I have 2 collection views in parallel. One of them is a menu and the other is a big one containing vertical table views that are populated with API queried data. 
When the user scrolls right, then Order History is highlighted and the next cell in the big collection view at the bottom takes over the rest of the screen and it shows its own tableview data. It's just like the implementation here.
Everything works EXCEPT when I tap on Order History in the state indicated in the attached image, the collection view at the bottom does not scroll to the 2nd cell in the big collection view. If I tap on the Current Orders cell then it does scroll to the 1st cell in the big collection view. I have no idea what's causing this problem after spending the last 48 hours on this.
This is the menu bar:
class MenuBar: UIView, UICollectionViewDelegate, UICollectionViewDataSource, UICollectionViewDelegateFlowLayout {
lazy var menuCollection: UICollectionView = {
let layout = UICollectionViewFlowLayout()
layout.scrollDirection = .horizontal
let collectionview = UICollectionView(frame: .zero, collectionViewLayout: layout)
collectionview.translatesAutoresizingMaskIntoConstraints = false
collectionview.backgroundColor = .white
collectionview.delegate = self
collectionview.dataSource = self
collectionview.register(InvoiceCell.self, forCellWithReuseIdentifier: InvoiceCell.identifier)
return collectionview
}()
var leftIndicatorConstraint: NSLayoutConstraint?
private let menuOptions = ["Current Orders", "Order History"]
var containerViewController: ContainerViewController? // I use an instance of the parent VC to pass on the value
override init(frame: CGRect) {
super.init(frame: frame)
addSubview(menuCollection)
menuCollection.topAnchor.constraint(equalTo: self.topAnchor, constant: 0).isActive = true
menuCollection.bottomAnchor.constraint(equalTo: self.bottomAnchor, constant: 0).isActive = true
menuCollection.leadingAnchor.constraint(equalTo: self.leadingAnchor, constant: 0).isActive = true
menuCollection.trailingAnchor.constraint(equalTo: self.trailingAnchor, constant: 0).isActive = true
menuCollection.selectItem(at: IndexPath(row: 0, section: 0), animated: true, scrollPosition: .left)
setupIndicator()
}
required init?(coder: NSCoder) {
fatalError("init(coder:) has not been implemented")
}
func setupIndicator() {
let indicatorBar = UIView()
indicatorBar.backgroundColor = UIColor(hex: Constants.Colors.primary)
addSubview(indicatorBar)
indicatorBar.translatesAutoresizingMaskIntoConstraints = false
leftIndicatorConstraint = indicatorBar.leadingAnchor.constraint(equalTo: self.leadingAnchor)
leftIndicatorConstraint?.isActive = true
indicatorBar.bottomAnchor.constraint(equalTo: self.bottomAnchor).isActive = true
indicatorBar.heightAnchor.constraint(equalToConstant: 5).isActive = true
indicatorBar.widthAnchor.constraint(equalTo: self.widthAnchor, multiplier: 0.5).isActive = true
}
func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
containerViewController?.scrollToMenu(menuIndex: indexPath.item) // Here I pass on the value of the menu item that's tapped
}
func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
return menuOptions.count
}
func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
guard let cell = collectionView.dequeueReusableCell(withReuseIdentifier: InvoiceCell.identifier, for: indexPath) as? InvoiceCell else { return UICollectionViewCell() }
cell.thisOption = menuOptions[indexPath.row]
return cell
}
func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
return CGSize(width: menuCollection.frame.size.width / CGFloat(menuOptions.count), height: menuCollection.frame.size.height)
}
func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, minimumInteritemSpacingForSectionAt section: Int) -> CGFloat {
return 0
}
func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, minimumLineSpacingForSectionAt section: Int) -> CGFloat {
return 0
}
}
And this is the parent UIView Controller that contains the menu collection view as well as the big base collection view
