How can I Init a Class with a Web Service Call in Swift? - ios

Question

I have a drug class which I would like to init by passing in a upc: Int. Inside the init I'd like to make a web service call, and populate the values in the class with the returned JSON (NSDictionary).
I have a start, but not a finish. I'm getting errors everywhere and can't seem to figure out how to best accomplish this. Maybe I'm going about this the whole wrong way. Could someone help?
Here's the code ..
init(upc: Int) {
let apiKey = "xx"
let baseURL = NSURL(string: "http://something.com/\(apiKey)/")
let getDrugByUpcURL = NSURL(string: "\(upc).json", relativeToURL: baseURL)
let config = NSURLSessionConfiguration.defaultSessionConfiguration()
let usernamePasswordString = "user:pass"
let usernamePasswordData = usernamePasswordString.dataUsingEncoding(NSUTF8StringEncoding)
let base64EncodedCredential = usernamePasswordData!.base64EncodedStringWithOptions(nil)
let authString = "Basic \(base64EncodedCredential)"
config.HTTPAdditionalHeaders = ["Authorization": authString]
let session = NSURLSession(configuration: config)
let downloadTask: NSURLSessionDownloadTask = session.downloadTaskWithURL(getDrugByUpcURL!, completionHandler: { (location: NSURL!, response: NSURLResponse!, error: NSError!) -> Void in
if (error == nil) {
let dataObject = NSData(contentsOfURL: location)
println(dataObject)
let drugDictionary: NSDictionary = NSJSONSerialization.JSONObjectWithData(dataObject!, options: nil, error: nil) as NSDictionary
println(drugDictionary["din"])
drug_id = drugDictionary["drug_id"] as Int
din = drugDictionary["din"] as String
packsize = drugDictionary["packsize"] as Double
brand = drugDictionary["brand"] as String
//generic = drugDictionary["generic"] as String
strength = drugDictionary["strength"] as String
form = drugDictionary["form"] as String
upc = drugDictionary["upc"] as String
//priceGroup = drugDictionary["price_group"] as String
//manufacturer = drugDictionary["manufacturer"] as String
onHandQuantity = drugDictionary["onhand"] as Double
//vendorCost = drugDictionary["vendor_cost"] as Double
//basePrice = drugDictionary["base_price"] as Double
//discount = drugDictionary["discount"] as Double
//price = drugDictionary["price"] as Double
} else {
println(error)
}
})
downloadTask.resume()
}
An error I'm getting is on all property assignment lines: Cannot assign to 'drug_id' in 'self'.

Answer 1

The problem is that you are accessing those instance variables from a closure, the completion handler of downloadTaskWithURL:.
This is easily fixed by prefixing the variables with self.. So drug_id becomes self.drug_id.
Note however that what you are doing in the above code is probably a bad idea. Or at least a very uncommon design: I don't think it is a good idea to do asynchronous network requests in your class' initializer.
Since the call to NSURLSessionDownloadTask is asynchronous, your init() will return immediately with uninitialized data and then at some unspecified moment in time your class will be fully populated with the result from the web service call. You don't know what that moment in time is though, so you really have no good way of knowing when your instance is ready.
This is most likely not what you had in mind.