Swift: Trying to retrieve the data from a dataSource Struct - ios

Question

Here is my code below:
class DataSource: NSObject {
var categories = [String]()
var items = [Item]()
private override init() {
super.init()
}
class var sharedDataSource: DataSource {
struct Static {
static var onceToken: dispatch_once_t = 0
static var instance: DataSource!
}
dispatch_once(&Static.onceToken) {
let dataSource = DataSource()
Static.instance = dataSource
let urlPath = "myUrlPathString"
let endpoint = NSURL(string: urlPath)
let request = NSMutableURLRequest(URL: endpoint!)
NSURLSession.sharedSession().dataTaskWithRequest(request,completionHandler: { (data, response, error) in
let json = JSON(data: data!)
print(json)
for obj in json.arrayValue {
let item: Item = Item()
item.itemID = obj["item"].stringValue
item.price = obj["price"].floatValue
item.title = obj["title"].stringValue
item.category = obj["category"].stringValue
item.available = obj["available"].boolValue
item.image = obj["image"].stringValue
print(item.title)
dataSource.items.append(item)
print(dataSource.items)
print("STOP")
}
}).resume()
}
return Static.instance
}
}
I am trying to use the result of this dataSource in a UICollectionView, by assigning its result to an Item array. I am successfully grabbing the data in my NSURLSession, and its local list, 'items', is being populated.
In my UICollectionView, in my viewDidLoad, I am assigning my local variable as follows:
let dataSource = DataSource()
items = dataSource.sharedInstance.items
Printing the value within the viewDidLoad always results in an empty array with no values, but I know the values are there by the time NURLSession is finished. I'm not sure how to write a completionhandler for this. This is my first time doing this kind of thing with a sharedDataSource that is a struct.
Any ideas anyone?
Thanks,
Sean

Answer 1

If you are doing this in viewDidLoad then it's likely the NSURL session has not completed by the time you try to get your items array. Send a notification in your dataSource class after the data has been received and then set items.
Eg
NSURLSession.sharedSession().dataTaskWithRequest(request,completionHandler: { (data, response, error) in
let json = JSON(data: data!)
print(json)
for obj in json.arrayValue {
let item: Item = Item()
item.itemID = obj["item"].stringValue
item.price = obj["price"].floatValue
item.title = obj["title"].stringValue
item.category = obj["category"].stringValue
item.available = obj["available"].boolValue
item.image = obj["image"].stringValue
print(item.title)
dataSource.items.append(item)
print(dataSource.items)
print("STOP")
}
NSNotificationCenter.defaultCenter().postNotificationName("dataRecievedNotification", object: nil)
}).resume()
But you also need to be able to receive this notification in your ViewController with the collectionView. So in viewDidLoad add
NSNotificationCenter.defaultCenter().addObserver(self, selector: "dataRecieved", name: "dataRecievedNotification", object: nil)
Then add a function in the same ViewController:
func dataRecieved() {
print("data received")
items = dataSource.sharedInstance.items
collectionView.reloadData()
}
Where dataSource is a variable declared above viewDidLoad:
let dataSource = DataSource()
Don't forget that if you are using observers you need to remove them when the class is removed from memory, so in the ViewController add this deinit function:
deinit {
NSNotificationCenter.defaultCenter().removeObserver(self)
}
EDIT: I think your singleton pattern can be simplified using modern swift. You don't have to use a struct or dispatch_once any more, just declaring a static let shared instance will handle all that for you. with that in mind, I tried to simplify your dataSource class:
class DataSource {
static let sharedInstance = DataSource()
var items = [Item]()
private init() {}
func retrieveItems() {
let urlPath = "myUrlPathString"
let endpoint = NSURL(string: urlPath)
let request = NSMutableURLRequest(URL: endpoint!)
NSURLSession.sharedSession().dataTaskWithRequest(request,completionHandler: { (data, response, error) in
let json = JSON(data: data!)
print(json)
for obj in json.arrayValue {
let item: Item = Item()
item.itemID = obj["item"].stringValue
item.price = obj["price"].floatValue
item.title = obj["title"].stringValue
item.category = obj["category"].stringValue
item.available = obj["available"].boolValue
item.image = obj["image"].stringValue
print(item.title)
items.append(item)
print(items)
print("STOP")
}
NSNotificationCenter.defaultCenter().postNotificationName("dataRecievedNotification", object: nil)
}).resume()
}
}
where you, in viewDidLoad, add the following logic:
if DataSource.sharedInstance.items.count == 0 {
DataSource.sharedInstance.retrieveItems()
}
then change dataRecieved to
func dataRecieved() {
print("data received")
items = DataSource.sharedInstance.items //Notice the capital
collectionView.reloadData()
}
and delete your declaration of var dataSource = DataSource() above viewDidLoad