Pass asynchronously fetched data to another class - xcode

Question

I'm designing an iOS App using Swift. I have a View Controller, DisplayBrand that's displaying data fetched from a backend. I have made another class, FetchAndParseBrand, for fetching and parsing data from the backend.
The view controller is calling a method, getBrand, in the fetching class. This method then revieces data asynchronously from the backend.
How do I return the received data to the View Controller? I guess there are multiple ways of solving this problem, do you have any suggestion for the best practice?
Here is some code for context:
DisplayBrand
class DisplayBrand: UIViewController {
override func viewDidLoad() {
super.viewDidLoad()
var brand = Brand()
FetchAndParseBrand().getBrand()
self.updateUI()
}
...
}
FetchAndParseBrand:
class FetchAndParseBrand: NSObject {
// Fetches one brand on static url
func getBrand() -> Brand{
let qos = Int(QOS_CLASS_USER_INITIATED.value)
var jsonError: NSError?
var brand = Brand()
if let url = NSURL(string: "http://backend.com/api/brand")
{
dispatch_async(dispatch_get_global_queue(qos, 0))
{
// simulate long load delay
println("going to sleep")
sleep(2)
if let brandData = NSData(contentsOfURL: url)
{
dispatch_async(dispatch_get_main_queue())
{
if let jsonBrand = NSJSONSerialization.JSONObjectWithData(brandData, options: nil, error: &jsonError) as? NSDictionary {
if let newName = jsonBrand.valueForKey("Name") as? String {
brand.name = newName
}
if let newGender = jsonBrand.valueForKey("Gender") as? Int {
if newGender == 0 {brand.gender = "male"}
else if newGender == 1 {brand.gender = "female"}
}
if let newId = jsonBrand.valueForKey("Id") as? Int {
brand.id = newId
}
brand.printBrand()
}
}
}
}
}
return brand
}
}
Brand
class Brand: NSObject {
var id: Int?
var name: String?
var image: UIImage?
var gender: String?
func printBrand() {
if id != nil { println("ID:\t\t\(self.id!)") }
if name != nil { println("Name:\t\(self.name!)") }
if image != nil { println("Image:\t\(self.image?.description)") }
if gender != nil { println("Gender:\t\(self.gender!)") }
}
}

Answer 1

You can use a completion handler for this, this will return the variable when it is read:
class FetchAndParseBrand: NSObject {
class func getBrand(completion: (brand: Brand) -> ()) {
let qos = Int(QOS_CLASS_USER_INITIATED.value)
var jsonError: NSError?
var brand = Brand()
if let url = NSURL(string: "http://backend.com/api/brand")
{
dispatch_async(dispatch_get_global_queue(qos, 0))
{
// simulate long load delay
println("going to sleep")
sleep(2)
if let brandData = NSData(contentsOfURL: url)
{
dispatch_async(dispatch_get_main_queue())
{
if let jsonBrand = NSJSONSerialization.JSONObjectWithData(brandData, options: nil, error: &jsonError) as? NSDictionary {
if let newName = jsonBrand.valueForKey("Name") as? String {
brand.name = newName
}
if let newGender = jsonBrand.valueForKey("Gender") as? Int {
if newGender == 0 {brand.gender = "male"}
else if newGender == 1 {brand.gender = "female"}
}
if let newId = jsonBrand.valueForKey("Id") as? Int {
brand.id = newId
}
completion(brand: brand)
brand.printBrand()
}
}
}
}
}
}
}
Then you can call the function like so:
FetchAndParseBrand.getBrand { (brand) -> () in
println(brand)
}

Answer 2

there are many ways. with callback function and delegates. you can also use singleton if you will call back only to a specific view controller:
class DisplayBrand: UIViewController {
static var sharedInstance :DisplayBrand?
override func viewDidLoad() {
super.viewDidLoad()
DisplayBrand.sharedInstance = self;
}
.
.
.
//on async over:
displayBrand.sharedInstance?.someFunction();

Answer 3

There is a major issue in your code:
FetchAndParseBrand.getBrand() is supposed to be a class method and has no parameter
The easiest way to get a asynchronous callback is a closure
Try this
Class DisplayBrand
class DisplayBrand: UIViewController {
override func viewDidLoad() {
super.viewDidLoad()
FetchAndParseBrand.getBrand() { (brand) -> () in
// do something with variable brand
self.updateUI() }
}
...
}
Class FetchAndParseBrand
class FetchAndParseBrand: NSObject {
// Fetches one brand on static url
class func getBrand(completion: (Brand)->()) {
let qos = Int(QOS_CLASS_USER_INITIATED.value)
var jsonError: NSError?
var brand = Brand()
if let url = NSURL(string: "http://backend.com/api/brand")
{
dispatch_async(dispatch_get_global_queue(qos, 0))
{
// simulate long load delay
println("going to sleep")
sleep(2)
if let brandData = NSData(contentsOfURL: url)
{
dispatch_async(dispatch_get_main_queue())
{
if let jsonBrand = NSJSONSerialization.JSONObjectWithData(brandData, options: nil, error: &jsonError) as? NSDictionary {
if let newName = jsonBrand.objectForKey("Name") as? String {
brand.name = newName
}
if let newGender = jsonBrand.objectForKey("Gender") as? Int {
if newGender == 0 {brand.gender = "male"}
else if newGender == 1 {brand.gender = "female"}
}
if let newId = jsonBrand.objectForKey("Id") as? Int {
brand.id = newId
}
brand.printBrand()
completion(brand)
}
}
}
}
}
}
}
Side note: valueForKey is a key value coding method. The standard method to get a value from a dictionary by key is objectForKey or use subscripting