPoint
get yCoordinate of myPoint

AppleScript initializes the script object point when it executes the makePoint command. The parameter variables in the makePoint handler, in this case, x and y, become local variables of the script object point. The initial value of x is 10, and the initial value of y is 20, because those are the parameters of the makePoint command that initialized the script object.

One way to use script object definitions in handlers is to define constructor functions, that is, handlers that create script objects. The following script uses
a constructor function to create three script objects.

on makePoint(x, y)
   script
      property xCoordinate:x
      property yCoordinate:y
   end script
end makePoint

set PointA to makePoint(10,20)
set PointB to makePoint(100,200)
set PointC to makePoint(1,1)

As in the previous example, you can retrieve the coordinates of the three script objects using the Get command.

Note

The distinction between defining a script object and initializing a script object is similar to the distinction between a class and an instance in object-oriented design. When you define a script object, you define a class of objects that respond to the same commands. When AppleScript initializes a script object, it creates an instance of the class. The script object gets its initial context (property values and handlers) from the script object definition, but its context can change as it responds to commands.