无法推断复杂闭包 return 类型；添加显式类型以消除歧义

Question

有人知道如何解决我遇到的这个错误吗？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 return 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，问题是你做了未指定变量的通用占位符类型。

您可以向闭包添加显式 return 类型，如@Mukesh 建议，在这种情况下，变量上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，然后闭包return 自动推断类型：

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。

NSFetchedResultsController 的声明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
}()

无法推断复杂闭包 return 类型；添加显式类型以消除歧义

Unable to infer complex closure return type; add explicit type to disambiguate

ios

swift

nsfetchedresultscontroller

type-inference