Firestore pagination using MVVM architecture swift - swift

Question

I don't quite understand what I am doing wrong since I am very new to MVVM. It worked in MVC architecture. I've setup my VM and am able to get the first set of results and even then that's not working properly. I get 4 results instead of 10 which is what LOADLIMIT is set as. I was able to get it to work in an MVC architecture without any issues. The VM function which triggers the query is called multiple (3) times instead of just once i.e. even prior to scrolling.
Here is my VM:
enum FetchRestaurant {
case success
case error
case location
case end
}
class ListViewModel {
let restaurant: [Restaurant]?
let db = Firestore.firestore()
var restaurantArray = [Restaurant]()
var lastDocument: DocumentSnapshot?
var currentLocation: CLLocation?
typealias fetchRestaurantCallback = (_ restaurants: [Restaurant]?, _ message: String?, _ status: FetchRestaurant) -> Void
var restaurantFetched: fetchRestaurantCallback?
var fetchRestaurant: FetchRestaurant?
init(restaurant: [Restaurant]) {
self.restaurant = restaurant
}
func fetchRestaurantCallback (callback: #escaping fetchRestaurantCallback) {
self.restaurantFetched = callback
}
func fetchRestaurants(address: String) {
print("address received: \(address)")
getLocation(from: address) { location in
if let location = location {
self.currentLocation = location
self.queryGenerator(at: location)
} else {
self.restaurantFetched?(nil, nil, .location)
}
}
}
func queryGenerator(at location: CLLocation) {
var query: Query!
if restaurantArray.isEmpty {
query = db.collection("Restaurant_Data").whereField("distributionType", isLessThanOrEqualTo: 2).limit(to: Constants.Mealplan.LOADLIMIT)
} else {
print("last document:\(String(describing: lastDocument?.documentID))")
query = db.collection("Restaurant_Data").whereField("distributionType", isLessThanOrEqualTo: 2).start(afterDocument: lastDocument!).limit(to: Constants.Mealplan.LOADLIMIT)
}
batchFetch(query: query)
}
func batchFetch(query: Query) {
query.getDocuments { (querySnapshot, error) in
if let error = error {
self.restaurantFetched?(nil, error.localizedDescription, .error)
} else if querySnapshot!.isEmpty {
self.restaurantFetched?(nil, nil, .end)
} else if !querySnapshot!.isEmpty {
let queriedRestaurants = querySnapshot?.documents.compactMap { querySnapshot -> Restaurant? in
return try? querySnapshot.data(as: Restaurant.self)
}
guard let restaurants = queriedRestaurants,
let currentLocation = self.currentLocation else {
self.restaurantFetched?(nil, nil, .end)
return }
self.restaurantArray.append(contentsOf: self.applicableRestaurants(allQueriedRestaurants: restaurants, location: currentLocation))
DispatchQueue.main.asyncAfter(deadline: .now(), execute: {
self.restaurantFetched?(self.restaurantArray, nil, .success)
})
self.lastDocument = querySnapshot!.documents.last
}
}
}
func getLocation(from address: String, completionHandler: #escaping (_ location: CLLocation?) -> Void) {
let geocoder = CLGeocoder()
geocoder.geocodeAddressString(address) { (placemarks, error) in
guard let placemarks = placemarks,
let location = placemarks.first?.location else {
completionHandler(nil)
return
}
completionHandler(location)
}
}
}
And in the VC viewDidLoad:
var fetchMore = false
var reachedEnd = false
let leadingScreensForBatching: CGFloat = 5.0
var searchController = UISearchController(searchResultsController: nil)
var currentAddress : String?
var listViewModel = ListViewModel(restaurant: [Restaurant]())
override func viewDidLoad() {
super.viewDidLoad()
listViewModel.fetchRestaurantCallback { (restaurants, error, result) in
switch result {
case .success :
self.loadingShimmer.stopShimmering()
self.loadingShimmer.removeFromSuperview()
guard let fetchedRestaurants = restaurants else { return }
self.restaurantArray.append(contentsOf: fetchedRestaurants)
self.tableView.reloadData()
self.fetchMore = false
case .location :
self.showAlert(alertTitle: "No businesses nearby", message: "Try going back and changing the address")
case .error :
guard let error = error else { return }
self.showAlert(alertTitle: "Error", message: error)
case .end :
self.fetchMore = false
self.reachedEnd = true
}
}
if let currentAddress = currentAddress {
listViewModel.fetchRestaurants(address: currentAddress)
}
}
I would really appreciate links or resources for implementing MVVM in Swift for a Firestore back-end. I'm coming up short on searches here and on Google. Even tried medium.
EDIT
class ListViewController: UITableViewController {
lazy var loadingShimmer: UIImageView = {
let image = UIImage(named: "shimmer_background")
let imageview = UIImageView(image: image)
imageview.contentMode = .top
imageview.translatesAutoresizingMaskIntoConstraints = false
return imageview
}()
var restaurantArray = [Restaurant]()
var planDictionary = [String: Any]()
var fetchMore = false
var reachedEnd = false
let leadingScreensForBatching: CGFloat = 5.0
var searchController = UISearchController(searchResultsController: nil)
var currentAddress : String?
var listViewModel = ListViewModel(restaurant: [Restaurant]())
override func viewDidLoad() {
super.viewDidLoad()
setupTable()
}
override func viewWillAppear(_ animated: Bool) {
clearsSelectionOnViewWillAppear = false
}
func setupTable() {
navigationItem.backBarButtonItem = UIBarButtonItem(title: "Restaurant", style: .plain, target: nil, action: nil)
tableView.register(RestaurantCell.self, forCellReuseIdentifier: "Cell")
tableView.delegate = self
tableView.dataSource = self
let navigationBarHeight: CGFloat = self.navigationController!.navigationBar.frame.height
tableView.contentInset = UIEdgeInsets(top: 0, left: 0, bottom: -navigationBarHeight, right: 0)
tableView.separatorStyle = .none
tableView.showsVerticalScrollIndicator = false
tableView.addSubview(loadingShimmer)
loadingShimmer.topAnchor.constraint(equalTo: tableView.safeAreaLayoutGuide.topAnchor).isActive = true
loadingShimmer.leadingAnchor.constraint(equalTo: tableView.leadingAnchor).isActive = true
loadingShimmer.trailingAnchor.constraint(equalTo: tableView.trailingAnchor).isActive = true
loadingShimmer.startShimmering()
initialSetup()
}
func initialSetup() {
let addressOne = planDictionary["addressOne"] as! String + ", "
let city = planDictionary["city"] as! String + ", "
let postalCode = planDictionary["postalCode"] as! String
currentAddress = addressOne + city + postalCode
setupSearch()
listViewModel.fetchRestaurantCallback { (restaurants, error, result) in
switch result {
case .success :
self.loadingShimmer.stopShimmering()
self.loadingShimmer.removeFromSuperview()
guard let fetchedRestaurants = restaurants else { return }
self.restaurantArray.append(contentsOf: fetchedRestaurants)
self.tableView.reloadData()
self.fetchMore = false
case .location :
self.showAlert(alertTitle: "No businesses nearby", message: "Try going back and changing the address")
case .error :
guard let error = error else { return }
self.showAlert(alertTitle: "Error", message: error)
case .end :
self.fetchMore = false
self.reachedEnd = true
}
}
if let currentAddress = currentAddress {
listViewModel.fetchRestaurants(address: currentAddress)
}
}
override func scrollViewDidEndDecelerating(_ scrollView: UIScrollView) {
let off = scrollView.contentOffset.y
let off1 = scrollView.contentSize.height
if off > off1 - scrollView.frame.height * leadingScreensForBatching {
print("\(fetchMore), \(reachedEnd)")
if !fetchMore && !reachedEnd {
if let address = self.currentAddress {
print("address sent: \(address)")
listViewModel.fetchRestaurants(address: address)
}
}
}
}
}

