As I noted in another comment, the documentation for SwiftUI is actually pretty good, especially if you take into account how quickly the framework has changed. It includes some rich, progressive tutorials. [0]

The author states that they are planning to write an app from scratch with SwiftUI. I think most iOS developers would hesitate to build an entire app on a beta framework.

[0] - https://developer.apple.com/tutorials/swiftui/tutorials