ios - 难以在 Swift 3 中配置 NSFetchedResultsController

Question

我正在将一个现有项目从 Swift 2 重构到 Swift 3。在我开始重构 Core Data 之前，一切都很简单。我能够创建托管对象并将它们保存在 managedObjectContext 中，但我很难让 NSFetchedResultsController 工作。我看了一下 this post ，但这并没有让我越过终点线。

从 JSON 导入记录后，我使用以下代码验证 managedObjectContext 中是否存在对象:

func recordCount() -> Int {
    let context = (UIApplication.shared.delegate as! AppDelegate).persistentContainer.viewContext
    let fetchRequest: NSFetchRequest<NSFetchRequestResult> = NSFetchRequest(entityName: "MyEntity")
    let count = try! context.count(for: fetchRequest)
    return count
}

当我创建一个 fetchedResultsController 时，我遇到了麻烦。我的代码没有崩溃，但它没有返回 NSManagedObjects，尽管存在与我的搜索匹配的对象。

下面是我如何在 UIViewController 中创建我的 NSFetchedResultsController。

class MyViewController: UIViewController, UITableViewDelegate, UITableViewDataSource, NSFetchedResultsControllerDelegate {
    // This is set on a prior viewController before segue.
    // I've verified it's not nil
    var selectedEquipmentString: String?

    let managedObjectContext = (UIApplication.shared.delegate as! AppDelegate).persistentContainer.viewContext

    lazy var fetchedResultsController: NSFetchedResultsController<MyEntity> = {
        // I've tried altering the syntax of the fetchRequest
        // let fetchRequest: NSFetchRequest<MyEntity> = MyEntity.fetchRequest()
        let fetchRequest: NSFetchRequest<NSFetchRequestResult> = NSFetchRequest(entityName: "MyEntity")
        let sortDescriptor = NSSortDescriptor(key: "generalArea", ascending: true)
        fetchRequest.sortDescriptors = [sortDescriptor]
        fetchRequest.predicate = NSPredicate(format: "equipmentDescription == %@", self.selectedEquipmentString!)
        let frc: NSFetchedResultsController<MyEntity> = NSFetchedResultsController(fetchRequest: fetchRequest as! NSFetchRequest<MyEntity>, managedObjectContext: self.managedObjectContext, sectionNameKeyPath: "generalArea", cacheName: nil)
        frc.delegate = self
        return frc
    }()

    // MARK: - View Lifecycle Methods (abbreviated)
    override func viewDidLoad() {
        super.viewDidLoad()
        // I've tried moving this call to viewWillAppear and viewDidAppear without success
        fetchObjectsFromManagedObjectContext()
    }

    // MARK: - Core Data Methods (abbreviated)
    func fetchObjectsFromManagedObjectContext() {
        do {
            try fetchedResultsController.performFetch()
        } catch {
            print("error: \(error)")
            return
        }
        print ("There are \(fetchedResultsController.fetchedObjects!.count) returned from fetchObjectsFromManagedObjectContext")
    }
}

此代码不会崩溃，但不会从 fetchRequest 返回任何记录。我能够通过谓词中的拼写错误强制崩溃，但如果没有拼写错误，尽管对象与谓词匹配，但不会返回任何对象。

我欢迎任何关于我的错误的建议。我放心，我知道这将是一个非常愚蠢的疏忽。感谢您的阅读。

Answer 1

你的 NSFetchRequest 应该有一个类型 NSFetchRequest<MyEntity> , 但您指定 NSFetchRequest<NSFetchRequestResult> .尝试更改它，让我知道它是否有帮助