Answer 1

That you're only getting back 4 results instead of 10 is not due to a faulty query or get-document request—those are coded properly. You're either losing documents when you parse them (some are failing Restaurant initialization), Constants.Mealplan.LOADLIMIT is wrong, or there aren't more than 4 documents in the collection itself that satisfy the query.
That the query is executed 3 times instead of once is also not due to anything in this code—viewDidLoad is only called once and geocodeAddressString only returns once. You're making a fetch request elsewhere that we can't see.
In the batchFetch method, you have a guard that returns out of the function without ever calling its completion handler. This will leave the UI in a state of limbo. I'd recommend always calling the completion handler no matter why the function returns.
You never manage the document cursor. If the get-document return has less documents than the load limit, then nil the last-document cursor. This way, when you attempt to get the next page of documents, guard against a nil cursor and see if there is even more to fetch.
There's no need to pass in an empty array and have your function fill it; simply construct and return an array of results within ListViewModel itself.
We can't see how you trigger pagination. Is it through a scroll delegate when the user reaches the bottom or through a button tap, for example? If it's through a scroll delegate, then I'd disable that for now and see how many returns you get—I suspect one, instead of 3.
What is the particular reason you've ditched MVC for MVVM here? With MVC, you can get pagination up with just a few lines of code. I think MVVM is overkill for iOS applications and would advise against using it unless you have a compelling reason.