swift - 获取请求时返回错误？

Question

我正在尝试弄清楚如何返回 do/catch 语句的错误部分，因为它不在我没有完成发送到的函数中，并且它只想接受 NSFetchedResultsController ？

这是代码

    fileprivate lazy var fetchedExercisesTodayController: NSFetchedResultsController<UserExercise> = {

    let fetchRequest: NSFetchRequest<UserExercise> = UserExercise.fetchRequest()
    fetchRequest.predicate = NSPredicate(format: "usersroutine == %@", self.routineName)

    do {
        let appDelegate = UIApplication.shared.delegate as! AppDelegate
        let context = appDelegate.persistentContainer.viewContext
        let queryResults = try context.fetch(fetchRequest)

        fetchRequest.sortDescriptors = [NSSortDescriptor(key: "id", ascending: true)]
        let fetchedResultsController = NSFetchedResultsController(fetchRequest: fetchRequest, managedObjectContext: self.persistentContainer.viewContext, sectionNameKeyPath: nil, cacheName: nil)
        fetchedResultsController.delegate = self
        return fetchedResultsController

    } catch {
        print("Error with request: \(error)")
        return error
    }
}()

错误出现在“返回错误”行

Cannot convert return expression of type 'Error' to return type 'NSFetchedResultsController'

Answer 1

如果它不会失败，则根本不需要 try/catch，因此只需将其删除即可。

fileprivate lazy var fetchedExercisesTodayController: NSFetchedResultsController<UserExercise> = {

    let fetchRequest: NSFetchRequest<UserExercise> = UserExercise.fetchRequest()
    fetchRequest.predicate = NSPredicate(format: "usersroutine == %@", self.routineName)

    let appDelegate = UIApplication.shared.delegate as! AppDelegate
    let context = appDelegate.persistentContainer.viewContext
    let queryResults = context.fetch(fetchRequest)

    fetchRequest.sortDescriptors = [NSSortDescriptor(key: "id", ascending: true)]
    let fetchedResultsController = NSFetchedResultsController(fetchRequest: fetchRequest, managedObjectContext: self.persistentContainer.viewContext, sectionNameKeyPath: nil, cacheName: nil)
    fetchedResultsController.delegate = self
    return fetchedResultsController
}() 

但是，如果您想防止失败(这可能在这里发生)，您可以设置 fetchedExcercisesTodayController一个可选变量。您必须将其设置为可选，因为如果不是，您需要返回 NSFetchedResultsController<UserExercise> 类型的对象无论。错误不是对象类型 NSFetchedResultsController<UserExercise> 。它是对象类型Error 。

如果您将其设置为可选，则在 catch 中您可以返回 nil 并执行以下三件事之一:

• 调用另一个方法，在尝试失败时显示/执行某些操作
• 抛出一个错误，您可以在此之外的地方进行处理
• 让它为您打印一条错误消息

这样做的唯一问题是，如果您已经初始化并使用此变量，则必须开始解包可选的 fetchedExercisesTodayController您使用过它的每个地方。本质上，这两件事都会起作用，您只需决定什么最适合您的代码库。

//changed to NSFetchedResultsController<UserExercise>?
fileprivate lazy var fetchedExercisesTodayController: NSFetchedResultsController<UserExercise>? = {

  let fetchRequest: NSFetchRequest<UserExercise> = UserExercise.fetchRequest()
  fetchRequest.predicate = NSPredicate(format: "usersroutine == %@", self.routineName)

  do {
    let appDelegate = UIApplication.shared.delegate as! AppDelegate
    let context = appDelegate.persistentContainer.viewContext
    let queryResults = try context.fetch(fetchRequest)

    fetchRequest.sortDescriptors = [NSSortDescriptor(key: "id", ascending: true)]
    let fetchedResultsController = NSFetchedResultsController(fetchRequest: fetchRequest, managedObjectContext: self.persistentContainer.viewContext, sectionNameKeyPath: nil, cacheName: nil)
    fetchedResultsController.delegate = self
    return fetchedResultsController

  } catch {
    print("Error with request: \(error)")
    //handle error in some way here
    return nil
  }
}()