Codable Error - Expected to decode Dictionary<String, Any> but found an array instead

Question

I am trying to decode a JSON with the help of codable - https://pastebin.com/Xfjj2XiP

However I'm getting this error when I do so.

typeMismatch(Swift.Dictionary<Swift.String, Any>, Swift.DecodingError.Context(codingPath: [CodingKeys(stringValue: "txt_forecast", intValue: nil)], debugDescription: "Expected to decode Dictionary<String, Any> but found an array instead.", underlyingError: nil))

Here's the code I'm using:

struct container: Decodable {
    var days: [forecastDay]

    //Coding keys
    enum CodingKeys: String, CodingKey {
        case forecast = "forecast"
        case txt_forecast = "txt_forecast"
        case forecastday = "forecastday"
    }

    // Decoding
    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        let forecast = try container.nestedContainer(keyedBy: CodingKeys.self, forKey: .forecast)
        let txt_forecast = try forecast.nestedContainer(keyedBy: CodingKeys.self, forKey: .txt_forecast)
        let forecastdays = try txt_forecast.nestedContainer(keyedBy: CodingKeys.self, forKey: .forecastday)

        let forecastdaysData = try forecastdays.decode(String.self, forKey: .forecastday)

        days = try JSONDecoder().decode([forecastDay].self, from: forecastdaysData.data(using: .utf8)!)

        print(days)

    }
}

struct forecastDay: Decodable {
    var period: Int?
    var icon: String?
    var title: String?
    var fcttext: String?

    //Coding keys
    enum CodingKeys: String, CodingKey {
            case period = "period"
            case icon = "icon"
            case title = "title"
            case fcttext = "fcttext"
    }

    // Decoding
    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        period = try container.decode(Int.self, forKey: .period)

        icon = try container.decode(String.self, forKey: .icon)

        title = try container.decode(String.self, forKey: .title)

        fcttext = try container.decode(String.self, forKey: .fcttext)
    }
}

Answer 1

The variable names in forecastDay do not differ from the keys in the JSON and init(from decoder:) does nothing custom either, so the struct for a forecast day period can be simplified.

struct ForecastDayPeriod: Decodable {
    let period: Int
    let icon: String
    let title: String
    let fcttext: String
}

Now it's best to use an enum with key(s) for each level in the JSON. Also, init(from decoder:) should not create a new JSONDecoder.

struct ForecastDay: Decodable {

    let periods: [ForecastDayPeriod]

    enum CodingKeys: String, CodingKey {
        case forecast
    }

    enum ForecastKeys: String, CodingKey {
        case txtForecast = "txt_forecast"
    }

    enum TxtForecastKeys: String, CodingKey {
        case forecastday
    }

    init(from decoder: Decoder) throws {
        let values = try decoder.container(keyedBy: CodingKeys.self)
        let forecast = try values.nestedContainer(keyedBy: ForecastKeys.self,
                                                  forKey: .forecast)
        let txtForecast = try forecast.nestedContainer(keyedBy: TxtForecastKeys.self,
                                                       forKey: .txtForecast)

        periods = try txtForecast.decode([ForecastDayPeriod].self,
                                         forKey: .forecastday)        
    }

}

Now it should be possible to decode the JSON from the pastebin example.

do {
    let jsonData: Data = ...
    let forecastDay = try JSONDecoder().decode(ForecastDay.self, from: jsonData)
} catch {
    print("Error: \(error)")
}