Proper Error handling in Swift - ios

Question

Hey stackoverflow members,
I work really hard on getting better on Swift, now I have a trivial problem. I'm a former C# developer so Error Handling until now was try {}... catch {}... Message... Done!
Now I'm developing an App which uses some JSON APIs. It's all working, downloading JSON Data, pack them into my Objects but there is one problem. The proper Error Handling..
I have the following code to download & parse JSON:
//Download & Parse JSON
func getJSON(urlToRequest: String) -> NSDictionary {
var url: NSURL = NSURL(string: urlToRequest)
var jsonRequest: NSURLRequest = NSURLRequest(URL: url)
var jsonResponse: AutoreleasingUnsafePointer<NSURLResponse?> = nil
var error: NSError?
var dataValue: NSData = NSURLConnection.sendSynchronousRequest(jsonRequest, returningResponse: jsonResponse, error:&error)
if error.description.isEmpty {
var jsonResult: NSDictionary = NSJSONSerialization.JSONObjectWithData(dataValue, options: NSJSONReadingOptions.MutableContainers, error: &error) as NSDictionary
if error.description.isEmpty {
return jsonResult
}
else {
return NSDictionary(object: "Error: Something with parsing went wrong :(", forKey: "error")
}
}
else {
return NSDictionary(object: "Error: There was an error with your connection :(", forKey: "error")
}
}
The error part is just temporary, the problem is my ViewController just calls one function to get the whole data for the week (days, matchups and so on)
I call it this way:
var rWrapper = RiotWrapper()
let lcsWeek: Week = rWrapper.getWeek("2014-07-07")
My getWeek method calls 3-4 functions which all parse JSON data in relation to the previous responses.
Ok, long story short question: I want to abort all Tasks if JSON or HTTP fails and fill my TableView just with an error message, how can I achieve this?
Something like: if error occurred -> Stop whatever you are doing -> return for example a null erm.. nil for week -> print error
Can someone help me? If someone have some lecture according this topic it will be fine also :D
Thanks in advance!
Btw: sorry for my "bad" english

Answer 1

All of your JSON parsing methods including getWeek should return a tuple with an optional return value and an error. If at any point you get an error in one of your methods from the JSON parsing, just immediately return the error with nil for the return value. Each method up the chain should check for an error from the previous methods and immediately return the error if it finds one:
func getWeek(string : String) -> (Week?, NSError) {
let (result, error) = self.otherMethod()
if error {
return (nil, error)
}
// continue happily ...
}

Answer 2

I would suggest having your function return an optional. It is easy to test and functions can be chained and shortcut.
func getWeek(string : String) -> Week? {
...
if error {
return nil
}
}
This can be quickly evaluated like this:
if let week = getWeek("2014-07-07") {
// handle success case
} else {
// handle nil
}
and check here for how to string together a number of functions that return optional:
https://developer.apple.com/library/prerelease/mac/documentation/Swift/Conceptual/Swift_Programming_Language/OptionalChaining.html