ios - 尽管表不为空，核心数据获取仍返回零记录

Question

我是 Swift 新手，正在构建一个 Core Data 应用程序。我已经构建并运行了 sample app from Ray Wenderlich's site其按预期工作。

我从 Ray 的应用程序中提取并修改了以下摘录:

var people: [NSManagedObject] = []

override func viewWillAppear(_ animated: Bool) {
    super.viewWillAppear(animated)

    //1
    guard let appDelegate = UIApplication.shared.delegate as? AppDelegate else {
        return
    }

    let managedContext = appDelegate.persistentContainer.viewContext

    // 2
    let fetchrequest = NSFetchRequest<NSManagedObject>(entityName: "Person")

    // 3
    do {
        people = try managedContext.fetch(fetchrequest)
        print("\(people.count) fetched from Person")
    }
    catch let error as NSError {
        print("Could not fetch. \(error), \(error.userInfo)")
    }
}

这按预期工作。但是，以下是我在我的应用程序中实现的内容:

var players: [NSManagedObject] = []

override func viewWillAppear(_ animated: Bool) {
    super.viewWillAppear(animated)

    guard let appDelegate = UIApplication.shared.delegate as? AppDelegate else {
        return
    }

    let managedContext = appDelegate.persistentContainer.viewContext

    let fetchRequest = NSFetchRequest<NSManagedObject>(entityName: "Golfer")

    do {
        players = try managedContext.fetch(fetchRequest)

        print("\(players.count) fetched from Golfer")
        // Load any saved players, otherwise load sample data.
        if players.count == 0 {
            // Load the sample data.
            // There are no players to load
            os_log("Loading sample players", log: OSLog.default, type: .debug)
            loadSamplePlayers()
        }
    }
    catch let error as NSError {
        print("Could not fetch Golfers. \(error), \(error.userInfo)")
    }
}

示例代码始终返回 Person 表的完整结果，并且 person 数组计数始终等于表中的行数。

但是，对于我的版本，它总是报告players数组的计数为零。因此，每次都会调用 loadSamplePlayers() 方法(将三个样本记录添加到 Golfer 表中)。

使用 SQL Lite 数据库检查器，我可以看到 Golfer 表在每次调用应用程序后继续增长。

我很困惑为什么这两个几乎相同的代码示例应该表现不同。

任何见解将不胜感激。

谢谢!

编辑:

这是还引用的 loadSamplePlayers() 方法和 printDatabaseStats():

private func loadSamplePlayers() {

    let sampleFirstNames: [String] = ["Casper", "Jocelyn", "Arthur"]
    let sampleLastNames: [String] = ["Vitturio", "Brown", "Mullard"]
    let sampleEmails: [String] = ["a@b.com", "fred@x.co.uk", "last@ohm.com"]
    let sampleHandicaps: [Double] = [16.2, 6.4, 27.2]

    let entity = NSEntityDescription.entity(forEntityName: "Golfer", in: managedObjectContext!)!

    for i in 1...3 {
        let player = NSManagedObject(entity: entity, insertInto: managedObjectContext)

        player.setValue(sampleFirstNames[i-1], forKeyPath: "firstName")
        player.setValue(sampleLastNames[i-1], forKey: "lastName")
        player.setValue(sampleEmails[i-1], forKey: "emailAddress")
        player.setValue(sampleHandicaps[i-1], forKey: "exactHandicap")
        // player.setValue(sampleMugshots[i-1], forKey: "mugShot")

        print("Prepare to save")

        do {
            try managedObjectContext?.save()
            players.append(player)
        }
        catch let error as NSError {
            print("Could not save. \(error), \(error.userInfo)")
        }

        print("There are now \(printDatabaseStats()) in the Golfers table")
    }
}

private func printDatabaseStats() -> Int {
    var count = 0
    managedObjectContext?.performAndWait {
        if let golferCount = try? self.managedObjectContext!.count(for: NSFetchRequest(entityName: "Golfer")) {
            print("\(golferCount) Payers")
            count = golferCount
        }
    }

    return count
}

这似乎工作成功，因为 print(...) 方法生成预期的控制台消息，并且 Golfer 表已填充并包含定义的行。

Answer 1

原始问题中未包含类定义，因此:

var managedObjectContext: NSManagedObjectContext? = (UIApplication.shared.delegate as? AppDelegate)?.managedObjectContext

这与以下定义不同:

let managedContext = appDelegate.persistentContainer.viewContext

但我使用它们就好像它们是一样的。我将两者标准化，因此它们是相同的，问题就消失了。