Section 12.4. Identifying References


12.4. Identifying References

AppleScript goes to some lengths to hide the existence of references, making it remarkably difficult to find out that a value is a reference. Properly speaking, a reference should be a class, a datatype like string or integer (see "Class" in Chapter 11, and Chapter 13). If you ask a string about its class, it says string. If you ask an integer about its class, it says integer. But if you ask a reference about its class, it will never tell the truth and say reference.

 set x to a reference to "hey" set y to a reference to 9 tell application "Finder" to set z to folder 1 class of x -- string class of y -- integer class of z -- folder

Here are a couple of tricks you can use to learn that a value is a reference. (I don't guarantee any of them, but they do seem mostly to work.)


The reference coercion trick

The only thing that can be coerced to a reference is a reference. If you try to coerce anything else to a reference, you'll get a runtime error. So try to coerce a value to a reference, and if there's no error, it is a reference. For example:

 tell application "Finder" to set x to folder 1 x as reference -- no error; it's a reference


The editor value trick

If a value, as shown in your script editor application, contains the word of, it is a reference. For example:

 tell application "Finder" to set x to folder 1 x -- folder "Mannie" of...; it's a reference set x to a reference to y x -- y of «script»; it's a reference

When I'm debugging or developing a script, I like the second method best; I look at a variable's value and I can usually see right away whether it's likely to be a reference. If I'm writing code where the code itself needs to test whether something is a reference, I like the first method best. Here's a general handler that returns a boolean value telling whether its parameter is a reference:

 on isRef(valueToTestAsRef)     try         valueToTestAsRef as reference         return true     on error         return false     end try end isRef -- and here's how to call it tell application "Finder"     set x to folder 1 end tell isRef(x) -- true set x to "haha" isRef(x) -- false isRef(a reference to x)) -- true




AppleScript. The Definitive Guide
AppleScript: The Definitive Guide, 2nd Edition
ISBN: 0596102119
EAN: 2147483647
Year: 2006
Pages: 267
Authors: Matt Neuburg

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