In Swift 2, how can I return JSON parsing errors to the completion block?

Question

I would like to create a function in Swift 2 that gets data from a URL and returns it as a JSON object using NSURLSession. At first, this seemed pretty straight forward. I wrote the following:

func getJson(url:NSURL, completeWith: (AnyObject?,NSURLResponse?,NSError?)->Void) -> NSURLSessionTask? {

    let session = NSURLSession.sharedSession()
    let task = session.dataTaskWithURL(url) {
        (data:NSData?, response:NSURLResponse?, error:NSError?) -> Void in

        if error != nil {
            completeWith(nil, response, error)
        }

        if let data = data {

            do {
                let object:AnyObject? = try NSJSONSerialization.JSONObjectWithData(data, options: .AllowFragments)
            } catch let caught as NSError {
                completeWith(nil, response, caught)
            }

            completeWith(object, response, nil)

        } else {
            completeWith(nil, response, error)
        }
    }

    return task
}

However, that doesn't compile because the completion block doesn't declare "throws". The exact error is Cannot invoke 'dataTaskWithURL' with an argument list of type '(NSURL, (NSData?, NSURLResponse?, NSError?) throws -> Void)'. Even though I'm catching all errors in my do/catch statement, Swift still wants to propagate the NSError up the chain. The only way I can see around it is to use try!, like this:

if let data = data {

    let object:AnyObject? = try! NSJSONSerialization.JSONObjectWithData(data, options: .AllowFragments)
    completeWith(object, response, nil)

} else {
    completeWith(nil, response, error)
}

Now everything compiles just fine, but I've lost the NSError that's thrown by NSJSONSerialization.JSONObjectWithData.

Is there was I can capture the NSError potentially thrown by NSJSONSerialization.JSONObjectWithData and propagate it to the completion block without modifying the completion block's signature?

Answer 1

I think, your catch is not exhaustive, so you need something like this:

do
{
  let object:AnyObject? = try NSJSONSerialization.JSONObjectWithData(data, options: .AllowFragments)
  completeWith(object, response, nil)
} catch let caught as NSError {
  completeWith(nil, response, caught)
} catch {
  // Something else happened.
  // Insert your domain, code, etc. when constructing the error.
  let error: NSError = NSError(domain: "<Your domain>", code: 1, userInfo: nil)
  completeWith(nil, nil, error)
}

Answer 2

to address the question from Jguffey. I saw the same error when I tried calling the function like this:

let taskResult = getJson(url!) { 
     (any: AnyObject,resp: NSURLResponse,error: NSError) in

it should be like this:

let taskResult = getJson(url!) { 
         (any: AnyObject?,resp: NSURLResponse?,error: NSError?) in