How to use Combine to assign the number of elements returned from a Core Data fetch request? - swift

Question

I want my app to periodically fetch new records and stores them in Core Data. I have a label on my UI that should display the number of elements for a particular record and I want that number to be updated as more records are added into the database. As an exercise, I want to use Combine to accomplish it.
I'm able to display the number of elements in the database when the app launches, but the number doesn't get updated when new data enters into the database (I verified that new data was being added by implementing a button that would manual refresh the UI).
Here's the code that displays the correct number of elements on launch but doesn't update when new records are added:
let replayRecordFetchRequest: NSFetchRequest<ReplayRecord> = ReplayRecord.fetchRequest()
_ = try? persistentContainer.viewContext.fetch(replayRecordFetchRequest).publisher.count().map { String(format: Constants.Strings.playsText, $0) }.assign(to: \.text, on: self.playsLabel)
Here's a code snippet from the WWDC 2019 Session 230 talk that I adapted but this doesn't work at all (the subscriber is never fired):
let replayRecordFetchRequest: NSFetchRequest<ReplayRecord> = ReplayRecord.fetchRequest()
if let replayRecords = try? replayRecordFetchRequest.execute() {
_ = replayRecords.publisher.count().map { String(format: Constants.Strings.playsText, $0) }.assign(to: \.text, on: self.playsLabel)
}

Answer 1

So, I didn't know this until now, but not all publishers are infinitely alive.
And the problem was that the NSFetchRequest.publisher is not a long-living publisher. It simply provides a way to iterate through the sequence of elements in the fetch request. As a result, the subscriber will cancel after the elements are iterated. In my case, I was counting the elements published until cancellation then assigning that value onto the UI.
Instead, I should be subscribing to changes to the managed object context and assigning that pipeline to my UI. Here's some example code:
extension NotificationCenter.Publisher {
func context<T>(fetchRequest: NSFetchRequest<T>) -> Publishers.CompactMap<NotificationCenter.Publisher, [T]> {
return compactMap { notification -> [T]? in
let context = notification.object as! NSManagedObjectContext
var results: [T]?
context.performAndWait {
results = try? context.fetch(fetchRequest)
}
return results
}
}
}
let playFetchRequest: NSFetchRequest<ReplayRecord> = ReplayRecord.fetchRequest()
let replayVideoFetchRequest: NSFetchRequest<ReplayVideo> = ReplayVideo.fetchRequest()
let playsPublisher = contextDidSavePublisher.context(fetchRequest: playFetchRequest).map(\.count)
let replayVideoPublisher = contextDidSavePublisher.context(fetchRequest: replayVideoFetchRequest).map(\.count)
playsSubscription = playsPublisher.zip(replayVideoPublisher).map {
String(format: Constants.Strings.playsText, $0, $1)
}.receive(on: RunLoop.main).assign(to: \.text, on: self.playsLabel)