JSON请求并不总是返回相同的响应类型（对象和数组）[重复]

Question

TL;DR：一个 JSON 请求有时会返回一个对象，有时会返回一个对象数组作为键，我不知道如何使用 Swift 4 正确解析它。

前言：我正在使用NextBus API 制作一个巴士服务应用程序，以提供教堂山地区巴士的巴士时刻表更新和预测。但是，我在获取公交车站的预测信息时遇到了问题（NextBus API PDF 第 13-15 页上的预测请求信息）。

问题：

停止预测的请求会返回两个键：“predictions”和“copyright”。虽然“copyright”键总是为预测请求返回一个字符串，但“predictions”键有时会返回一个对象，有时会返回一个数组，具体取决于路线上是否有两辆公共汽车。

这是用 Postman 可视化的问题：

预测返回一个数组：

{
"predictions": [
    {
        "agencyTitle": "Chapel Hill Transit",
        "routeTag": "N",
        "routeTitle": "N",
        "stopTitle": "Estes Park Apts - Departure",
        "stopTag": "estepark",
        "dirTitleBecauseNoPredictions": "To Family Practice Building"
    },
    {
        "agencyTitle": "Chapel Hill Transit",
        "routeTag": "N",
        "routeTitle": "N",
        "stopTitle": "Estes Park Apts - Arrival",
        "stopTag": "estepark_a",
        "dirTitleBecauseNoPredictions": "To Family Practice Building"
    }
],
"copyright": "All data copyright Chapel Hill Transit 2018." 
}

预测返回一个对象：

{
"predictions": {
    "agencyTitle": "Chapel Hill Transit",
    "routeTag": "A",
    "routeTitle": "A",
    "stopTitle": "Martin Luther King Jr Blvd  at Timber Hollow",
    "stopTag": "airptimb_s",
    "dirTitleBecauseNoPredictions": "To Northside"
},
"copyright": "All data copyright Chapel Hill Transit 2018."
}

我正在使用 Swift 4 在 Xcode 9.4.1 中制作这个应用程序。这是我当前处理请求的代码：

func fetchStopPrediction(stopId: String, routeTag: String, completion: @escaping (Predictions) -> Void) {
    let routeInfo = "\(stopId)&routeTag=\(routeTag)"
    let urlString = baseUrl + routeInfo
    print(urlString)

    Alamofire.request(urlString, method: .get).responseJSON { (response) in
        if let jsonResponse = response.result.value {
            print("JSON: \(jsonResponse)")
        }

        if let data = response.data {
            do {
                let predictions = try self.decoder.decode(Predictions.self, from: data)
                completion(predictions)
            } catch let error {
                print("Error", error)
            }
        }
    }
}

struct Predictions: Codable {
    let predictions: [Prediction?]
    let copyright: String?
}

Answer 1

您必须编写一个自定义初始化程序。首先解码一个字典，如果它解码一个数组失败

struct Predictions : Decodable {
    let predictions: [Prediction]
    let copyright: String

    private enum CodingKeys: String, CodingKey { case predictions, copyright }

    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        copyright = try container.decode(String.self, forKey: .copyright)
        do {
            let prediction = try container.decode(Prediction.self, forKey: .predictions)
            predictions = [prediction]
        } catch DecodingError.typeMismatch {
            predictions = try container.decode([Prediction].self, forKey: .predictions)
        }
    }
}