[CoreData] Transformable な Attribute の使い方

// Register the transformer at the very beginning.
// .colorToDataTransformer is a name defined with an NSValueTransformerName extension.
ValueTransformer.setValueTransformer(ColorToDataTransformer(), forName: .colorToDataTransformer)

public class MyClass: NSObject, Codable {
    var type: Int
    var title: String
    override init() {
        self.type = Int.random(in: 0...9)
        self.title = "Hello" + "\(Int.random(in: 10...19))"
    }
}

class MyClassTransformer: NSSecureUnarchiveFromDataTransformer {
    override class var allowedTopLevelClasses: [AnyClass] {
        return [MyClass.self]
    }
    override class func transformedValueClass() -> AnyClass {
        MyClass.self
    }
    override class func allowsReverseTransformation() -> Bool {
        true
    }
    
    override func transformedValue(_ value: Any?) -> Any? {
        guard let data = value as? Data else { fatalError("invalid data type") }
        let myClass = try! JSONDecoder().decode(MyClass.self, from: data)
        return myClass

    }
    override func reverseTransformedValue(_ value: Any?) -> Any? {
        guard let myClass = value as? MyClass else { fatalError("invalid data type") }
        return try! JSONEncoder().encode(myClass)
    }
}

//
//  ContentView.swift
//
//  Created by : Tomoaki Yagishita on 2022/09/05
//  © 2022  SmallDeskSoftware
//

import SwiftUI
import CoreData

struct ContentView: View {
    @Environment(\.managedObjectContext) private var viewContext

    @FetchRequest(
        sortDescriptors: [NSSortDescriptor(keyPath: \Item.timestamp, ascending: true)],
        animation: .default)
    private var items: FetchedResults

    var body: some View {
        NavigationView {
            List {
                ForEach(items) { item in
                    NavigationLink {
                        VStack {
                            Text("Item at \(item.timestamp!, formatter: itemFormatter)")
                            Text("MyClass type : \(item.myClass?.type ?? -5)")
                            Text("MyClass title: \(item.myClass?.title ?? "NoTitle")")
                        }
                    } label: {
                        Text(item.timestamp!, formatter: itemFormatter)
                    }
                }
                .onDelete(perform: deleteItems)
            }
            .toolbar {
                ToolbarItem {
                    Button(action: addItem) {
                        Label("Add Item", systemImage: "plus")
                    }
                }
            }
            Text("Select an item")
        }
    }

    private func addItem() {
        withAnimation {
            let newItem = Item(context: viewContext)
            newItem.timestamp = Date()
            newItem.myClass = MyClass()

            do {
                try viewContext.save()
            } catch {
                // Replace this implementation with code to handle the error appropriately.
                // fatalError() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development.
                let nsError = error as NSError
                fatalError("Unresolved error \(nsError), \(nsError.userInfo)")
            }
        }
    }

    private func deleteItems(offsets: IndexSet) {
        withAnimation {
            offsets.map { items[$0] }.forEach(viewContext.delete)

            do {
                try viewContext.save()
            } catch {
                // Replace this implementation with code to handle the error appropriately.
                // fatalError() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development.
                let nsError = error as NSError
                fatalError("Unresolved error \(nsError), \(nsError.userInfo)")
            }
        }
    }
}

private let itemFormatter: DateFormatter = {
    let formatter = DateFormatter()
    formatter.dateStyle = .short
    formatter.timeStyle = .medium
    return formatter
}()

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView().environment(\.managedObjectContext, PersistenceController.preview.container.viewContext)
    }
}

class MyClassTransformer: NSSecureUnarchiveFromDataTransformer {
    override class var allowedTopLevelClasses: [AnyClass] {
        return [MyClass.self]
    }
    override class func transformedValueClass() -> AnyClass {
        MyClass.self
    }
    override class func allowsReverseTransformation() -> Bool {
        true
    }
    
    override func transformedValue(_ value: Any?) -> Any? {
        guard let data = value as? Data else { fatalError("invalid data type") }
        let myClass = try! JSONDecoder().decode(MyClass.self, from: data)
        return myClass

    }
    override func reverseTransformedValue(_ value: Any?) -> Any? {
        guard let myClass = value as? MyClass else { fatalError("invalid data type") }
        return try! JSONEncoder().encode(myClass)
    }
}

public class MyClass: NSObject, Codable {
    var type: Int
    var title: String
    override init() {
        self.type = Int.random(in: 0...9)
        self.title = "Hello" + "\(Int.random(in: 10...19))"
    }
}

        if let url = FileManager.default.urls(for: .applicationSupportDirectory, in: .userDomainMask).first {            print("file exists at \(url)")
        }