Data Out of Order - ios

Question

I am using the Flickr API to get all albums within a collection and then using the album IDs from to get the primary photo for each of the albums. It works but the photos are not returned to me in order so the primary photo for each album does not match up to the album titles in my collection view.
func getPhotoCollection() {
let collectionURLString = "https://api.flickr.com/services/rest/?method=flickr.collections.getTree&api_key={API_KEY}&collection_id=72157676119666248&user_id={USER_ID}&format=json&nojsoncallback=1"
self.session = URLSession(configuration: .default, delegate: self, delegateQueue: nil)
let task = self.session?.dataTask(with: URL(string: collectionURLString)!, completionHandler: { (data, response, error) in
let json = self.getJSONFrom(urlString: collectionURLString)
let collections = json["collections"]
let collection = collections["collection"].arrayObject as! [[String:AnyObject]]
for collectionObject in collection {
let sets = collectionObject["set"] as! [[String: AnyObject]]
for set in sets {
let albumId = set["id"] as! String
let albumTitle = set["title"] as! String
self.albumIds.append(albumId)
self.albumTitles.append(albumTitle)
}
}
self.getAlbumPrimary()
})
task?.resume()
}
func getAlbumPrimary() {
for albumId in self.albumIds {
let apiURLString = "https://api.flickr.com/services/rest/?method=flickr.photosets.getPhotos&api_key={API_KEY}&photoset_id=\(albumId)&per_page=1&user_id={USER_ID}&format=json&nojsoncallback=1"
self.session = URLSession(configuration: .default, delegate: self, delegateQueue: nil)
let task = self.session?.dataTask(with: URL(string: apiURLString)!, completionHandler: { (data, response, error) in
let json = self.getJSONFrom(urlString: apiURLString)
let photos = json["photoset"]
let photo = photos["photo"].arrayObject as! [[String:AnyObject]]
let primaryPic = photo[0]
let farm = primaryPic["farm"] as! Int
let server = primaryPic["server"] as! String
let picId = primaryPic["id"] as! String
let secret = primaryPic["secret"] as! String
let urlString = String(format: "https://farm%d.static.flickr.com/%#/%#_%#_b.jpg", farm, server, picId, secret)
self.albumPrimaryURLs.append(urlString)
DispatchQueue.main.async {
self.collectionView.reloadData()
self.loaded = true
}
})
task?.resume()
}
}

Answer 1

This is the way multiple asynchronous tasks work ( out of order ) ,you need to create a model instead of seperate arrays then load from the data to fill the last property
class Item {
let id,title:String
var url:String?
init(id:String,title:String){
self.id = id
self.title = title
}
func loadUrl(completion:#escaping () -> () ) {
// here load and set the url
}
}
var items = [Item]() // declare main array
let albumId = set["id"] as! String
let albumTitle = set["title"] as! String
let item = Item(id:albumId,title:albumTitle)
self.items.append(item)
Then to load the collection finally
let g = DispatchGroup()
items.forEach {
g.enter()
$0.loadUrl {
g.leave()
}
}
g.notify(queue:.main) {
self.collectionView.reloadData()
self.loaded = true
}