SwiftData preview sample code

It seems to be a problem in the #Preview macro. In the demo app (SampleTrips), I've managed to get it working using the old style previews:

@MainActor
struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
            .modelContainer(PreviewSampleData.container)
    }
}

Hi @pjc83, this is a known issue. There is however a workaround. Just use MainActor.assumeIsolated like so:

#Preview {
    MainActor.assumeIsolated {
        ContentView()
            .modelContainer(previewContainer)
    }
}

Thanks both, the MainActor.assumeIsolated worked for me such I can still use the new #Preview macro, too.

Thanks for the help fixing the preview, however I'm hitting issues when running the app in the simulator. Maybe something isn't setup right for working with the backend? Getting these errors when trying to create a new file in the running app:

os_unix.c:46973: (0) open(/Users/myUserName/Library/Developer/CoreSimulator/Devices/17277B7F-0589-4D47-B3BF-C4C910FC23D6/data/Containers/Data/Application/C4A2EDA5-4BD0-4D9C-8437-75DFEC2B269F/Library/Application Support/default.store) - Undefined error: 0
API call with unopened database connection pointer
misuse at line 179760 of [554764a6e7]

on Xcode15 beta4, all workarounds above don’t work for me.

I notice that release notes mentioned that it is a known issue and recommend to run on device replacing preview, but it is not convenient.

is there any other workaround?

I found some working solutions. This is how I generate the preview containter:

let previewContainer: ModelContainer = {
    do {
        let container = try ModelContainer(for: Snippet.self,
                                           configurations: ModelConfiguration(isStoredInMemoryOnly: true))
        
        Task { @MainActor in
            
            let context = container.mainContext
            
            let snip = Snippet.example2()

            
            let folder = Folder(name: "folder with favorite snippet")
            context.insert(folder)
            folder.snippets.append(Snippet(isFavorite: true, 
                     title: "favorite snippet"))

            // add test data here
        }
        
        return container
    } catch {
        fatalError("Failed to create container: \(error.localizedDescription)")
    }
}()

#Preview {
     FolderListView()
        .modelContainer(previewContainer)
}

Using view where I need to pass in a SwiftData object is more complex. I created a demo project with more examples. All previews are working for Xcode 15 https://github.com/gahntpo/SnippetBox-SwiftData