RemObjects’ Elements compiler supports their own implementation of Swift called Silver across all major platforms/devices. It also supports four other languages.
I’ve just started looking into it, but the major caveat is you have to design and maintain a separate UI for each platform, even Mac vs Windows vs Linux. But that also means you should get much better control and performance on each platform vs Xojo (least common denominator).
And like Xojo, their proof is in “eating their own dogfood” - their IDEs for Mac and Windows are built with Elements. You can also use Elements from within VS.
As I said, I just started evaluating it. It is more complex to setup and use than Xojo, but I’m trying to ascertain what benefits it gives me over Xojo. Oh, and the Swift (Silver) only version is totally free!