collectionView doesn't reloadData in viewDidLoad but it works in viewWillAppear

Question

I showed a viewController named NewOrderVC from a tab bar with this code:

if let selectedAnn = mapView.selectedAnnotations[0] as? StoreAnnotation {
                let id = selectedAnn.storeModel!.id
                let vc = NewOrderVC(storeID: id)          
                vc.modalPresentationStyle = .fullScreen
                self.show(vc, sender: nil)
            }

With getStore function, I download some codes from firebase. NewOrderVC:

override func viewDidLoad() {
        super.viewDidLoad()

        DispatchQueue.main.async {
            self.getStore(id: self.id)
        }

        initializeHideKeyboard()
        let layout = UICollectionViewFlowLayout()
        layout.scrollDirection = .vertical
        layout.minimumLineSpacing = 1
        layout.minimumInteritemSpacing = 1
        layout.itemSize = CGSize(width: view.width / 2 - 15,
                                 height: view.width - 30)
        collectionView = UICollectionView(frame: .zero, collectionViewLayout: layout)      
        guard let collectionView = collectionView else {
            return
        }
        collectionView.register(ProductCollectionViewCell.self, forCellWithReuseIdentifier: ProductCollectionViewCell.identifier)
        collectionView.register(HeaderCollectionReusableView.self,
                                 forSupplementaryViewOfKind: UICollectionView.elementKindSectionHeader,
                                 withReuseIdentifier: HeaderCollectionReusableView.identifier) 
        collectionView.delegate = self
        collectionView.dataSource = self
        view.addSubview(collectionView)
        view.addSubview(finishButton)
        collectionView.backgroundColor = .systemBackground

In viewDidLoad, collectionView.reloadData does not work. I can't see collectionView when view controller appeared.

        DispatchQueue.main.async {
            self.collectionView.reloadData()
        }      
    } 

If I change the viewController with using tab bar icons, collectionView appear and objects loaded in it.

    override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)
        collectionView.delegate = self
        collectionView.dataSource = self

        DispatchQueue.main.async {         
            self.collectionView.reloadData()
        }
    }

This is getStore function. It works like this.

private func getStore(id: String)  {
        DatabaseManager.shared.showProducts(id: id) { downloaded in
            switch downloaded {
            case .failure(_):
                self.makeAlert(title: "Error", message: "Products could not download!")
                break
            case.success(let products):
                self.store = products
            }
        }
        DispatchQueue.main.async {
            self.collectionView.reloadData()
        }
        
}

Thank you for any help.

Answer 1

If the data you are displaying in collectionView is coming from getStore function, then you need to reload CollectionView after the data comes rather then in viewDidLoad and ViewWillAppear