Can't get value from Google-Books JSON API

The response you get from the API is an array of titles.

I suggest using if let to unwrap the Optional value you get from KVC, and typecasting the result as a Swift array of Strings.

Swift 1

func getBookInfo(isbn: String) {    var url: String = "https://www.googleapis.com/books/v1/volumes?q=isbn:" + isbn    var request: NSMutableURLRequest = NSMutableURLRequest()    request.URL = NSURL(string: url)    request.HTTPMethod = "GET"    NSURLConnection.sendAsynchronousRequest(request, queue: NSOperationQueue(), completionHandler:{ (response:NSURLResponse!, data: NSData!, error: NSError!) -> Void in        var error: AutoreleasingUnsafeMutablePointer<NSError?> = nil        if let jsonResult = NSJSONSerialization.JSONObjectWithData(data, options: nil, error: error) as? NSDictionary {            if let arrayOfTitles = jsonResult.valueForKeyPath("items.volumeInfo.title") as? [String] {                let titles = ", ".join(arrayOfTitles)                println(titles)            } else {                // error: no title found            }        } else {            GlobalConstants.AlertMessage.displayAlertMessage("Error fetching data from barcode, please try again.", view: self)        }    })}getBookInfo("0553283685")  // prints "Hyperion"

Swift 2

For this version we're using NSURLSession because NSURLConnection is now deprecated.

func getBookInfo(isbn: String) {    let urlString = "https://www.googleapis.com/books/v1/volumes?q=isbn:" + isbn    if let url = NSURL(string: urlString) {        NSURLSession.sharedSession().dataTaskWithURL(url, completionHandler: {data, _, error -> Void in            if let error = error {                print(error.localizedDescription)            } else {                if let data = data,                    jsonResult = try? NSJSONSerialization.JSONObjectWithData(data, options: []),                    arrayOfTitles = jsonResult.valueForKeyPath("items.volumeInfo.title") as? [String] {                    let titles = arrayOfTitles.joinWithSeparator(", ")                    print(titles)                } else {                    GlobalConstants.AlertMessage.displayAlertMessage("Error fetching data from barcode, please try again.", view: self)                }            }        }).resume()    }}getBookInfo("0553283685") // prints "Hyperion"

Swift 3

Same as Swift 2 with some syntax changes. I've also added the "authors" example, and I'm now using guard. Just for the sake of showing something different from the previous example.

func getBookInfo(isbn: String) {    guard let url = URL(string: "https://www.googleapis.com/books/v1/volumes?q=isbn:\(isbn)") else {        print("the url is not valid")        return    }    URLSession.shared().dataTask(with: url, completionHandler: {data, response, error -> Void in        guard error == nil else {            print(response)            print(error!.localizedDescription)            return        }        guard let data = data else {            print("no error but no data")            print(response)            return        }        guard let jsonResult = try? JSONSerialization.jsonObject(with: data, options: []) else {            print("the JSON is not valid")            return        }        if let arrayOfTitles = jsonResult.value(forKeyPath: "items.volumeInfo.title") as? [String] {            print(arrayOfTitles)        }        if let arrayOfAuthors = jsonResult.value(forKeyPath: "items.volumeInfo.authors") as? [[String]] {            print(arrayOfAuthors)        }    }).resume()}getBookInfo(isbn: "0553283685")