I am trying to translate code that looks like this:

static NSString *SKTWindowControllerCanvasSizeObservationContext = @"com.apple.SKTWindowController.canvasSize";

and later

[[self document] addObserver:self forKeyPath:SKTDocumentCanvasSizeKey options:NSKeyValueObservingOptionNew context:SKTWindowControllerCanvasSizeObservationContext];

to Ruby:

SKTWindowControllerCanvasSizeObservationContext = "com.apple.SKTWindowController.canvasSize"
document.addObserver(self, forKeyPath: SKTDocumentCanvasSizeKey, 
options: NSKeyValueObservingOptionNew,
context:SKTWindowControllerCanvasSizeObservationContext)

but when it runs I get this error:

 expected instance of Pointer, got `"com.apple.SKTWindowController.canvasSize"' (String) (TypeError)

How do I get the a pointer to the context string?  All the examples I have found for the Pointer class seem to expect an objective C routing to fill it in.

Thanks,

Dave.