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) } } }
上面的代码在 Swift3 (Xcode8-beta5) 中编译并运行,但不再适用于 beta 6。你能指出我的根本原因吗?
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 - 你必须标记它们@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。
@noescape
但是,now@noescape是函数类型参数的默认值。如果您想存储或捕获此类功能,您现在需要标记它们@escaping:
@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,请参阅Swift Evolution 提案。