无法将“NSNull”类型的值转换为“NSString”，然后应用程序崩溃

Question

我们正在尝试创建一个从 API 获取 JSON 的函数。我们知道这给了我们 NIL，但我们不知道为什么会发生错误。我们得到的确切错误消息是

[] 2020-08-01 16:29:26.501199-0400 HEFT[97766:2952325] [] nw_proxy_resolver_create_parsed_array [C1 代理 pac] 评估错误：NSURLErrorDomain：-1003 无法将“NSNull”（0x7fff87a92380）类型的值转换为“NSString”（0x7fff87b502e8）。 2020-08-01 16:29:26.670549-0400 HEFT [97766:2952139] 无法将类型“NSNull”（0x7fff87a92380）的值转换为“NSString”（0x7fff87b502e8）。 (lldb)

我们尝试过修改代码以找到解决方案，并尝试使用其他一些问题，但它们都与我们想要实现的目标无关。

func getJson() {
        if let url = URL(string: "https://api.weather.gov/alerts/active?area=GA") {
            URLSession.shared.dataTask(with: url) { (data:Data?, response:URLResponse?, error:Error?) in
                if error == nil {
                    if data != nil {
                        if let json = try? JSONSerialization.jsonObject(with: data!, options: []) as? [String:AnyObject] {
                            DispatchQueue.main.async {
                                
                                //if let rawfeatures = json["features"] {
                                var rawfeatures = json["features"] as! [Dictionary< String, AnyObject>]
                                var keepgoingfeatures = rawfeatures.count
                                var FeatureIndex = 0
                                while keepgoingfeatures != 0{
                                    let currentRawFeature = rawfeatures[FeatureIndex]
                                    let currentRawFeatureProperties = currentRawFeature["properties"]
                                    let currentFeature = Feature()
                                    currentFeature.event = currentRawFeatureProperties!["event"] as! String
                                    currentFeature.description = currentRawFeatureProperties!["description"] as! String
                                    currentFeature.instructions = currentRawFeatureProperties!["instruction"] as! String
                                    currentFeature.urgency = currentRawFeatureProperties!["urgency"] as! String
                                    keepgoingfeatures -= 1
                                    FeatureIndex += 1
                                }
                            }
                        }
                    }
                    
                    
                } else {
                    print("We have an error")
                }
            }.resume()
        }
    }

Answer 1

其中一些警报具有null 对应instructions。我建议定义您的对象以承认该字段是可选的，即它可能不存在。例如

struct Feature {
    let event: String
    let description: String
    let instruction: String?
    let urgency: String
}

而且，在解析它时，我可能会建议摆脱所有那些强制展开运算符，例如

enum NetworkError: Error {
    case unknownError(Data?, URLResponse?)
    case invalidURL
}

@discardableResult
func getWeather(area: String, completion: @escaping (Result<[Feature], Error>) -> Void) -> URLSessionTask? {
    // prepare request

    var components = URLComponents(string: "https://api.weather.gov/alerts/active")!
    components.queryItems = [URLQueryItem(name: "area", value: area)]
    var request = URLRequest(url: components.url!)
    request.setValue("(\(domain), \(email))", forHTTPHeaderField: "User-Agent")

    // perform request

    let task = URLSession.shared.dataTask(with: request) { data, response, error in
        guard
            error == nil,
            let httpResponse = response as? HTTPURLResponse,
            200 ..< 300 ~= httpResponse.statusCode,
            let responseData = data,
            let responseDictionary = try? JSONSerialization.jsonObject(with: responseData) as? [String: Any],
            let rawFeatures = responseDictionary["features"] as? [[String: Any]]
        else {
            DispatchQueue.main.async {
                completion(.failure(error ?? NetworkError.unknownError(data, response)))
            }
            return
        }

        let features = rawFeatures.compactMap { feature -> Feature? in
            guard
                let properties = feature["properties"] as? [String: Any],
                let event = properties["event"] as? String,
                let description = properties["description"] as? String,
                let urgency = properties["urgency"] as? String
            else {
                print("required string absent!")
                return nil
            }
            let instruction = properties["instruction"] as? String

            return Feature(event: event, description: description, instruction: instruction, urgency: urgency)
        }

        DispatchQueue.main.async {
            completion(.success(features))
        }
    }

    task.resume()

    return task
}

其他一些观察：

1. 我已删除所有强制转换（as!）。如果服务器出现问题，您不希望您的应用程序崩溃。例如，我经常收到 503 错误。如果服务器暂时不可用，您不希望崩溃。

2. The docs 说你应该设置User-Agent，所以我在上面这样做。显然，相应地设置domain 和email 字符串常量。

3. 虽然您可以手动构建 URL，但使用URLComponents 是最安全的，因为它可以处理可能需要的任何百分比转义。这里不需要，但如果您开始处理更复杂的请求（例如，需要指定一个包含空格的城市名称，例如“Los Angeles”），这将是一个有用的模式。

4. 我建议使用上述完成处理程序模式，以便调用者可以知道请求何时完成。所以你可能会这样做：

    getWeather(area: "GA") { result in
        switch result {
        case .failure(let error):
            print(error)
            // update UI accordingly
   
        case .success(let features):
            self.features = features        // update your model object
            self.tableView.reloadData()     // update your UI (e.g. I'm assuming a table view, but do whatever is appropriate for your app
        }
    }
   

5. 我将返回 URLSessionTask 以防您可能想要取消请求（例如，用户关闭相关视图），但我已将其标记为 @discardableResult，因此您不必这样做如果您不想要，请使用它。

6. 我已经用 guard 语句替换了 if 语句的塔。它使代码更易于遵循，并采用“提前退出”模式，您可以更轻松地将退出代码与失败（如果有）联系起来。

---

就个人而言，我建议您更进一步，不要手动解析 JSONSerialization 结果。让JSONDecoder 为您完成所有这些工作要容易得多。例如：

struct ResponseObject: Decodable {
    let features: [Feature]
}

struct Feature: Decodable {
    let properties: FeatureProperties
}

struct FeatureProperties: Decodable {
    let event: String?
    let description: String
    let instruction: String?
    let urgency: String
}

enum NetworkError: Error {
    case unknownError(Data?, URLResponse?)
    case invalidURL
}

@discardableResult
func getWeather(area: String, completion: @escaping (Result<[FeatureProperties], Error>) -> Void) -> URLSessionTask? {
    var components = URLComponents(string: "https://api.weather.gov/alerts/active")!
    components.queryItems = [URLQueryItem(name: "area", value: area)]
    var request = URLRequest(url: components.url!)
    request.setValue("(\(domain), \(email))", forHTTPHeaderField: "User-Agent")

    let task = URLSession.shared.dataTask(with: request) { data, response, error in
        guard
            error == nil,
            let httpResponse = response as? HTTPURLResponse,
            200 ..< 300 ~= httpResponse.statusCode,
            let responseData = data
        else {
            DispatchQueue.main.async {
                completion(.failure(error ?? NetworkError.unknownError(data, response)))
            }
            return
        }

        do {
            let responseObject = try JSONDecoder().decode(ResponseObject.self, from: responseData)
            DispatchQueue.main.async {
                completion(.success(responseObject.features.map { $0.properties }))
            }
        } catch let parseError {
            DispatchQueue.main.async {
                completion(.failure(parseError))
            }
        }
    }

    task.resume()

    return task
}

Answer 2

简短的回答是因为你强制转换所有内容并假设 json 没有的非常特定的格式。

所以在某些时候你会读到一个刚刚插入的值。 具体说明。

作为一个工作/非崩溃修复（我在本地运行！）：

let currentFeature = Feature()
currentFeature.event = currentRawFeatureProperties!["event"] as? String ?? ""
currentFeature.description = currentRawFeatureProperties!["description"] as? String ?? ""
currentFeature.instructions = currentRawFeatureProperties!["instruction"]  as? String ?? ""
currentFeature.urgency = currentRawFeatureProperties!["urgency"] as? String ?? ""

我敦促您广泛地重构您的功能