“(Class) -> () -> (Class)”类型的值没有成员 'variable'

Question

let fetchedResultsController: NSFetchedResultsController = { () -> NSFetchedResultsController<NSFetchRequestResult> in
    let fetchRequest = NSFetchRequest<NSFetchRequestResult>(entityName: "Message")
    fetchRequest.sortDescriptors = [NSSortDescriptor(key: "date", ascending: true)]
    fetchRequest.predicate = NSPredicate(format: "friend.name = %@", self.friend!.name!)
    let delegate = UIApplication.shared.delegate as! AppDelegate
    let context = delegate.persistentContainer.viewContext
    let frc = NSFetchedResultsController(fetchRequest: fetchRequest, managedObjectContext: context, sectionNameKeyPath: nil, cacheName: nil)
    return frc
}()

在显示以下内容的行中：fetchRequest.predicate = NSPredicate(format: "friend.name = %@", self.friend!.name!) 我收到错误：Value of type '(ChatLogController) -> () -> (ChatLogController)' has no member 'friend' 所以显然它无法在我的 class ChatLogController 中找到 friend某种原因，还是我错了？这就是我的猜测，因为它找不到 friend 作为 self 的成员。

这里是 friend 变量，它在 ChatLogController 里面 class:

var friend: Friend?
{
    didSet
    {
        navigationItem.title = friend?.name

        messages = friend?.messages?.allObjects as? [Message]

        messages = messages?.sorted(by: {[=14=].date!.compare(.date! as Date) == .orderedAscending})
    }
}

有人知道我为什么会收到这个错误以及如何解决它吗？

编辑： 我注意到，如果我从第一个代码片段的第一行删除 () -> NSFetchedResultsController<NSFetchRequestResult> in，错误就会消失，但是我会收到此错误：Unable to infer complex closure return type; add explicit type to disambiguate

Answer 1

您已将 friend 声明为 optional 属性，它在 fetchedResultsController 的 initialisation 时尚未初始化，它是一个常量，因此它给出了错误。要解决此问题，您可以使用 lazy initialisation，这意味着

delaying the creation of an object or some other expensive process until it’s needed

使 fetchedResultsController 属性延迟，这将延迟对象的创建，直到它需要

lazy var fetchedResultsController: NSFetchedResultsController = { [unowned self] () -> NSFetchedResultsController<NSFetchRequestResult> in
    let fetchRequest = NSFetchRequest<NSFetchRequestResult>(entityName: "Message")
    fetchRequest.sortDescriptors = [NSSortDescriptor(key: "date", ascending: true)]
    fetchRequest.predicate = NSPredicate(format: "friend.name = %@", self.friend!.name!)
    let delegate = UIApplication.shared.delegate as! AppDelegate
    let context = delegate.persistentContainer.viewContext
    let frc = NSFetchedResultsController(fetchRequest: fetchRequest, managedObjectContext: context, sectionNameKeyPath: nil, cacheName: nil)
    return frc
}()

“(Class) -> () -> (Class)”类型的值没有成员 'variable'

Value of type '(Class) -> () -> (Class)' has no member 'variable'

xcode

compiler-errors

nsfetchedresultscontroller

ios

swift