Applescript 如何读取通过引用传递的值的属性？

我正在尝试编写OmniGraffle 5.3.6脚本，其中我尝试读取已传递给函数（用AppleScript的说法是处理程序）的

画布的
层
元素。当我运行以下命令时：

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

我得到以下错误：

  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

但是，如果我要内联函数，那么一切都可以正常工作：

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

我读了，但我觉得没有帮助。它非常稀疏。
您可以通过引用传递。然而，无论您传递到哪里，它都必须具有“告诉应用程序”，以便应用程序能够处理传递的引用。所以你需要这样的函数

on exportToPng(theCanvas)
    tell application "OmniGraffle Professional 5"
      set layerCount to count of layers of theCanvas
    end tell
end exportToPng

毕竟，只有“OmniGraffle Professional 5”知道如何从Canvas获取图层