使用通用参数创建 async/await 函数

Question

我正在尝试将我的工作完成处理函数转换为新的 async/await 语法，但我在为使用通用参数 (T) 的函数找到正确语法时遇到问题：

我之前用过的和有效的：

typealias CurrentWeatherCompletionHandler = (CurrentWeather?, Error?) -> Void

private func weatherDownload<T: Codable>(weatherType: String, completionHandler completion:  @escaping (_ object: T?,_ error: Error?) -> ()) {
    let storage = Storage.storage()
    
    let pathReference = storage.reference(withPath: "\(weatherType)"+"-de.json")
    
    pathReference.getData(maxSize: 1 * 1024 * 1024) { data, error in
        
        if let error = error {
            print("Error beim \(weatherType) Download: \(error)")
            completion(nil, error)
        } else {
            do {
                let weather = try self.decoder.decode(T.self, from: data!)
                completion(weather, nil)
            } catch let error {
                completion(nil, error)
            }
        }
    }
}

func getCurrentWeather(completionHandler completion: @escaping CurrentWeatherCompletionHandler) {
    weatherDownload(weatherType: Constants.currentWeatherFolder) { (weather: CurrentWeather?, error) in
        completion(weather, error)
    }
}

我试过但没有奏效的方法：

private func weatherDownload<T: Codable>(weatherType: String) async -> (weather: T?, error: Error?) {
   //same network code as before but with return instead of completion:
   return (weather, nil) // Compiler Error: 'nil' requires a contextual type / Unexpected non-void return value in void function
}

func getCurrentWeather() async -> (weather: CurrentWeather?, error: Error?) {
    return await weatherDownload(weatherType: Constans.currentWeatherFolder)
}

欢迎任何想法。

Answer 1

本质上，如果您使用的 SDK 不支持并发 API，您可以使用它的唯一方法是调用 withCheckedContinuation 或 withCheckedThrowingContinuation（用于错误处理).您的代码的问题是我提到的问题，和/或 Swift 没有正确地从 return 类型推断出 T 类型的事实。
希望以下模式对您有用：

private func weatherDownload<T: Codable>(_ type: T.Type, weatherType: String) async throws -> T {
    return try await withCheckedThrowingContinuation { continuation in 
        // Your original code from before, 
        // with the following difference instead of calling the completion handler:
        ...
        do {
            let weather = try self.decoder.decode(T.self, from: data)
            continuation.resume(returning: weather)
        } catch {
            continuation.resume(throwing: error)
        }
    }
}

有关此模式的更多信息，请查看以下链接：link1, link2

使用通用参数创建 async/await 函数

Create async/await function with Generic Parameter

firebase

swift

firebase-storage