Want to view json metadata

问题

I am trying to build an app that displays Currency exchange rates using the Alpha Vantage API for iOS. I have built the functions but can't figure out how to access the exact json value which is "5. Exchange Rate".

Here is some of the code and the json data to help explain better:

The URL Built:

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()

    }

The Quotes in the view controller

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

        }
    }
}

// The Quotes being showing after touching the USD button:

@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)

}

The Raw JSON Data:The json data on the webpage

回答1:

The main error is that the value for key 5. Exchange Rate is String not Float and the code crashes reliably when being force unwrapped.

In Swift 4 the Decodable protocol is much more convenient than a dictionary.

Create two structs. The ExchangeRate struct contains a computed property floatRate to return the Float value

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
    }
}

Then make the download function more versatile by passing from and to currency as parameters. This version uses the JSONDecoder and returns either the ExchangeRate instance or the 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()
}

and use it

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
        }
    }
}

来源：https://stackoverflow.com/questions/51467195/want-to-view-json-metadata