Publish background context Core Data changes in a SwiftUI view without blocking the UI - core-data

Question

After running a background-context core data task, Xcode displays the following purple runtime warning when the updates are published in a SwiftUI view:
"[SwiftUI] Publishing changes from background threads is not allowed; make sure to publish values from the main thread (via operators like receive(on:)) on model updates."
Besides the ContentView.swift code below, I also added container.viewContext.automaticallyMergesChangesFromParent = true to init in the default Persistence.swift code.
How can I publish the background changes on the main thread to fix the warning? (iOS 14, Swift 5)
Edit: I've changed the code below, in response to the first answer, to clarify that I'm looking for a solution that doesn't block the UI when a lot of changes are saved.
struct PersistenceHelper {
private let context: NSManagedObjectContext
init(context: NSManagedObjectContext = PersistenceController.shared.container.viewContext) {
self.context = context
}
public func fetchItem() -> [Item] {
do {
let request: NSFetchRequest<Item> = Item.fetchRequest()
var items = try self.context.fetch(request)
if items.isEmpty { // Create items if none exist
for _ in 0 ..< 250_000 {
let item = Item(context: context)
item.timestamp = Date()
item.data = "a"
}
try! context.save()
items = try self.context.fetch(request)
}
return items
} catch { assert(false) }
}
public func updateItemTimestamp(completionHandler: #escaping () -> ()) {
PersistenceController.shared.container.performBackgroundTask({ backgroundContext in
let start = Date(), request: NSFetchRequest<Item> = Item.fetchRequest()
do {
let items = try backgroundContext.fetch(request)
for item in items {
item.timestamp = Date()
item.data = item.data == "a" ? "b" : "a"
}
try backgroundContext.save() // Purple warning appears here
let interval = Double(Date().timeIntervalSince(start) * 1000) // Artificial two-second delay so cover view has time to appear
if interval < 2000 { sleep(UInt32((2000 - interval) / 1000)) }
completionHandler()
} catch { assert(false) }
})
}
}
// A cover view with an animation that shouldn't be blocked when saving the background context changes
struct CoverView: View {
#State private var toggle = true
var body: some View {
Circle()
.offset(x: toggle ? -15 : 15, y: 0)
.frame(width: 10, height: 10)
.animation(Animation.easeInOut(duration: 0.25).repeatForever(autoreverses: true))
.onAppear { toggle.toggle() }
}
}
struct ContentView: View {
#State private var items: [Item] = []
#State private var showingCoverView = false
#State private var refresh = UUID()
let persistence = PersistenceHelper()
let formatter = DateFormatter()
var didSave = NotificationCenter.default
.publisher(for: .NSManagedObjectContextDidSave)
// .receive(on: DispatchQuene.main) // Doesn't help
var body: some View {
ScrollView {
LazyVStack {
Button("Update Timestamp") {
showingCoverView = true
persistence.updateItemTimestamp(completionHandler: { showingCoverView = false })
}
ForEach(items, id: \.self) { item in
Text(formatter.string(from: item.timestamp!) + " " + (item.data ?? ""))
}
}
}
.id(refresh)
.onAppear {
formatter.dateFormat = "HH:mm:ss"
items = persistence.fetchItem()
}
.onReceive(didSave) { _ in
items = persistence.fetchItem()
}
.fullScreenCover(isPresented: $showingCoverView) {
CoverView().onDisappear { refresh = UUID() }
}
}
}

Answer 1

Since you are performing a background task, you are on a background thread - rather than the main thread.
To switch to the main thread, change the line producing the runtime warning to the following:
DispatchQueue.main.async {
try backgroundContext.save()
}

Answer 2

You should use Combine and observe changes to your background context and update State values for your UI to react.
#State private var coreDataAttribute = ""
var body: some View {
Text(coreDataAttribute)
.onReceive(
CoreDataManager.shared.moc.publisher(for: \.hasChanges)
.subscribe(on: DispatchQueue.global())
.receive(on: DispatchQueue.global())
.map{_ in CoreDataManager.shared.fetchCoreDataValue()}
.filter{$0 != coreDataAttribute}
.receive(on: DispatchQueue.main))
{ value in
coreDataAttribute = value
}
}