Robust Feature Checking in SketchUp

When the SketchUp Ruby API introduces new features or changes a common pattern to ensure backward compatibility is:

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if Sketchup.version.split('.')[0].to_i < 7
  model.start_operation('FooBar')
else
  model.start_operation('FooBar', true)
end

if Sketchup.version.split('.')[0].to_i < 7
  model.start_operation('FooBar')
else
  model.start_operation('FooBar', true)
end

One might even need to check the minor and revision number in some cases. But doing so require you to split and parse strings as well as remembering which version supported what feature.

Instead, one can check for the feature itself. In the case of start_operation‘s disable_ui flag, you can check the method’s arity: Sketchup::Model.instance_method(:start_operation).arity

The rewritten code would look like this:

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if model.method(:start_operation).arity == 1
  model.start_operation('FooBar')
else
  model.start_operation('FooBar', true)
end

if model.method(:start_operation).arity == 1
  model.start_operation('FooBar')
else
  model.start_operation('FooBar', true)
end

In SketchUp 6 and older this returns 1 while under SketchUp 7 and newer it returns -1. (-1 is returned for methods that accept variable number of arguments.)

Another useful check is .respond_to? – which you can use to check for  features like so:

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if model.materials.respond_to?(:remove)
  model.materials.remove( material )
else
  self.remove_material_hack( material )
end

if model.materials.respond_to?(:remove)
  model.materials.remove( material )
else
  self.remove_material_hack( material )
end

Feature testing like this is much more robust as you do not have to look up the exact version number for when SketchUp started supporting the feature. Simply test if the feature exist.

It’s the same type of design encouraged webdesign – where in the early days one relied on sniffing out the user agent string, but that has proven to be terrible unreliable. Instead, best practices now encourages people to test against features.

This method of designing is called Duck typing.

When I see a bird that walks like a duck and swims like a duck and quacks like a duck, I call that bird a duck.