In my app, I've been using ModelActors in SwiftData, and using actors with a custom executor in Core Data to create concurrency safe services.
I have multiple actor services that relate to different data model components or features, each that have their own internally managed state (DocumentService, ImportService, etc).
The problem I've ran into, is that I need to be able to use multiple of these services within another service, and those services need to share the same context. Swift 6 doesn't allow passing contexts across actors.
The specific problem I have is that I need a master service that makes multiple unrelated changes without saving them to the main context until approved by the user.
I've tried to find a solution in SwiftData and Core Data, but both have the same problem which is contexts are not sendable. Read the comments in the code to see the issue:
/// This actor does multiple things without saving, until committed in SwiftData.
@ModelActor
actor DatabaseHelper {
func commitChange() throws {
try modelContext.save()
}
func makeChanges() async throws {
// Do unrelated expensive tasks on the child context...
// Next, use our item service
let service = ItemService(modelContainer: SwiftDataStack.shared.container)
let id = try await service.expensiveBackgroundTask(saveChanges: false)
// Now that we've used the service, we need to access something the service created.
// However, because the service created its own context and it was never saved, we can't access it.
let itemFromService = context.fetch(id) // fails
// We need to be able to access changes made from the service within this service,
/// so instead I tried to create the service by passing the current service context, however that results in:
// ERROR: Sending 'self.modelContext' risks causing data races
let serviceFromContext = ItemService(context: modelContext)
// Swift Data doesn't let you create child contexts, so the same context must be used in order to change data without saving.
}
}
@ModelActor
actor ItemService {
init(context: ModelContext) {
modelContainer = SwiftDataStack.shared.container
modelExecutor = DefaultSerialModelExecutor(modelContext: context)
}
func expensiveBackgroundTask(saveChanges: Bool = true) async throws -> PersistentIdentifier? {
// Do something expensive...
return nil
}
}
Core Data has the same problem:
/// This actor does multiple things without saving, until committed in Core Data.
actor CoreDataHelper {
let parentContext: NSManagedObjectContext
let context: NSManagedObjectContext
/// In Core Data, I can create a child context from a background context.
/// This lets you modify the context and save it without updating the main context.
init(progress: Progress = Progress()) {
parentContext = CoreDataStack.shared.newBackgroundContext()
let childContext = NSManagedObjectContext(concurrencyType: .privateQueueConcurrencyType)
childContext.parent = parentContext
self.context = childContext
}
/// To commit changes, save the parent context pushing them to the main context.
func commitChange() async throws {
// ERROR: Sending 'self.parentContext' risks causing data races
try await parentContext.perform {
try self.parentContext.save()
}
}
func makeChanges() async throws {
// Do unrelated expensive tasks on the child context...
// As with the Swift Data example, I am unable to create a service that uses the current actors context from here.
// ERROR: Sending 'self.context' risks causing data races
let service = ItemService(context: self.context)
}
}
Am I going about this wrong, or is there a solution to fix these errors?
Some services are very large and have their own internal state. So it would be very difficult to merge all of them into a single service. I also am using Core Data, and SwiftData extensively so I need a solution for both.
I seem to have trapped myself into a corner trying to make everything concurrency save, so any help would be appreciated!
