Error handling in block with Swift syntax - ios

Question

The error handling here doesn't feel right. Anyone have any suggests for how to improve it? Using optional binding to establish errors and a return value variable.
That cool?
class ChargePointsFetcher {
func getDevices(location: NSURL, completion handler:([ChargeDevice]?, error: NSError?) -> Void) {
let request = NSURLRequest(URL: location)
let session = NSURLSession.sharedSession()
let task = session.dataTaskWithRequest(request, completionHandler: { (let data, let response, let error) -> Void in
var returnValue: NSError?
if let e = error {
returnValue = e
}
var collection: [ChargeDevice]?
if returnValue == nil {
collection = [ChargeDevice]()
var parsingError: NSError?
if let json: NSDictionary = NSJSONSerialization.JSONObjectWithData(data, options: .AllowFragments, error: &parsingError) as? NSDictionary {
if let chargeDevices = json.valueForKey("ChargeDevice") as? NSArray {
for chargeDevice in chargeDevices {
let device = ChargeDevice()
collection!.append(device)
}
}
}
if let e = parsingError {
returnValue = e
}
}
dispatch_async(dispatch_get_main_queue(), { () -> Void in
handler(collection, error: returnValue)
})
})
}
}

Answer 1

In my opinion, you should try very hard to avoid the (Value?, Error?) pattern when you can. It creates bad corner cases since two combinations are illegal. Instead, when possible, I suggest (Value, Error?) any time Value has a sensible "zero". In this case "zero" is "empty array". This matches the ObjC sense very closely, since a nil NSArray is very similar to an empty array.
Doing that, you can substantially simplify this code:
func getDevices(location: NSURL, completion handler:([ChargeDevice], error: NSError?) -> Void) {
// Note that error is marked "var" so we can modify it, and switched from NSError! to NSError?
let task = session.dataTaskWithRequest(request, completionHandler: { (let data, let response, var error: NSError?) -> Void in
var result = [ChargeDevice]()
if error == nil {
// Avoid NSArray and NSDictionary wherever you can in Swift
if let
json = NSJSONSerialization.JSONObjectWithData(data,
options: .AllowFragments,
error: &error
) as? [String:AnyObject],
chargeDevices = json["ChargeDevice"] as? [AnyObject] {
// map is much simpler in this case, but in your full code, for may fine fine
result = chargeDevices.map{ _ in ChargeDevice() }
}
}
dispatch_async(dispatch_get_main_queue(), { () -> Void in
handler(result, error: error)
})
})
}
Note that in your code and my code, incorrect but valid JSON will not generate an error of any kind. It'll just quietly skip through the as? calls. I would probably move all of this JSON work into another function to keep this closure from getting out of hand.
Another approach here is called a Result. My preferred example is Rob Rix's. Even though I've done a lot of work on Result, I've personally find it difficult to bridge to Cocoa cleanly. It's useful if your entire program uses it, and if you encapsulate most Cocoa interaction, but I've found it cumbersome to use as a one-off solution. (Many people would disagree with me here, it's just my experience. Result is definitely worth exploring to form your own opinion.)