The following code doesn't work. The diagnostics says something like: "An NSManagedObject of class 'Book' must have a valid NSEntityDescription."

struct DetailView_Previews: PreviewProvider {
    static let moc = NSManagedObjectContext(concurrencyType: .mainQueueConcurrencyType)

    static var previews: some View {
        let book = Book(context: moc)
        book.title = "Test book"
        book.author = "Test author"
        book.genre = "Fantasy"
        book.rating = 4
        book.review = "This was a great book; I really enjoyed it."

        return NavigationView {
            DetailView(book: book)
        }
    }
}

Does somebody have a solution?

Hello ! I have the same problem, does anyone have a solution ?

Same here... I am using Xcode 13.4, built for iOS 15.5, tried simulating on iPod touch (7th generation), iPhone 12, iPad (9th generation).

I was able to fix the issue by adding a Persistence.swift file following the example code here: https://www.hackingwithswift.com/forums/100-days-of-swiftui/day-54-bookworm-an-nsmanagedobject-of-class-book-must-have-a-valid-nsentitydescription/5043

I really like the solution from the video however which is just to delete all the preview code from DetailView.swift.

The general question here is:

How do I show mock core data objects in a SwiftUI preview provider?

I have another suggestion.

Check out Mark Moeykens' excellent eBook titled, Core Data Quick Start in SwiftUI dated, 22 June 2022.

Our own @twoStraws is mentioned in the opening credits.

Mark writes nearly nine pages with plenty of notes and graphics explaining how to populate a temporary CoreData datastore with sample objects so that you can view fake CoreData records in your previews. Sweet!

The book is excellent and is free.

See -> Mark's Excellent Core Data Book

Thanks @Obelix, was looking how to do this, and downloaded that book and now I can happily design my screen in the preview with fake core data :)

So you don't have to go digging in the book you can create a PreviewDataController under the Preview Content folder of your project...

class PreviewDataController {

    private let container = NSPersistentContainer(name: "DiceRolls")

    var viewContext: NSManagedObjectContext {
        container.viewContext
    }

    init() {
        container.persistentStoreDescriptions.first!.url = URL(fileURLWithPath: "/dev/null")

        container.loadPersistentStores { description, error in
            if let error = error {
                print("Core Data failed to load: \(error.localizedDescription)")
            }
        }
    }

    func savePreviewData(diceRolls: [[String : Any]]) {

        diceRolls.forEach { diceRollDict in
            let diceRoll = DiceRoll(context: container.viewContext)
            diceRoll.id = UUID()
            diceRoll.totalRolled = diceRollDict["totalRolled"] as! Int16
            diceRoll.numberOfSides = diceRollDict["numberOfSides"] as! Int16
            diceRoll.numberOfDice = diceRollDict["numberOfDice"]  as! Int16
            diceRoll.dateOfRoll = diceRollDict["dateOfRoll"] as? Date
        }
    }
}

Then in your preview...

struct PreviousRollsView_Previews: PreviewProvider {

    private static let diceRolls = [
        [
            "totalRolled": Int16(4),
            "numberOfSides": Int16(6),
            "numberOfDice": Int16(1),
            "dateOfRoll": Date()
        ],
        [
            "totalRolled": Int16(12),
            "numberOfSides": Int16(6),
            "numberOfDice": Int16(2),
            "dateOfRoll": Date()
        ],
        [
            "totalRolled": Int16(14),
            "numberOfSides": Int16(4),
            "numberOfDice": Int16(4),
            "dateOfRoll": Date()
        ],
    ]

    static var previews: some View {
        let previewDataController = PreviewDataController()

        previewDataController.savePreviewData(diceRolls: diceRolls)

        return PreviousRollsView(isPresented: .constant(true))
            .environment(
                \.managedObjectContext,
                 previewDataController.viewContext
            )
    }
}