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

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.

CodePudding user response：

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