Somewhat confusingly, Swift drops the prefix for ObjC enum values:
imageView.contentMode = .scaleAspectFill
This is because Swift already knows what enum type is being used. Alternatively, you can specify the enum too:
imageView.contentMode = UIViewContentMode.scaleAspectFill
Note: In versions of Swift before version 3, “scaleAspectFill” will need to be capitalized.