CoreDataRepository

📣 Checkout the discussion for SwiftData

CoreDataRepository is a library for using CoreData on a background queue. It features endpoints for CRUD, batch, fetch, and aggregate operations. Also, it offers a stream like subscription for fetch and read.

Since NSManagedObjects are not thread safe, a value type model must exist for each NSMangaedObject subclass.

Motivation

CoreData is a great framework for local persistence on Apple's platforms. However, it can be tempting to create strong dependencies on it throughout an app. Even worse, the viewContext runs on the main DispatchQueue along with the UI. Even fetching data from the store can be enough to cause performance problems.

The goals of CoreDataRepository are:

Ease isolation of CoreData related code away from the rest of the app.
Improve ergonomics by providing an asynchronous API.
Improve usability of private contexts to relieve load from the main DispatchQueue.
Make local persistence with CoreData feel more 'Swift-like' by allowing the model layer to use value types.

Mapping `NSManagedObject`s to value types

It may feel convoluted to add this layer of abstraction over local persistence and the overhead of mapping between objects and value types. Similar to the motivation for only exposing views to the minimum data they need, why should the model layer be concerned with the details of the persistence layer? NSManagedObjects are complicated types that really should be isolated as much as possible.

To give some weight to this idea, here's a quote from the Q&A portion of this talk by Andy Matuschak:

Q: How do dependencies work out? It seems like the greatest value of using values is in the model layer, yet that’s the layer at which you have the most dependencies across the rest of your app, which is probably in Objective-C.

Andy: In my experience, we had a CoreData stack, which is the opposite of isolation. Our strategy was putting a layer about the CoreData layer that would perform queries and return values. But where would we add functionality in the model layer? As far as using values in the view layer, we do a lot of that actually. We have a table view cell all the way down the stack that will render some icon and a label. The traditional thing to do would be to pass the ManagedObject for that content to the cell, but it doesn’t need that. There’s no reason to create this dependency between the cell and everything the model knows about, and so we make these lightweight little value types that the view needs. The owner of the view can populate that value type and give it to the view. We make these things called presenters that given some model can compute the view data. Then the thing which owns the presenter can pass the results into the view.

Basic Usage

Model Bridging

There are two protocols that handle bridging between the value type and managed models.

RepositoryManagedModel

@objc(RepoMovie)
public final class RepoMovie: NSManagedObject {
    @NSManaged var id: UUID?
    @NSManaged var title: String?
    @NSManaged var releaseDate: Date?
    @NSManaged var boxOffice: NSDecimalNumber?
}

extension RepoMovie: RepositoryManagedModel {
    public func create(from unmanaged: Movie) {
        update(from: unmanaged)
    }

    public typealias Unmanaged = Movie
    public var asUnmanaged: Movie {
        Movie(
            id: id ?? UUID(),
            title: title ?? "",
            releaseDate: releaseDate ?? Date(),
            boxOffice: (boxOffice ?? 0) as Decimal,
            url: objectID.uriRepresentation()
        )
    }

    public func update(from unmanaged: Movie) {
        id = unmanaged.id
        title = unmanaged.title
        releaseDate = unmanaged.releaseDate
        boxOffice = NSDecimalNumber(decimal: unmanaged.boxOffice)
    }

    static func fetchRequest() -> NSFetchRequest<RepoMovie> {
        let request = NSFetchRequest<RepoMovie>(entityName: "RepoMovie")
        return request
    }
}

UnmanagedModel

public struct Movie: Hashable {
    public let id: UUID
    public var title: String = ""
    public var releaseDate: Date
    public var boxOffice: Decimal = 0
    public var url: URL?
}

extension Movie: UnmanagedModel {
    public var managedRepoUrl: URL? {
        get {
            url
        }
        set(newValue) {
            url = newValue
        }
    }

    public func asRepoManaged(in context: NSManagedObjectContext) -> RepoMovie {
        let object = RepoMovie(context: context)
        object.id = id
        object.title = title
        object.releaseDate = releaseDate
        object.boxOffice = boxOffice as NSDecimalNumber
        return object
    }
}

CRUD

var movie = Movie(id: UUID(), title: "The Madagascar Penguins in a Christmas Caper", releaseDate: Date(), boxOffice: 100)
let result: Result<Movie, CoreDataRepositoryError> = await repository.create(movie)
if case let .success(movie) = result {
    os_log("Created movie with title - \(movie.title)")
}

Fetch

let fetchRequest = NSFetchRequest<RepoMovie>(entityName: "RepoMovie")
fetchRequest.sortDescriptors = [NSSortDescriptor(keyPath: \RepoMovie.title, ascending: true)]
fetchRequest.predicate = NSPredicate(value: true)
let result: Result<[Movie], CoreDataRepositoryError> = await repository.fetch(fetchRequest)
if case let .success(movies) = result {
    os_log("Fetched \(movies.count) movies")
}

Fetch Subscription

Similar to a regular fe:

let result: AnyPublisher<[Movie], CoreDataRepositoryError> = repository.fetchSubscription(fetchRequest)
let cancellable = result.subscribe(on: userInitSerialQueue)
            .receive(on: mainQueue)
            .sink(receiveCompletion: { completion in
                switch completion {
                case .finished:
                    os_log("Fetched a bunch of movies")
                default:
                    fatalError("Failed to fetch all the movies!")
                }
        }, receiveValue: { value in
            os_log("Fetched \(value.items.count) movies")
        })
...
cancellable.cancel()

Aggregate

let result: Result<[[String: Decimal]], CoreDataRepositoryError> = await repository.sum(
    predicate: NSPredicate(value: true),
    entityDesc: RepoMovie.entity(),
    attributeDesc: RepoMovie.entity().attributesByName.values.first(where: { $0.name == "boxOffice" })!
)
if case let .success(values) = result {
    os_log("The sum of all movies' boxOffice is \(values.first!.values.first!)")
}

Batch

let movies: [[String: Any]] = [
    ["id": UUID(), "title": "A", "releaseDate": Date()],
    ["id": UUID(), "title": "B", "releaseDate": Date()],
    ["id": UUID(), "title": "C", "releaseDate": Date()],
    ["id": UUID(), "title": "D", "releaseDate": Date()],
    ["id": UUID(), "title": "E", "releaseDate": Date()]
]
let request = NSBatchInsertRequest(entityName: RepoMovie.entity().name!, objects: movies)
let result: Result<NSBatchInsertResult, CoreDataRepositoryError> = await repository.insert(request)

OR

let movies: [[String: Any]] = [
    Movie(id: UUID(), title: "A", releaseDate: Date()),
    Movie(id: UUID(), title: "B", releaseDate: Date()),
    Movie(id: UUID(), title: "C", releaseDate: Date()),
    Movie(id: UUID(), title: "D", releaseDate: Date()),
    Movie(id: UUID(), title: "E", releaseDate: Date())
]
let result: (success: [Movie], failed: [Movie]) = await repository.create(movies)
os_log("Created these movies: \(result.success)")
os_log("Failed to create these movies: \(result.failed)")

TODO

Add a subscription feature for aggregate functions
Migrate subscription endpoints to AsyncSequence instead of Publisher
Simplify model protocols (require only one protocol for the value type)
Allow older platform support by working around the newer variants of NSManagedObjectContext.perform and NSManagedObjectContext.performAndWait

Contributing

I welcome any feedback or contributions. It's probably best to create an issue where any possible changes can be discussed before doing the work and creating a PR.

The above TODO section is a good place to start if you would like to contribute but don't already have a change in mind.