[codecamper]

It would be so good to be able to use just one language (swift, scala, or java preferably) to write portable but the native UI.

however, what happens when I have written my app & this company goes belly up. It isn't open source so I guess then I'd be sol.

for now I use j2objc & it is working pretty well. My only big wish would be for some way to write persistence logic just once too.

[efdee]

Take a look at React Native. Develop using Javascript, but render native components - not webbased stuff like Cordova.

[badlogic]

Check out RoboVM. We don't do portable UI layers, but you can reuse all your Java/Scala/Kotlin/Clojure code for business logic.