ios - Swift - ViewController のオブジェクトの配列への AlamofireObjectMapper

Question

JSON データが与えられた場合:

{
    "location": "Toronto, Canada",    
    "three_day_forecast": [
        { 
            "conditions": "Partly cloudy",
            "day" : "Monday",
            "temperature": 20 
        },
        { 
            "conditions": "Showers",
            "day" : "Tuesday",
            "temperature": 22 
        },
        { 
            "conditions": "Sunny",
            "day" : "Wednesday",
            "temperature": 28 
        }
    ]
}

そして、次のクラス:

class WeatherResponse: Mappable {
    var location: String?
    var threeDayForecast: [Forecast]?

    required init?(_ map: Map){

    }

    func mapping(map: Map) {
        location <- map["location"]
        threeDayForecast <- map["three_day_forecast"]
    }
}

class Forecast: Mappable {
    var day: String?
    var temperature: Int?
    var conditions: String?

    required init?(_ map: Map){

    }

    func mapping(map: Map) {
        day <- map["day"]
        temperature <- map["temperature"]
        conditions <- map["conditions"]
    }
}

私は私の中に持っていますViewController：

class ViewController: UIViewController {

    var threeDayForecastArray = [Forecast]()

    override func viewDidLoad() {
        super.viewDidLoad()

        getWeatherReport()

        print(threeDayForecastArray.count) // 0
        print(threeDayForecastArray[0].conditions) //fatal error: Index out of range
    }

    func getWeatherReport() {
        let URL = "https://raw.githubusercontent.com/tristanhimmelman/AlamofireObjectMapper/d8bb95982be8a11a2308e779bb9a9707ebe42ede/sample_json"
        Alamofire.request(.GET, URL).responseObject { (response: Response<WeatherResponse, NSError>) in

            let weatherResponse = response.result.value
            print(weatherResponse?.location)

            if let threeDayForecast = weatherResponse?.threeDayForecast {
                self.threeDayForecastArray = threeDayForecast
                for forecast in threeDayForecast {
                    print(forecast.day)
                    print(forecast.temperature)
                }
            }
        }
    }

}

にデータを入力しようとしていますがthreeDayForecastArray、count = 0 またはインデックス不足エラーが発生します。私が間違っていることや助けについてのアドバイスは大歓迎です。