Execution order of closures - ios

Question

Here's my code:
I'm just a beginner in programming with this language and I have a problem with a dynamic collection view
My problem is that the the first print is executed after the second one and I'm wondering why...
class ViewController: UICollectionViewController, UICollectionViewDelegateFlowLayout {
#IBOutlet weak var menuButton: UIBarButtonItem!
let db = Firestore.firestore()
let cellId: String = "cellId"
var news = [[String: Any]]()
override func viewDidLoad(){
super.viewDidLoad()
navigationItem.title = "Novità"
sideMenu()
customizeNavBar()
collectionView?.register(NovitaCellView.self, forCellWithReuseIdentifier: cellId)
}
override func didReceiveMemoryWarning(){
super.didReceiveMemoryWarning()
}
override func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
db.collection("News").getDocuments(){(querySnapshot, err) in
if let err = err{
print("Errore: \(err)")
}else{
for document in (querySnapshot?.documents)!{
let cell = document.data()
self.news.append(cell)
print ("document number: \(self.news.count)")
}
}
}
print("exe return with value: \(self.news.count)")
return self.news.count
}
edit: I tried setting it into the viewDidLoad func as well as setting it both as a sync queue and an async queue and it either doesn't works.
edit 2: i made this working by adding the closure into a max priority queue and reloading the view after in the main thread but it takes a long time in order to work..

Answer 1

The idea is that this line
db.collection("News").getDocuments(){(querySnapshot, err) in
is asynchronous (runs in another queue other than the main queue ) it's a network request that will run after the below line (which runs in main queue)

Answer 2

You are going to background thread mode when you call getDocuments which have less priority than Main thread.So replace below part of code into viewDidLoad or a method which is called from viewDidLoad
db.collection("News").getDocuments(){(querySnapshot, err) in
if let err = err{
print("Errore: \(err)")
}else{
for document in (querySnapshot?.documents)!{
let cell = document.data()
self.news.append(cell)
print ("document number: \(self.news.count)")
}
DispatchQueue.main.async {
tableview.reloadData()
}
}
}
and when you get your data you go back to main thread and reload your tableview using this code.
DispatchQueue.main.async {
tableView.reloadData()
}
You can learn more about threading from apple documentation and this tutorial also.