CoreDataAdapter.swift

import UIKit
import Combine
import CoreData
import OSLog

public typealias CoreDataModelTransformer<T: NSManagedObject, M: Hashable> = (T) -> M?

public final class CoreDataAdapter<T: NSManagedObject, M: Hashable>: NSObject, NSFetchedResultsControllerDelegate {

    private let logger = Logger(subsystem: "codes.rambo.CombineCoreData", category: "CoreDataAdapter")

    public typealias SnapshotType = NSDiffableDataSourceSnapshot<Int, M>

    public let container: NSPersistentContainer
    public var request: NSFetchRequest<T> {
        didSet {
            setupController()
        }
    }
    public let transformer: CoreDataModelTransformer<T,M>

    @Published public private(set) var snapshot = SnapshotType()

    private var controller: NSFetchedResultsController<T>

    public init(with request: NSFetchRequest<T>, in container: NSPersistentContainer, transformer: @escaping CoreDataModelTransformer<T,M>) {
        self.container = container
        self.request = request
        self.transformer = transformer
        self.controller = NSFetchedResultsController(
            fetchRequest: request,
            managedObjectContext: container.viewContext,
            sectionNameKeyPath: nil,
            cacheName: "\(String(describing: T.self))"
        )

        super.init()

        setupController()
    }

    private func setupController() {
        controller = NSFetchedResultsController(
            fetchRequest: request,
            managedObjectContext: container.viewContext,
            sectionNameKeyPath: nil,
            cacheName: nil
        )

        do {
            controller.delegate = self
            try controller.performFetch()
            updateSnapshot()
        } catch {
            assertionFailure("Failed to start NSFetchedResultsController: \(String(describing: error))")
            logger.fault("Couldn't start the fetched results controller: \(String(describing: error), privacy: .public)")
        }
    }

    public func controllerDidChangeContent(_ controller: NSFetchedResultsController<NSFetchRequestResult>) {
        logger.debug("\(#function, privacy: .public)")

        updateSnapshot()
    }

    private var managedObjects: [T] { controller.fetchedObjects ?? [] }

    public private(set) var models: [M] = []

    private let updateQueue = DispatchQueue(label: "AdapterUpdate", qos: .userInteractive)

    private func updateSnapshot() {
        updateQueue.async {
            let ctx = self.container.newBackgroundContext()
            let objs = self.managedObjects.compactMap { ctx.object(with: $0.objectID) as? T }
            self.models = objs.compactMap(self.transformer)

            var snap = NSDiffableDataSourceSnapshot<Int, M>()
            snap.appendSections([0])
            snap.appendItems(self.models)

            DispatchQueue.main.async {
                self.snapshot = snap
            }
        }
    }

}