致命错误：解开Optional值时意外发现nil

Question

I am attempting to parse a JSON document and enter its information into a UICollectionView. I had tested the parsing before working on the UICollectionViewDelegate/Flowlayout/DataSource etc. It was returning the correct information, however now I am getting this error. Any idea what I am doing wrong here?

class ViewModel {

let urlString = "https://s3.amazonaws.com/nxtapnxtap/clubsinfo.json"

var clubNames = [String]()
var smImg  = [UIImage]()
var lgImg = [String]()

func fetchItems(success: () -> ()) {
    let url = NSURL(string: urlString)
    let session = NSURLSession(configuration: NSURLSessionConfiguration.defaultSessionConfiguration())
    let task = session.dataTaskWithURL(url!) { (data, response, error) in
        var jsonError: NSError?
    let json = NSJSONSerialization.JSONObjectWithData(data, options: nil, error: &jsonError) as NSDictionary // Error here --> fatal error: unexpectedly found nil while unwrapping an Optional value

        if let unwrappedError = jsonError {
            println("jsonError: \(unwrappedError)")
        } else {
            self.clubNames = json.valueForKeyPath("entry.name.label") as [String]
            self.smImg = json.valueForKeyPath("entry.smimg.label") as [UIImage]
            self.lgImg = json.valueForKeyPath("entry.lgimg.label") as [String]
            success()
        }
    }
    task.resume()
}

Answer 1

The object coming back from JSONObjectWithData is nil. You are trying to force cast it as an NSDictionary. You need to check if it can be cast to an NSDictionary before you act on it:

func fetchItems(success: () -> ()) {
    let url = NSURL(string: urlString)
    let session = NSURLSession(configuration: NSURLSessionConfiguration.defaultSessionConfiguration())
    let task = session.dataTaskWithURL(url!) { (data, response, error) in
        var jsonError: NSError?
        if let json = NSJSONSerialization.JSONObjectWithData(data, options: nil, error: &jsonError) as? NSDictionary {
            self.clubNames = json.valueForKeyPath("entry.name.label") as [String]
            self.smImg = json.valueForKeyPath("entry.smimg.label") as [UIImage]
            self.lgImg = json.valueForKeyPath("entry.lgimg.label") as [String]
            success()
        } else if let unwrappedError = jsonError {
            println("jsonError: \(unwrappedError)")
        }
    }

    task.resume()
}