class ContainerViewController: UIViewController, UICollectionViewDelegate, UICollectionViewDataSource, UICollectionViewDelegateFlowLayout {
lazy var menuBar: MenuBar = {
let menuBar = MenuBar()
menuBar.containerViewController = self
menuBar.translatesAutoresizingMaskIntoConstraints = false
return menuBar
}()
lazy var baseCollectionView: UICollectionView = {
let layout = UICollectionViewFlowLayout()
layout.scrollDirection = .horizontal
let collectionView = UICollectionView(frame: .zero, collectionViewLayout: layout)
collectionView.register(PastCell.self, forCellWithReuseIdentifier: PastCell.pastCellId)
collectionView.register(CurrentCell.self, forCellWithReuseIdentifier: CurrentCell.currentCellId)
collectionView.isPagingEnabled = true
collectionView.delegate = self
collectionView.dataSource = self
collectionView.translatesAutoresizingMaskIntoConstraints = false
collectionView.showsHorizontalScrollIndicator = false
collectionView.backgroundColor = .white
return collectionView
}()
var orderViewModel = OrderViewModel(order: Order(addressOne: "", addressTwo: "", city: "", postalCode: "", mpName: "", planType: 0, restaurantAddress: "", restaurantId: "", restaurantName: "", timeOfCreation: nil, currentTotal: 0.00, orderMenu: [], status: 0, itemsHTML: ""))
override func viewDidLoad() {
super.viewDidLoad()
view.backgroundColor = .white
setupView()
}
override func viewDidDisappear(_ animated: Bool) {
super.viewDidDisappear(animated)
orderViewModel.removeListener()
}
private func setupView() {
view.addSubview(menuBar)
menuBar.topAnchor.constraint(equalTo: view.safeAreaLayoutGuide.topAnchor, constant: 0).isActive = true
menuBar.leadingAnchor.constraint(equalTo: view.safeAreaLayoutGuide.leadingAnchor, constant: 0).isActive = true
menuBar.trailingAnchor.constraint(equalTo: view.safeAreaLayoutGuide.trailingAnchor, constant: 0).isActive = true
menuBar.heightAnchor.constraint(equalToConstant: 40).isActive = true
view.addSubview(baseCollectionView)
baseCollectionView.topAnchor.constraint(equalTo: menuBar.bottomAnchor, constant: 0).isActive = true
baseCollectionView.leadingAnchor.constraint(equalTo: view.leadingAnchor, constant: 0).isActive = true
baseCollectionView.trailingAnchor.constraint(equalTo: view.trailingAnchor, constant: 0).isActive = true
baseCollectionView.bottomAnchor.constraint(equalTo: view.bottomAnchor, constant: 0).isActive = true
}
func scrollToMenu(menuIndex: Int) {
// Here I am using the passed on menu index to scroll to the appropriate cell in the big collection view.
let indexPath = IndexPath(item: menuIndex, section: 0)
baseCollectionView.scrollToItem(at: indexPath, at: .left, animated: true) // For some reason it ALWAYS SCROLL TO THE FIRST CELL }
func scrollViewDidScroll(_ scrollView: UIScrollView) {
menuBar.leftIndicatorConstraint?.constant = scrollView.contentOffset.x / 2
}
func scrollViewWillEndDragging(_ scrollView: UIScrollView, withVelocity velocity: CGPoint, targetContentOffset: UnsafeMutablePointer<CGPoint>) {
let index = Int(targetContentOffset.pointee.x / view.frame.width)
menuBar.menuCollection.selectItem(at: IndexPath(item: index, section: 0), animated: true, scrollPosition: .left)
}
func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
return 2
}
func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
switch indexPath.row {
case 0:
if let cell = collectionView.dequeueReusableCell(withReuseIdentifier: CurrentCell.currentCellId, for: indexPath) as? CurrentCell {
if let tabBarHeight = self.tabBarController?.tabBar.frame.size.height {
cell.adjustSize(top: menuBar.frame.height * 1.2, bottom: tabBarHeight * 0.5)
return cell
}
}
case 1:
if let cell = collectionView.dequeueReusableCell(withReuseIdentifier: PastCell.pastCellId, for: indexPath) as? PastCell {
cell.backgroundColor = .systemOrange
return cell
}
default:
return UICollectionViewCell()
}
return UICollectionViewCell()
}
func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
return CGSize(width: view.frame.size.width, height: view.frame.size.height)
}
func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, minimumLineSpacingForSectionAt section: Int) -> CGFloat {
return 0
}
}