Accessing Array Outside Closure in Swift 3 - ios

Question

I have this closure which I use to populate my array and dictionary. However, when I try to use it outside the function, it's empty. I understand that this closure works on an asynchronous thread, so is it right to assume that I try to access that variable before it's been populated? As a result, I get an empty array. Here is my code.
class HomeCollectionViewController: UICollectionViewController, UICollectionViewDelegateFlowLayout, UISearchBarDelegate, UIGestureRecognizerDelegate {
var entries = [String: DiaryEntry]()
var entryIDS = [String]()
var searchController: UISearchController!
override func viewDidLoad() {
super.viewDidLoad()
// Register cell classes
self.collectionView!.register(DiaryCell.self, forCellWithReuseIdentifier: "homeCell")
collectionView?.backgroundColor = UIColor.white
navigationController?.hidesBarsOnSwipe = true
if let userID = FIRAuth.auth()?.currentUser?.uid {
FirebaseService.service.getUserEntriesRef(uid: userID).observe(.value, with: { [weak weakSelf = self] (snapshot) in
let enumerator = snapshot.children
while let entry = enumerator.nextObject() as? FIRDataSnapshot {
weakSelf?.entryIDS.append(entry.key)
weakSelf?.entries[entry.key] = DiaryEntry(snapshot: entry)
}
weakSelf?.entryIDS.reverse()
weakSelf?.collectionView?.reloadData()
})
print("Entries: \(entryIDS.count) ")
}
// Do any additional setup after loading the view.
}
What's the best way to deal with such a multithreaded execution?

Answer 1

I follow the coding standards (for Swift) of Raywenderlich and his team. If I'm going to re-write your code to have a strong self, it would be like this:
class HomeCollectionViewController: UICollectionViewController, UICollectionViewDelegateFlowLayout, UISearchBarDelegate, UIGestureRecognizerDelegate {
var entries = [String: DiaryEntry]()
var entryIDS = [String]()
var searchController: UISearchController!
override func viewDidLoad() {
super.viewDidLoad()
// Register cell classes
self.collectionView!.register(DiaryCell.self, forCellWithReuseIdentifier: "homeCell")
collectionView?.backgroundColor = UIColor.white
navigationController?.hidesBarsOnSwipe = true
if let userID = FIRAuth.auth()?.currentUser?.uid {
FirebaseService.service.getUserEntriesRef(uid: userID).observe(.value, with: {
[weak self] (snapshot) in
guard let strongSelf = self else {
return
}
let enumerator = snapshot.children
while let entry = enumerator.nextObject() as? FIRDataSnapshot {
strongSelf.entryIDS.append(entry.key)
strongSelf.entries[entry.key] = DiaryEntry(snapshot: entry)
}
strongSelf.entryIDS.reverse()
strongSelf.collectionView?.reloadData()
})
print("Entries: \(entryIDS.count) ")
}
// Do any additional setup after loading the view.
}
I hope this works. I'm writing my code like this too when connecting to any APIs, such as Firebase and Alamofire.

Answer 2

Use Dispatch Groups to keep track of when you're done appending all the elements to your array, then make a notify callback that'll automatically be called when they're all added.
class HomeCollectionViewController: UICollectionViewController, UICollectionViewDelegateFlowLayout, UISearchBarDelegate, UIGestureRecognizerDelegate {
var entries = [String: DiaryEntry]()
var entryIDS = [String]()
let dispatchGroup = DispatchGroup()
var searchController: UISearchController!
override func viewDidLoad() {
super.viewDidLoad()
// Register cell classes
self.collectionView!.register(DiaryCell.self, forCellWithReuseIdentifier: "homeCell")
collectionView?.backgroundColor = UIColor.white
navigationController?.hidesBarsOnSwipe = true
self.dispatchGroup.enter()
if let userID = FIRAuth.auth()?.currentUser?.uid {
FirebaseService.service.getUserEntriesRef(uid: userID).observe(.value, with: { [weak weakSelf = self] (snapshot) in
let enumerator = snapshot.children
while let entry = enumerator.nextObject() as? FIRDataSnapshot {
weakSelf?.entryIDS.append(entry.key)
weakSelf?.entries[entry.key] = DiaryEntry(snapshot: entry)
}
self.dispatchGroup.leave()
})
self.dispatchGroup.notify(queue: DispatchQueue.main, execute: {
print("Entries: \(entryIDS.count) ")
weakSelf?.entryIDS.reverse()
weakSelf?.collectionView?.reloadData()
})
}
// Do any additional setup after loading the view.
}
I'd also recommend just using self instead of weakSelf?.

Answer 3

I just came across your problem, while searching for a solution of the same problem and i finally managed to figure out. so, i will try to answer it would be helpful for many others coming behind us.
The problem is that the array exists only inside the closure. So, the solution is to make an array outside of viewDidLoad and set it using once you have the complete array, then use didSet to set entryIDS
var entryIDS = [String]() {
didSet {
//something
}
}