New project fails using SwiftUI, XCode 12 and Catalina

Question

I'm trying to create a new XCode project, plain brand new and I can run it on the simulator, but on the canvas it says: Cannot preview in this file - active scheme does not build this file If on top of that I start adding macOS support, it says:

'appName' is annotated with @main and must provide a main static function of type () -> Void or () throws -> Void. It also says:

Cannot find type 'App' in scope

Cannot find type 'Scene' in scope

Regarding mac support I was reading there is a workaround, even though I tried and the error remains.

As for the active scheme, I read this, but doesn't help. I mean... there is only 1 scheme in a new project and it's selected automatically

I would assume a plain hello world project would work, what's going on?

Answer 1

Regarding the macOS support is what Asperi mentioned, on Catalina has problems supporting mac on XCode 12.

As for the new project failing, I found that if I press Cmd + Opt + P, it would build it on the Canvas fine. I had tried clicking "Try again" on the canvas which I would assume is the same and didn't work. You don't need to delete the file and create a new one nor select any target since it's a new project and the target is selected automatically. Also the @Main doesn't have to be changed to run on iOS and iPad, for macOS support just wait until Big Sur releases.

Answer 2

SwiftUI 2.0 on macOS is available only starting from Big-Sur macOS 11.0

@available(iOS 14.0, macOS 11.0, tvOS 14.0, watchOS 7.0, *)
public protocol App {
                    ^^^^^^^^^^^^