从dataTask completeHandler引发错误

Question

如何迅速在这样的完成处理程序中引发错误：

    let task = URLSession.shared.dataTask(with: request as URLRequest, completionHandler: {
        (data, response, error) in
        do {
            //something
            completion(result)
        } catch let jsonError {
            throw CustomError.myerror //THIS DOESN'T WORK
        }
    })
    task.resume()

因为错误是

从类型为（（_，_，_）的throwing函数将->（）无效转换为非抛出函数类型为'（Data ?, URLResponse ?, Error？）-> Void'

Answer 1

简单地说：你不能throw在dataTask完成关闭

您可以在完成处理程序中返回两个值

...completion: @escaping (ResultType?, Error?)->Void

然后返回

completion(result, nil)
completion(nil, CustomError.myerror)

或更方便地使用具有关联类型的枚举

enum Result {
    case success(ResultType), failure(Error)
}

...completion: @escaping (Result)->Void

然后返回

completion(.success(result))
completion(.failure(CustomError.myerror))

您可以处理结果

foo() { result in
    switch result {
    case .success(let resultType): // do something with the result
    case .failure(let error): // Handle the error
    }
}

更新：

在使用新的内置Result类型的Swift 5中，它更加舒适，因为它Result可以捕获throwing表达式的结果

...completion: @escaping (Result<MyType,Error>)->Void

let task = URLSession.shared.dataTask(with: request as URLRequest, completionHandler: {
    (data, response, error) in

    completion(Result { try something()})
})
task.resume()