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
}()

在说明的行fetchRequest.predicate = NSPredicate(format: "friend.name = %@", self.friend!.name!)我收到错误：Value of type '(ChatLogController) -> () -> (ChatLogController)' has no member 'friend'显然，由于某种原因，我的班级friend中找不到ChatLogController，或者我是错误？这就是我猜的，因为它不会friend成为self的成员。

以下是friend变量，它位于ChatLogController类：

之内

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

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

        messages = messages?.sorted(by: {$0.date!.compare($1.date! as Date) == .orderedAscending})
    }
}

任何人都知道我为什么会收到此错误以及如何解决？

修改：我注意到如果我从第一个代码段的第一行删除() -> NSFetchedResultsController<NSFetchRequestResult> in，则错误消失，但是我收到此错误：Unable to infer complex closure return type; add explicit type to disambiguate < / p>

Answer 1

您已声明friend为optional属性，该属性在fetchedResultsController的initialisation时尚未初始化，这是一个常数，因此它会给出错误。要解决此问题，您可以使用lazy initialisation，这意味着

延迟创建对象或其他一些昂贵的进程直到它需要

使fetchedResultsController属性变为惰性，这将延迟对象的创建，直到其所需的

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
}()

类型的值'（类） - ＆gt; （） - ＆gt; （类）'没有成员'变量'

1 个答案: