json - 解码 Json 不工作，也不报错

Question

我正在尝试解码以下 json:

    [
        {
            "breeds":[
            {
            "weight":{
            "imperial":"65 - 75",
            "metric":"29 - 34"
            },
            "height":{
            "imperial":"21 - 28",
            "metric":"53 - 71"
            },
            "id":14,
            "name":"American Foxhound",
            "country_code":"US",
            "bred_for":"Fox hunting, scent hound",
            "breed_group":"Hound",
            "life_span":"8 - 15 years",
            "temperament":"Kind, Sweet-Tempered, Loyal, Independent, Intelligent, Loving"
            }
            ],
            "id":"p4xvDeEpW",
            "url":"https://cdn2.thedogapi.com/images/p4xvDeEpW.jpg",
            "width":680,
            "height":453
        }
    ]

我将我的模型设置为:

struct Request: Decodable {
    let response: [Response]
}

struct Response: Decodable {
    let breeds: [Dog]
    let url: String
}

struct Dog: Decodable {
    let weight: Measurement
    let height: Measurement
    let name: String
    let countryCode: String?
    let breedGroup: String
    let lifeSpan: String
    let temperament: String
    let origin: String?
}

struct Measurement: Decodable {
    let imperial: String
    let metric: String
}

我调用 API 如下:

func fetchDogs(with request: URLRequest, completion: @escaping (([Dog]?, String?, Error?) -> ())) {
    URLSession.shared.dataTask(with: request) { (data, response, error) in
        let decoder = JSONDecoder()
        decoder.keyDecodingStrategy = .convertFromSnakeCase

        guard let data = data,
            let request = try? decoder.decode(Request.self, from: data) else {
                completion(nil, nil, error)
                return
        }

        completion(request.response[0].breeds, request.response[0].url, nil)
        }.resume()
}

但它总是命中守卫 completion(nil, nil, error) 行，我不确定为什么我不确定如何将 JSON 的前几行分开以开始关闭。使用:

struct Request: Decodable {
    let response: [Response]
}

好像不太对。如果我更新请求以解码 Response 结构，它仍然会触发 guard 。

我的请求生成器如下所示:

struct RequestBuilder {
    private let baseURLString: String

    static var defaultBuilder: RequestBuilder {
        return RequestBuilder(
            baseURLString: Constant.URL.dogsBase
        )
    }

    func dogsRequest() -> URLRequest {
        return dogRequest(path: Constant.Path.search)
    }

    private func dogRequest(path: String) -> URLRequest {
        guard var components = URLComponents(string: baseURLString) else {
            preconditionFailure(Constant.PreconditionFailure.baseURL)
        }

        components.path = path

        guard let url = components.url else {
            preconditionFailure(Constant.PreconditionFailure.baseURL)
        }
        var request = URLRequest(url: url)
        request.setValue(Constant.Key.dogs, forHTTPHeaderField: "x-api-key")

        return request
    }
}

可以找到api here

Answer 1

问题是，当您将 Request.self 传递给 decode.decode 时，JSONDecoder 期望找到一个 Dictionary 的键是 response 因为你声明了 Request。

您只需将[Response].self 传递给decode 方法即可。

guard let data = data,
    let request = try? decoder.decode([Response].self, from: data) else {
        completion(nil, nil, error)
        return
}

如果您没有使用 try? 屏蔽解码错误，您会立即从错误消息中看到问题:

typeMismatch(Swift.Dictionary, Swift.DecodingError.Context(codingPath: [], debugDescription: "Expected to decode Dictionary but found an array instead.", underlyingError: nil))

当您不确定您的代码是否有效时，您应该始终在 do-catch block 中使用普通的 try 语句，因为这样您实际上可以查看错误消息。 JSONDecoder 抛出非常有用的错误，所以我建议您永远不要使用 try? 将它们屏蔽为 nil。