Reliably using UICollectionView indexPath to access dictionary keys? - swift

Question

I'm trying to use the current CollectionViewcell's IndexPath to access data from a dictionary. The dictionary's keys are of type Int.
This CollectionView has "full page" cells, meaning that each cell takes up the full view area where I am using horizontal scrolling (paging enabled) to navigate between cells.
The dictionary is :
var dataFromServer: [Int: [VillageFestival]]?
Each CollectionViewCell has a TableView inside it, where I plan to have a variable number of rows, depending on how many items there are in [VillageFestival]
However, in CollectionView cellForItemAt indexPath the behaviour of the method is causing some trouble, as printing indexPath.item or setting it as my navigationController's title, returns odd but "understandable" results given how I think dequeueReusableCell works?...
Example: the current index is 0. When I scroll to the right, the current index is now 2. If I navigate to page 6 and then one page back, the current index indicates 3.
I have changed my dictionary keys from Date to String and now to Int, in an attempt to simplify logic.. But the problem persists.
I am using a global pageIndex: Int that is being updated inside CollectionView cellForItemAt
override func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
pageIndex = indexPath.item
self.navigationController?.navigationBar.topItem?.title = String(pageIndex)
let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "collectionViewCell", for: indexPath) as! CollectionViewCell
// CollectionViewCell resize to CollectionView Bounds
cell.tableViewCellOffsets = (UIApplication.shared.statusBarFrame.size.height + (self.navigationController?.navigationBar.frame.height ?? 0.0) , self.tabBarController?.tabBar.frame.height ?? 0)
return cell
}
In Tableview numberOfRowsInSection, I'm using pageIndex to access the dictionary values.
func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
guard let festivals = dataFromServer?[pageIndex] else {return 0}
return festivals.count
}
With my current code, the app displays 0 rows for some pages and 1 row for others. My guess is that the collectionView's cellForItemAt is called before (also possibly after?) the tableView's methods and this makes using a global pageIndex unreliable...
Thanks!

Answer 1

Try this playground, it might help you:
import UIKit
import PlaygroundSupport
class Cell: UICollectionViewCell, UITableViewDataSource {
var data: [Int]! = [] {
didSet {
tableView.reloadData()
}
}
private let tableView: UITableView
override init(frame: CGRect) {
tableView = UITableView(frame: .zero, style: .plain)
super.init(frame: frame)
tableView.register(UITableViewCell.self, forCellReuseIdentifier: "Cell")
tableView.dataSource = self
tableView.translatesAutoresizingMaskIntoConstraints = false
contentView.addSubview(tableView)
NSLayoutConstraint.activate([
tableView.leadingAnchor.constraint(equalTo: contentView.leadingAnchor),
tableView.trailingAnchor.constraint(equalTo: contentView.trailingAnchor),
tableView.topAnchor.constraint(equalTo: contentView.topAnchor),
tableView.bottomAnchor.constraint(equalTo: contentView.bottomAnchor)
])
}
required init?(coder aDecoder: NSCoder) {
fatalError()
}
func numberOfSections(in tableView: UITableView) -> Int {
return 1
}
func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
return data.count
}
func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
let cell = tableView.dequeueReusableCell(withIdentifier: "Cell", for: indexPath)
cell.textLabel?.text = String(data[indexPath.item])
return cell
}
}
class VC: UICollectionViewController {
let data = [
0: [1, 2, 3, 4, 5],
1: [6, 4],
2: [5, 5, 5, 5, 6],
3: [9, 9, 8, 4, 5, 5, 5]
]
override init(collectionViewLayout layout: UICollectionViewLayout) {
super.init(collectionViewLayout: layout)
collectionView.isPagingEnabled = true
collectionView.register(Cell.self, forCellWithReuseIdentifier: "Cell")
}
required init?(coder aDecoder: NSCoder) {
fatalError()
}
override func numberOfSections(in collectionView: UICollectionView) -> Int {
return 1
}
override func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
return data.count
}
override func viewDidLayoutSubviews() {
(collectionViewLayout as? UICollectionViewFlowLayout)?.itemSize = collectionView.bounds.size
}
override func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "Cell", for: indexPath) as! Cell
cell.data = data[indexPath.item]
return cell
}
}
let layout = UICollectionViewFlowLayout()
layout.scrollDirection = .horizontal
PlaygroundPage.current.liveView = VC(collectionViewLayout: layout)