非转义参数的关闭使用可能允许它转义 [英] Closure use of non-escaping parameter may allow it to escape

问题描述

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?
}

使用示例实现:

    /// 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)
            }
        }
    }

上面的代码在 Swift3 (Xcode8-beta5) 中编译和运行，但不再适用于 beta 6.你能指出根本原因吗?

The code above compiled and worked in Swift3 (Xcode8-beta5) but does not work with beta 6 anymore. Can you point me to the underlying cause?

推荐答案

这是由于函数类型参数的默认行为发生了变化.在 Swift 3 之前(特别是 Xcode 8 beta 6 附带的构建)，它们将默认为转义 - 您必须标记它们 @noescape 以防止它们被存储或捕获，这保证它们不会超过函数调用的持续时间.

This is due to a change in the default behaviour for parameters of function type. Prior to Swift 3 (specifically the build that ships with Xcode 8 beta 6), they would default to being escaping – you would have to mark them @noescape in order to prevent them from being stored or captured, which guarantees they won't outlive the duration of the function call.

然而，现在 @noescape 是函数类型参数的默认值.如果要存储或捕获此类函数，现在需要将它们标记为 @escaping:

However, now @noescape is the default for function-typed parameters. If you want to store or capture such functions, you now need to mark them @escaping:

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

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

请参阅 Swift Evolution 提案 有关此更改的更多信息.

See the Swift Evolution proposal for more info about this change.

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