ios - 如何: Using Combine to react to CoreData changes in the background

Question

我想实现以下目标:每当有人触发 CoreData 保存(即发送 NSManagedObjectContextDidSave 通知)，我想执行一些 背景 基于更改的 NSManagedObject 的计算。具体示例:假设在一个笔记应用程序中，我想异步计算所有笔记中的单词总数。
当前的问题在于 NSManagedObject 上下文显式绑定(bind)到线程并且不鼓励您使用 NSManagedObject。在这个线程之外。
我设置了两个 NSManagedObjectContext s 在我的 SceneDelegate :

let context = (UIApplication.shared.delegate as! AppDelegate).persistentContainer.viewContext
let backgroundContext = (UIApplication.shared.delegate as! AppDelegate).persistentContainer.newBackgroundContext()

我还通过 NotificationCenter.default.publisher(for: .NSManagedObjectContextDidSave) 订阅了通知。并且在我只触发一个 managedObjectContext.save() 后收到两次保存通知.但是，这两个通知都是从同一个线程(即 UIThread)发送的，所有 NSManagedObjects在用户字典中有一个 .managedObjectContext这是 viewContext而不是 backgroundContext .
我的想法是根据关联的 NSManagedObjectContext 是否过滤通知。是背景通知，因为我假设通知也是在(私有(private))DispatchQueue 上发送的，但似乎所有通知都是在 UIThread 上发送的，并且从未使用过后台上下文。
关于如何解决这个问题的任何想法？这是一个错误吗？如何根据 backgroundContext 检索通知在关联的 DispatchQueue 上运行下游任务？

Answer 1

您可以创建一个发布者，当 Core Data 中与您相关的内容发生更改时，它会通知您。

我为此写了一篇文章。 Combine, Publishers and Core Data.

import Combine
import CoreData
import Foundation

class CDPublisher<Entity>: NSObject, NSFetchedResultsControllerDelegate, Publisher where Entity: NSManagedObject {
    typealias Output = [Entity]
    typealias Failure = Error

    private let request: NSFetchRequest<Entity>
    private let context: NSManagedObjectContext
    private let subject: CurrentValueSubject<[Entity], Failure>
    private var resultController: NSFetchedResultsController<NSManagedObject>?
    private var subscriptions = 0

      init(request: NSFetchRequest<Entity>, context: NSManagedObjectContext) {
        if request.sortDescriptors == nil { request.sortDescriptors = [] }
        self.request = request
        self.context = context
        subject = CurrentValueSubject([])
        super.init()
    }

      func receive<S>(subscriber: S)
        where S: Subscriber, CDPublisher.Failure == S.Failure, CDPublisher.Output == S.Input {
        var start = false

        synchronized(self) {
            subscriptions += 1
            start = subscriptions == 1
        }

        if start {
            let controller = NSFetchedResultsController(fetchRequest: request, managedObjectContext: context, 
                                                        sectionNameKeyPath: nil, cacheName: nil)
            controller.delegate = self

            do {
                try controller.performFetch()
                let result = controller.fetchedObjects ?? []
                subject.send(result)
            } catch {
                subject.send(completion: .failure(error))
            }
            resultController = controller as? NSFetchedResultsController<NSManagedObject>
        }
        CDSubscription(fetchPublisher: self, subscriber: AnySubscriber(subscriber))
    }

      func controllerDidChangeContent(_ controller: NSFetchedResultsController<NSFetchRequestResult>) {
        let result = controller.fetchedObjects as? [Entity] ?? []
        subject.send(result)
    }

      private func dropSubscription() {
        objc_sync_enter(self)
        subscriptions -= 1
        let stop = subscriptions == 0
        objc_sync_exit(self)

        if stop {
            resultController?.delegate = nil
            resultController = nil
        }
    }

    private class CDSubscription: Subscription {
        private var fetchPublisher: CDPublisher?
        private var cancellable: AnyCancellable?

        @discardableResult
        init(fetchPublisher: CDPublisher, subscriber: AnySubscriber<Output, Failure>) {
            self.fetchPublisher = fetchPublisher

            subscriber.receive(subscription: self)

            cancellable = fetchPublisher.subject.sink(receiveCompletion: { completion in
                subscriber.receive(completion: completion)
            }, receiveValue: { value in
                _ = subscriber.receive(value)
            })
        }

        func request(_ demand: Subscribers.Demand) {}

        func cancel() {
            cancellable?.cancel()
            cancellable = nil
            fetchPublisher?.dropSubscription()
            fetchPublisher = nil
        }
    }

}