封闭使用非转义参数可能会使它转义

Question

我有一个协议：

enum DataFetchResult {
    case success(data: Data)
    case failure
}

protocol DataServiceType {
    func fetchData(location: String, completion: (DataFetchResult) -> (Void))
    func cachedData(location: String) -> Data?
}

通过示例实现：

    /// An implementation of DataServiceType protocol returning predefined results using arbitrary queue for asynchronyous mechanisms.
    /// Dedicated to be used in various tests (Unit Tests).
    class DataMockService: DataServiceType {

        var result      : DataFetchResult
        var async       : Bool = true
        var queue       : DispatchQueue = DispatchQueue.global(qos: .background)
        var cachedData  : Data? = nil

        init(result : DataFetchResult) {
            self.result = result
        }

        func cachedData(location: String) -> Data? {
            switch self.result {
            case .success(let data):
                return data
            default:
                return nil
            }
        }

        func fetchData(location: String, completion: (DataFetchResult) -> (Void)) {

            // Returning result on arbitrary queue should be tested,
            // so we can check if client can work with any (even worse) implementation:

            if async == true {
                queue.async { [weak self ] in
                    guard let weakSelf = self else { return }

                    // This line produces compiler error: 
                    // "Closure use of non-escaping parameter 'completion' may allow it to escape"
                    completion(weakSelf.result)
                }
            } else {
               completion(self.result)
            }
        }
    }

上面的代码已在Swift3（Xcode8-beta5）中编译并工作，但不再与beta 6一起工作。您能指出我的根本原因吗？

Answer 1

这是由于功能类型参数的默认行为发生了变化。在Swift 3（特别是Xcode 8 beta 6附带的内部版本）之前，它们将默认转义-
您必须对其进行标记@noescape，以防止它们被存储或捕获，从而确保它们不会超过使用寿命函数调用。

但是，现在@noescape是函数类型参数的默认设置。如果要存储或捕获此类功能，则现在需要对其进行标记@escaping：

protocol DataServiceType {
  func fetchData(location: String, completion: **@escaping** (DataFetchResult) -> Void)
  func cachedData(location: String) -> Data?
}

func fetchData(location: String, completion: **@escaping** (DataFetchResult) -> Void) {
  // ...
}

有关此更改的更多信息，请参见Swift Evolution提案。