AppleScript: The Definitive Guide

Scoping of Handlers

A handler definition may appear only at the top level of a script or script object. (Therefore you can’t nest handlers, except indirectly, by having a handler inside a script object inside a handler.)

A handler is visible to code in the scope where it is defined, even code that precedes the definition (see Section 4.4.1). For example:

myHandler(  ) -- Howdy
on myHandler(  )
        display dialog "Howdy"
end myHandler

A handler is visible to scopes within the scope where it is defined. Remarkably, this works even before the handler is defined. For example:

script x
        myHandler(  )
end script
run x -- Howdy
on myHandler(  )
        display dialog "Howdy"
end myHandler

A handler is visible on demand from outside the script object where it is defined. Code that can see a script object can refer to its handlers, using essentially the same two kinds of syntax by which it would refer to its properties (see Section 7.5.1). You don’t need to use the word its to call a script object’s handler from within a tell block addressed to that script object. This example illustrates both kinds of syntax:

script x
        on myHandler(  )
                display dialog "Howdy"
        end myHandler
end script
x's myHandler(  ) -- Howdy
tell x
        myHandler(  ) -- Howdy
end tell

The comparison between handlers and properties is apt. The scoping of handlers is very similar to the scoping of properties. They are both top-level entities of a script or script object; they are visible on demand wherever that script or script object is visible; and ...