swift - 无法推断通用参数“实体”

我不确定为什么Swift无法将我显式传递的类型作为参数进行推断。

在下面的提取管理器上，尝试加载“实体”类型的对象时出现错误。


  不能推断通用参数“实体”

class EventsViewController: UIViewController {
  private let fetchManager = FetchManager()
  override func viewDidLoad() {
    super.viewDidLoad()
    fetchManager.willPerformQuery = {}
    // generic parameter 'Entity' could not be inferred
    fetchManager.loadObjects(ofType: Event) { (outcome) in
      switch outcome {
      case .success(let events):
        print("Entities: \(events.count)")
      case .failure(let code):
        switch code {
        case .connectionUnavailable:
          print("1")
        case .notAuthenticated:
          print("2")
        case .retry:
          print("3")
        case .serviceUnavailable:
          print("4")
        case .unknown:
          print("5")
        }
      }
    }
  }
}

class FetchManager {
  private let database: ReadableDatabase = CloudKitDatabase()
  var willPerformQuery: () -> Void = {}
  func loadObjects<Entity: Queryable & CloudKitInitable>(ofType type: Entity, withCompletion completion: @escaping (FetchOutcome<Entity>) -> Void) {
    database.loadObjects(matching: Entity.query, willPerformQuery: { [weak self] (_) in
      self?.willPerformQuery()
    }, didPerformQuery: { (outcome) in
      switch outcome {
      case .success(let files):
        let entities: [Entity] = files.compactMap(Entity.init)
        completion(FetchOutcome<Entity>.success(entities))
      case .failure(let code):
        completion(FetchOutcome<Entity>.failure(code))
      }
    })
  }
}

最佳答案

引用类型本身（以访问其“元类型”）的语法为TypeName.self。在这种情况下，您将使用Event.self：

fetchManager.loadObjects(ofType: Event.self) { (outcome) in
                                      ^~~~~

方法签名也应该用Entity.Type更新，以便编译器可以告诉它想要类型information而不是类型instance：

func loadObjects<Entity: Queryable & CloudKitInitable>(ofType type: Entity.Type, ...) {
...
}

关于swift - 无法推断通用参数“实体”，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/49854186/