Knowing when lightweight migration finished - swift

Question

Setup:
I'm trying to do a lightweight Core Data migration in my app. I added a new Core Data model version and added the new improvements (added 9 attributes to existing entities, 2 new entities, relationships between the new and existing entities).
Issue:
Switching from a build with the old database to the new one causes the app to have no data. But once I quit the app and come back, the data is all there (I'm assuming it was just being migrated).
Question:
Is there a way to know when the Core Data is starting a migration (to let the users know) and when the migration is finished (to refresh their view with the new data)? I've done so much searching this past couple of weeks and have not come with much.
Thank you in advance!
Code:
This is how I set up Core Data (specifically the setupContainer() code):
final class CoreDataManager {
static let sharedManager = CoreDataManager()
lazy var persistentContainer: NSPersistentContainer = {
setupContainer()
}()
private func setupContainer() -> NSPersistentContainer {
useCloudSync = UserDefaults.standard.bool(forKey: UserDefaults.Keys.useCloudSync)
let containerToUse: NSPersistentContainer?
if useCloudSync {
containerToUse = NSPersistentCloudKitContainer(name: "appName")
} else {
containerToUse = NSPersistentContainer(name: "appName")
}
//check if we have a container
guard let container = containerToUse else {
fatalError("Hey Listen! Could not get a container!!")
}
// Enable history tracking and remote notifications
guard let description = container.persistentStoreDescriptions.first else {
fatalError("Hey Listen! ###\(#function): Failed to retrieve a persistent store description.")
}
//be notified of change
description.setOption(true as NSNumber, forKey: NSPersistentStoreRemoteChangeNotificationPostOptionKey)
description.setOption(true as NSNumber, forKey: NSMigratePersistentStoresAutomaticallyOption)
description.setOption(true as NSNumber, forKey: NSInferMappingModelAutomaticallyOption)
//set tracking history if we're using local container
if !useCloudSync {
description.setOption(true as NSNumber, forKey: NSPersistentHistoryTrackingKey)
}
container.loadPersistentStores(completionHandler: { (storeDescription, error) in
if let error = error as NSError? {
fatalError("Hey Listen! Couldn't load persistent store. Unresolved error \(error), \(error.userInfo)")
}
})
container.viewContext.mergePolicy = NSMergeByPropertyObjectTrumpMergePolicy
container.viewContext.transactionAuthor = appTransactionAuthorName
container.viewContext.automaticallyMergesChangesFromParent = true
//Remote changes notification
NotificationCenter.default.addObserver(
self, selector: #selector(type(of: self).storeRemoteChange(_:)),
name: .NSPersistentStoreRemoteChange, object: container.persistentStoreCoordinator)
return container
}//end of setup container
}

Answer 1

Presumably you're calling loadPersistentStores on your NSPersistentContainer. That method accepts a completion handler (closure). That's where you'll get the callback that the migration has finished (or has failed). And you can tell when the migration is about to begin by adding a call just before loadPersistentStores.
If you create your own NSPersistentStoreDescription instance and add it to the NSPersistentContainer, you can control whether the migration happens and whether it runs synchronously or asynchronously.
See Enabling core data lightweight migration in Swift 3 (particularly answer https://stackoverflow.com/a/53607127/719690) for an example of using the completion handler.
Does the completionHandler of loadPersistentStores of NSPersistentContainer run synchronously? is also useful to understanding the flow.
Working from your code sample, you might revise it to (and I've switch to shortened closure syntax):
container.loadPersistentStores { storeDescription, error in
if let error = error as NSError? {
// error path, but should be nicer than just crashing
fatalError("Hey Listen! Couldn't load persistent store. Unresolved error \(error), \(error.userInfo)")
} else {
// happy path
print("unicorns and rainbows, migration succeeded, load the tableview")
}
}
(typed in browser, beware typos)
A Swiftier version would use a guard let on the error checking, but I wanted to minimize changes I made to your sample.