json - 想查看json元数据

Question

我正在尝试使用适用于 iOS 的 Alpha Vantage API 构建一个显示货币汇率的应用程序。我已经构建了函数，但无法弄清楚如何访问确切的 json 值，即“5. 汇率”。

下面是一些代码和 json 数据，以帮助更好地解释:

构建的 URL:

func USDtoEUR(_ completionHandler: @escaping (_ success: Bool, _ quotes: [String:AnyObject]?, _ error: String?) -> Void) {

    let urlString = "https://www.alphavantage.co/query?function=CURRENCY_EXCHANGE_RATE&from_currency=USD&to_currency=EUR&apikey=NP3M8LL62YJDO0YX"
        let session = URLSession.shared
        let url = URL(string: urlString)!

        let request = URLRequest(url: url)
        let task = session.dataTask(with: request, completionHandler: { data, response, error in
            if error != nil {
                completionHandler(false, nil, error!.localizedDescription)
            }
            else {
                do {
                    let result = try JSONSerialization.jsonObject(with: data!, options: JSONSerialization.ReadingOptions.allowFragments) as! NSDictionary
                    if let dictionary = result["Realtime Currency Exchange Rate"] as? [String:AnyObject]! {
                        completionHandler(true, dictionary, nil)
                    }
                    else {
                        completionHandler(false, nil, nil)
                    }
                } catch {
                    completionHandler(false, nil, "Unable to process retrieved data.")
                }
            }

        })
        task.resume()

    }

View Controller 中的引号

func usdQUotesRequest() {

  USDClient().USDtoEUR() { success, newQuote, error in

    if success {
        self.usdtoeurquote = newQuote
        DispatchQueue.main.async {
            self.stopActivityIndicator()
            self.Refresh.isEnabled = true
        }

    } else {
        DispatchQueue.main.async {
            self.displayAlert("Unable to Retrieve Latest Conversion Rates", message: "\(error!)")
            self.stopActivityIndicator()
            self.Refresh.isEnabled = true

        }
    }
}

//触摸美元按钮后显示的报价:

@IBAction func usdConversions(_ sender: Any) {

    self.displayAlert("Alert!", message: "USD Selected")

    let usdVal = (outputCurrency1.text! as NSString).floatValue

    let euroValue = usdVal * (usdtoeurquote["5. Exchange Rate"] as! Float)
    outputCurrency2.text = String(format: "%.2f", euroValue)

    let gbpVal = usdVal * (usdtogbpquote["5. Exchange Rate"] as! Float)
    outputCurrency3.text = String(format: "%.2f", gbpVal)

    let cnyVal = usdVal * (usdtocnyquote["5. Exchange Rate"] as! Float)
    outputCurrency2.text = String(format: "%.2f", cnyVal)

    let cadVal = usdVal * (usdtocadquote["5. Exchange Rate"] as! Float)
    outputCurrency2.text = String(format: "%.2f", cadVal)

    let inrVal = usdVal * (usdtoinrquote["5. Exchange Rate"] as! Float)
    outputCurrency2.text = String(format: "%.2f", inrVal)

    let sekVal = usdVal * (usdtosekquote["5. Exchange Rate"] as! Float)
    outputCurrency2.text = String(format: "%.2f", sekVal)

    let rubVal = usdVal * (usdtorubquote["5. Exchange Rate"] as! Float)
    outputCurrency2.text = String(format: "%.2f", rubVal)

    let nzdVal = usdVal * (usdtonzdquote["5. Exchange Rate"] as! Float)
    outputCurrency2.text = String(format: "%.2f", nzdVal)

}

原始 JSON 数据:The json data on the webpage

Answer 1

主要错误是键 5 的值。 Exchange Rate 是 String 而不是 Float 并且在强制解包时代码会可靠地崩溃。

在 Swift 4 中，Decodable 协议(protocol)比字典方便得多。

创建两个结构。 ExchangeRate 结构包含计算属性 floatRate 以返回 Float 值

struct Root : Decodable {
    let exchangeRate : ExchangeRate
    private enum  CodingKeys: String, CodingKey { case exchangeRate = "Realtime Currency Exchange Rate" }
}

struct ExchangeRate : Decodable {

    let fromCode, fromName, toCode, toName, rate : String
    let lastRefreshed, timeZone : String

    private enum  CodingKeys: String, CodingKey {
        case fromCode = "1. From_Currency Code"
        case fromName = "2. From_Currency Name"
        case toCode = "3. To_Currency Code"
        case toName = "4. To_Currency Name"
        case rate = "5. Exchange Rate"
        case lastRefreshed = "6. Last Refreshed"
        case timeZone = "7. Time Zone"
    }

    var floatRate : Float {
        return Float(rate) ?? 0.0
    }
}

然后通过将 from 和 to 货币作为参数传递，使下载功能更加通用。此版本使用 JSONDecoder 并返回 ExchangeRate 实例或 Error

func downloadRate(from: String, to: String, completionHandler: @escaping (ExchangeRate?, Error?) -> Void) {

    let urlString = "https://www.alphavantage.co/query?function=CURRENCY_EXCHANGE_RATE&from_currency=\(from)&to_currency=\(to)&apikey=NP3M8LL62YJDO0YX"
    let task = URLSession.shared.dataTask(with: URL(string: urlString)!, completionHandler: { data, response, error in
        if error != nil {
            completionHandler(nil, error!)
        } else {
            do {
                let result = try JSONDecoder().decode(Root.self, from: data!)
                completionHandler(result.exchangeRate, nil)
            } catch {
                completionHandler(nil, error)
            }
        }
    })
    task.resume()
}

并使用它

func usdQUotesRequest() {

  USDClient().downloadRate(from:"USD", to: "EUR") { exchangeRate, error in

    if let exchangeRate = exchangeRate {
        self.usdtoeurquote = exchangeRate.floatRate
        DispatchQueue.main.async {
            self.stopActivityIndicator()
            self.Refresh.isEnabled = true
        }
    } else {
        DispatchQueue.main.async {
            self.displayAlert("Unable to Retrieve Latest Conversion Rates", message: "\(error!)")
            self.stopActivityIndicator()
            self.Refresh.isEnabled = true
        }
    }
}