How to read a property of a value passed by reference? - applescript

Question

I am trying to script OmniGraffle 5.3.6 where I try to read the layers element of a canvas that I have passed to a function (or handler, in AppleScript parlance). When I run the following:
on exportToPng(theCanvas)
set layerCount to count of layers of theCanvas
end exportToPng
tell application "OmniGraffle Professional 5"
set theDocument to front document
set allCanvases to canvases of theDocument
set theCanvas to item 1 of allCanvases
my exportToPng(theCanvas)
end tell
I get the following error:
error "OmniGraffle Professional 5 got an error: Can’t make |layers| of
canvas id 1 of document \"base-dependency-diagram.graffle\" into type reference."
number -1700 from |layers| of canvas id 1 of document
"base-dependency-diagram.graffle" to reference
However, if I were to inline my function, then everything works fine:
tell application "OmniGraffle Professional 5"
set theDocument to front document
set allCanvases to canvases of theDocument
set theCanvas to item 1 of allCanvases
set layerCount to count of layers of theCanvas
end tell
I read Passing by Reference Versus Passing by Value, but I did not find it helpful. It's pretty sparse.

Answer 1

You can pass by reference. However, wherever you pass it must also have "tell application" so that application can handle the passed reference. So you will want your function like this...
on exportToPng(theCanvas)
tell application "OmniGraffle Professional 5"
set layerCount to count of layers of theCanvas
end tell
end exportToPng
After all, only "OmniGraffle Professional 5" knows how to get the layers from theCanvas.