Question

有人知道我怎么能解决这个错误吗？在以下代码块的第一行收到错误：

let fetchedResultsController: NSFetchedResultsController = {
    let fetchRequest = NSFetchRequest<NSFetchRequestResult>(entityName: "Message")
    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
}()

Answer 1

尝试在闭包中添加返回类型，如下代码：

let fetchedResultsController: NSFetchedResultsController = { () -> NSFetchedResultsController<NSFetchRequestResult> in
    let fetchRequest = NSFetchRequest<NSFetchRequestResult>(entityName: "Message")
    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
}()

Answer 2

错误信息有点误导，问题是你做错了不指定变量的通用占位符类型。

您可以向闭包添加显式返回类型，如@Mukesh 建议，在这种情况下，变量的类型注释不是需要：

let fetchedResultsController = { () -> NSFetchedResultsController<NSFetchRequestResult> in
    // ...
    return frc
}()

或者完全指定变量的类型，然后关闭返回类型是自动推断的：

let fetchedResultsController: NSFetchedResultsController<NSFetchRequestResult> = {
    // ...
    return frc
}()

Answer 3

两个问题：

如其他答案中所述，您必须指定泛型类型，在您的情况下，理想类型是具体的NSManagedObject子类。

声明NSFetchedResultsController至少需要一个排序描述符。

let fetchedResultsController: NSFetchedResultsController<Message> = {
    let fetchRequest = NSFetchRequest<Message>(entityName: "Message")
    fetchRequest.sortDescriptors = [NSSortDescriptor(key: "somekey", ascending: true]
    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
}()

无法推断出复杂的闭包返回类型;添加显式类型以消除歧义

3 个答案: