try-catch - 在 Swift 2 中，如何将 JSON 解析错误返回到完成 block ？

Question

我想在 Swift 2 中创建一个函数，该函数从 URL 获取数据并使用 NSURLSession 将其作为 JSON 对象返回。起初，这似乎很简单。我写了以下内容:

func getJson(url:NSURL, completeWith: (AnyObject?,NSURLResponse?,NSError?)->Void) -> NSURLSessionTask? {

    let session = NSURLSession.sharedSession()
    let task = session.dataTaskWithURL(url) {
        (data:NSData?, response:NSURLResponse?, error:NSError?) -> Void in

        if error != nil {
            completeWith(nil, response, error)
        }

        if let data = data {

            do {
                let object:AnyObject? = try NSJSONSerialization.JSONObjectWithData(data, options: .AllowFragments)
            } catch let caught as NSError {
                completeWith(nil, response, caught)
            }

            completeWith(object, response, nil)

        } else {
            completeWith(nil, response, error)
        }
    }

    return task
}

但是，这不会编译，因为完成块没有声明“throws”。确切的错误是 Cannot invoke 'dataTaskWithURL' with an argument list of type '(NSURL, (NSData?, NSURLResponse?, NSError?) throws -> Void)' .即使我在 do/catch 中发现了所有错误声明，Swift 仍然希望将 NSError 向上传播。我能看到它周围的唯一方法是使用 try! ， 像这样:

if let data = data {

    let object:AnyObject? = try! NSJSONSerialization.JSONObjectWithData(data, options: .AllowFragments)
    completeWith(object, response, nil)

} else {
    completeWith(nil, response, error)
}

现在一切都编译得很好，但是我丢失了 NSJSONSerialization.JSONObjectWithData 抛出的 NSError .

我是否可以捕获 NSJSONSerialization.JSONObjectWithData 可能引发的 NSError并将其传播到完成块而不修改完成块的签名？

Answer 1

我认为，您的捕获并不详尽，因此您需要这样的东西:

do
{
  let object:AnyObject? = try NSJSONSerialization.JSONObjectWithData(data, options: .AllowFragments)
  completeWith(object, response, nil)
} catch let caught as NSError {
  completeWith(nil, response, caught)
} catch {
  // Something else happened.
  // Insert your domain, code, etc. when constructing the error.
  let error: NSError = NSError(domain: "<Your domain>", code: 1, userInfo: nil)
  completeWith(nil, nil, error)
}