SwiftUI CoreData NSManagedObjectContext in the Environment

Question

I have an issue exposing my NSManagedObjectContext to SwiftUI's Environment. Here's my code:

extension SceneDelegate: UIWindowSceneDelegate {

    func scene(_ scene: UIScene, willConnectTo session: UISceneSession, options connectionOptions: UIScene.ConnectionOptions) {
        guard let windowScene = scene as? UIWindowScene else { return }

        let window = UIWindow(windowScene: windowScene)
        let context = PersistentContainer.shared.viewContext

        let rootView = TabBarView().environment(\.managedObjectContext, context)
        window.rootViewController = UIHostingController(rootView: rootView)

        self.window = window
        window.makeKeyAndVisible()
    }
}

class PersistentContainer: NSPersistentContainer {
    static let shared = PersistentContainer()

    private convenience init() {
        self.init(name: "App")

        viewContext.mergePolicy = NSMergePolicy(merge: .mergeByPropertyStoreTrumpMergePolicyType)
        viewContext.automaticallyMergesChangesFromParent = true

        loadPersistentStores { description, error in
            if let error = error {
                fatalError("Unable to load persistent stores: \(error)")
            }
        }
    }
}

struct CategoriesView: View {
    @Environment(\.managedObjectContext) var context

    @FetchRequest(entity: CoreCategory.entity(),
                  sortDescriptors: [
                    NSSortDescriptor(keyPath: \CoreCategory.createdAt, ascending: true)
                  ]
                 ) var categories: FetchedResults<CoreCategory>
}

My CategoriesView is my root view, so the context is accessed on app launch. I get the following error when my app launches ...

[error] warning:  View context accessed for persistent container App with no stores loaded

... yet the view is able to display results fine. Furthermore, if I present a modal view that contains a SwiftUI @FetchRequest in the same format as my CategoriesView, the app crashes with the following error:

[SwiftUI] Context in environment is not connected to a persistent store coordinator: <NSManagedObjectContext: 0x600002348b60>

From all the tutorials I found online I'm exposing my NSManagedObjectContext the recommended way. I have a feeling it's related to loadPersistentStores being asynchronous, but the context being set in the Environment synchronously. Anyone else able to get CoreData to work in SwiftUI?

Answer 1

Figured out what's causing both error messages here. The first error is coming from my PersistentContainer singleton. I moved the two lines configuring the viewContext into the loadPersistentStores completion block, and those warnings went away:

private convenience init() {
    self.init(name: "App")

    loadPersistentStores { description, error in
        viewContext.mergePolicy = NSMergePolicy(merge: .mergeByPropertyStoreTrumpMergePolicyType)
        viewContext.automaticallyMergesChangesFromParent = true

        if let error = error {
            fatalError("Unable to load persistent stores: \(error)")
        }
    }
}

The second issue is apparently an issue with SwiftUI since the betas. There's a thread about it on the Apple Dev Forums here. You can do this as a workaround:

.sheet(isPresented: $isPresentingCategoryPicker) {
    CategoriesView()
        .environment(\.managedObjectContext, self.context)
}

It seems the environment gets cleared for modally presented view controllers.