Using AudioConverterFillComplexBuffer in Swift

I'm trying to port some old Objective-C code to Swift, and I ran into an issue while using `AudioToolbox` (who knew you'd have issues using a C API in swift!) Anyways, I can't seem to get the signature for the callback correct, and the documentation is no help. Does anyone have experience using this in Swift?

https://developer.apple.com/library/IOs/documentation/MusicAudio/Reference/AudioConverterServicesReference/index.html#//apple_ref/swift/tdef/AudioConverterComplexInputDataProc