Isolated managed object context for FetchRequest in SwiftUI

Question

In a SwiftUI view, by default FetchRequest fetches from the managedObjectContext environment value. If instead the view wants to fetch into an isolated context, for example to make discardable edits without polluting the context of other views, how can it change the context that FetchRequest uses?

One option is to wrap the view in an outer view that creates the isolated context and then calls the wrapped view using it:

var body: some View {
    WrappedView().environment(\.managedObjectContext, isolatedContext)
}

This is tedious, however. You have to create two views and pass all the wrapped views' arguments through the wrapper. Is there a better way to tell FetchRequest which context to use?

Answer 1

If you use the standard PersistentController that Apple gives as a startup you could try using

.environment(\.managedObjectContext, privateContext)

Your View would need this property to make it work. @State shouldn't be necessary since the changes are being done in the background by other means such as notifications.

let privateContext = PersistenceController.shared.container.newBackgroundContext()

Invoking newBackgroundContext() method causes the persistent container to create and return a new NSManagedObjectContext with the concurrencyType set to NSManagedObjectContextConcurrencyType.privateQueueConcurrencyType. This new context will be associated with the NSPersistentStoreCoordinator directly and is set to consume NSManagedObjectContextDidSave broadcasts automatically.

Then to test it out using most of the sample code from Apple.

struct SampleSharedCloudKitApp: App {
    let privateContext = PersistenceController.shared.container.newBackgroundContext()
    var body: some Scene {
        WindowGroup {
            VStack{
                Text(privateContext.description) //Added this to match with ContentView
                ContentView()
                    .environment(\.managedObjectContext, privateContext)
                    //Once you pass the privateContext here everything below it will have the privateContext
                    //You don't need to connect it with @FetchRequest by any other means
            }
        }
    }
}

struct ContentView: View {
    @Environment(\.managedObjectContext) private var viewContext
    @FetchRequest(
        sortDescriptors: [NSSortDescriptor(keyPath: \Item.timestamp, ascending: true)],
        animation: .default)
    private var items: FetchedResults<Item>

    var body: some View {
        List {
            Text((items.first!.managedObjectContext!.concurrencyType == NSManagedObjectContextConcurrencyType.privateQueueConcurrencyType).description) //This shows true
            Text(items.first!.managedObjectContext!.description)// This description matches the parent view
            Text(viewContext.description)// This description matches the parent view

Also, something to note is that you have to set

container.viewContext.automaticallyMergesChangesFromParent = true
container.viewContext.mergePolicy = NSMergePolicy.mergeByPropertyStoreTrump

In order for the main context to show the changes done after saving the privateContext. I put it in the PersistenceController init right after the loadPersistentStores closure.