将 ManagedObject 集合从获取结果传递到闭包 ViewController

Question

我想执行后台提取并将结果传递给闭包。目前我正在使用 NSPersistentContainer 中的 performBackgroundTask 方法，它给出了 NSManagedObjectContext 作为闭包。然后使用该上下文我正在执行获取请求。获取完成后，我将结果传递给完成处理程序。

func getAllWorkspacesAsync(completion: @escaping ([Workspace]) -> Void) {
    CoreDataStack.shared.databaseContainer.performBackgroundTask { childContext in
        let workspacesFetchRequest: NSFetchRequest<Workspace> = NSFetchRequest(entityName: "Workspace")
        workspacesFetchRequest.predicate = NSPredicate(format: "remoteId == %@", "\(UserDefaults.lastSelectedWorkspaceId))")
        do {
            let workspaces: [Workspace] = try childContext.fetch(workspacesFetchRequest)
            completion(workspaces)
        } catch let error as NSError {
            // Handle error
        }
    }
}

我将从 ViewModel 调用此方法并使用 Combine PassthroughSubject 通知 ViewController 事件。

class WorkspaceViewModel {

    private var cancellables: Set<AnyCancellable> = []
    let resultPassthroughObject: PassthroughSubject<[Workspace], Error> = PassthroughSubject()

    private let cacheManager = WorkspaceCacheProvider.shared

    static public let shared: WorkspaceViewModel = {
        let instance = WorkspaceViewModel()
        return instance
    }()

    func fetchWorkspaces() {
        cacheManager.getAllWorkspacesAsync { [weak self] workspaces in
            guard let self = self else { return }
            self.resultPassthroughObject.send(workspaces)
        }
    }
}

和 ViewController 代码：

class WorkspaceViewController: UIViewController {

    private var cancellables: Set<AnyCancellable> = []

    override func viewDidLoad() {
        super.viewDidLoad()
    
        WorkspaceViewModel.shared.resultPassthroughObject
            .receive(on: RunLoop.main)
            .sink { _ in } receiveValue: { workspaces in
            // update the UI
        }
        .store(in: &cancellables)

    }
}

我的问题是：传递 NSManagedObject 项目安全吗？

Answer 1

没有，it is not

Do not pass NSManagedObject instances between queues. Doing so can result in corruption of the data and termination of the app. When it is necessary to hand off a managed object reference from one queue to another, use NSManagedObjectID instances.

你会想要这样的东西：

func getAllWorkspacesAsync(completion: @escaping ([NSManagedObjectID]) -> Void) {
    CoreDataStack.shared.databaseContainer.performBackgroundTask { childContext in
        let workspacesFetchRequest: NSFetchRequest<Workspace> = NSFetchRequest(entityName: "Workspace")
        workspacesFetchRequest.predicate = NSPredicate(format: "remoteId == %@", "\(UserDefaults.lastSelectedWorkspaceId))")
        do {
            let workspaces: [Workspace] = try childContext.fetch(workspacesFetchRequest)
            completion(workspaces.map { [=10=].objectID } )
        } catch let error as NSError {
            // Handle error
        }
    }
}

您需要对您的发布商进行相应的更改，以便发布 [NSManagedObjectID]。

在视图控制器中收到此数组后，您需要在视图上下文中调用 object(with:) 以获取 Workspace 本身。无论如何，这将在对象的视图上下文中执行提取。

您可能需要考虑获取工作区所花费的时间是否值得使用后台上下文。默认情况下，Core Data 对象作为 faults 检索，并且仅在通过访问对象的属性触发故障时才获取实际属性值。

或者，如果视图控制器的目的是呈现用户可以从中 select 的工作区列表，您可以 return 包含工作区名称和对象 ID。然而，这一切对我来说听起来像 pre-optimisation。

将 ManagedObject 集合从获取结果传递到闭包 ViewController

Passing ManagedObject collection from fetch result to closure up to ViewController

core-data

nsmanagedobject

nsmanagedobjectcontext

ios

swift