Nested CollectionView doesn't receive delegate events? - ios

Question

I have a horizontal collection view embedded in the cell of a vertical collection view (a carousel in a feed unit). For some strange reason, the inner scroll view is not receiving scroll view delegate events like scrollViewDidScroll. It does scroll, I just don't get any notifications about it.
The outer scrollview does, however, receive these events. I suspected the gesture recognizers might be interfering with eachother but I'm not sure. Do you have any idea what might be going wrong here or how I could get the inner collection view to behave normally?
EDIT: I determined that the source of the bug is actually a bug / undefined behavior with compositional layouts with orthogonal scrolling behavior:
https://developer.apple.com/forums/thread/127825
Basically they internally create additional horizontal collection views whose delegate events are not propagated upwards. That said, the approach in the accepted answer does work since it's not using this behavior.

Answer 1

Quick testing -- "horizontal collection view embedded in the cell of a vertical collection view" -- no trouble getting scrollViewDidScroll events for both.
Here's a quick example:
// simple collection view cell with centered label
class HorizItemCell: UICollectionViewCell {
let theLabel: UILabel = {
let v = UILabel()
v.textAlignment = .center
v.backgroundColor = UIColor(white: 0.95, alpha: 1.0)
v.translatesAutoresizingMaskIntoConstraints = false
return v
}()
override init(frame: CGRect) {
super.init(frame: frame)
commonInit()
}
required init?(coder: NSCoder) {
super.init(coder: coder)
commonInit()
}
func commonInit() {
contentView.addSubview(theLabel)
let g = contentView.layoutMarginsGuide
NSLayoutConstraint.activate([
theLabel.leadingAnchor.constraint(equalTo: g.leadingAnchor, constant: 12.0),
theLabel.trailingAnchor.constraint(equalTo: g.trailingAnchor, constant: -12.0),
theLabel.centerYAnchor.constraint(equalTo: g.centerYAnchor),
])
}
func fillData(_ str: String, bk: UIColor) {
theLabel.text = str
contentView.backgroundColor = bk
}
}
// collection view cell with horizontal scrolling collection view
class VertItemCell: UICollectionViewCell, UICollectionViewDataSource, UICollectionViewDelegate {
var cv: UICollectionView!
var nItems: Int = 0
var bkColor: UIColor = .white
var vertRow: Int = 0
let theLabel: UILabel = {
let v = UILabel()
v.font = .systemFont(ofSize: 14.0, weight: .light)
v.translatesAutoresizingMaskIntoConstraints = false
return v
}()
override init(frame: CGRect) {
super.init(frame: frame)
commonInit()
}
required init?(coder: NSCoder) {
super.init(coder: coder)
commonInit()
}
func commonInit() {
let fl = UICollectionViewFlowLayout()
fl.scrollDirection = .horizontal
fl.estimatedItemSize = CGSize(width: 120, height: 36)
cv = UICollectionView(frame: .zero, collectionViewLayout: fl)
cv.translatesAutoresizingMaskIntoConstraints = false
contentView.addSubview(theLabel)
contentView.addSubview(cv)
let g = contentView.layoutMarginsGuide
NSLayoutConstraint.activate([
theLabel.topAnchor.constraint(equalTo: g.topAnchor, constant: 0.0),
theLabel.leadingAnchor.constraint(equalTo: g.leadingAnchor, constant: 0.0),
theLabel.trailingAnchor.constraint(equalTo: g.trailingAnchor, constant: 0.0),
cv.topAnchor.constraint(equalTo: theLabel.bottomAnchor, constant: 4.0),
cv.leadingAnchor.constraint(equalTo: g.leadingAnchor, constant: 4.0),
cv.trailingAnchor.constraint(equalTo: g.trailingAnchor, constant: -4.0),
cv.bottomAnchor.constraint(equalTo: g.bottomAnchor, constant: -4.0),
])
cv.register(HorizItemCell.self, forCellWithReuseIdentifier: "hc")
cv.dataSource = self
cv.delegate = self
contentView.layer.borderWidth = 2
contentView.layer.borderColor = UIColor.lightGray.cgColor
contentView.layer.cornerRadius = 12
}
func fillData(rowNum: Int, numItems: Int, bk: UIColor) {
nItems = numItems
bkColor = bk
vertRow = rowNum
theLabel.text = "Vertical Row: \(vertRow)"
}
func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
return nItems
}
func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
let c = collectionView.dequeueReusableCell(withReuseIdentifier: "hc", for: indexPath) as! HorizItemCell
c.fillData("\(indexPath.item)", bk: bkColor)
return c
}
func scrollViewDidScroll(_ scrollView: UIScrollView) {
print(type(of: self), "Horizontal Scrolling - Row:", vertRow, "contentOffset:", scrollView.contentOffset)
}
}
class TestingVC: UIViewController, UICollectionViewDataSource, UICollectionViewDelegate {
var cv: UICollectionView!
let colors: [UIColor] = [
.systemRed, .systemGreen, .systemBlue,
.cyan, .magenta, .yellow,
.red, .green, .blue,
.orange, .purple,
]
let counts: [Int] = [
10, 12, 19, 13, 8, 11, 17, 14, 16, 9, 15,
]
override func viewDidLoad() {
super.viewDidLoad()
let fl = UICollectionViewFlowLayout()
fl.scrollDirection = .vertical
cv = UICollectionView(frame: .zero, collectionViewLayout: fl)
cv.translatesAutoresizingMaskIntoConstraints = false
view.addSubview(cv)
let safeG = view.safeAreaLayoutGuide
NSLayoutConstraint.activate([
cv.topAnchor.constraint(equalTo: safeG.topAnchor, constant: 20.0),
cv.leadingAnchor.constraint(equalTo: safeG.leadingAnchor, constant: 20.0),
cv.trailingAnchor.constraint(equalTo: safeG.trailingAnchor, constant: -20.0),
cv.bottomAnchor.constraint(equalTo: safeG.bottomAnchor, constant: -20.0),
])
cv.register(VertItemCell.self, forCellWithReuseIdentifier: "vc")
cv.dataSource = self
cv.delegate = self
}
override func viewDidLayoutSubviews() {
super.viewDidLayoutSubviews()
if let fl = cv.collectionViewLayout as? UICollectionViewFlowLayout {
fl.itemSize = CGSize(width: cv.frame.width, height: 80.0)
}
}
func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
return 12
}
func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
let c = collectionView.dequeueReusableCell(withReuseIdentifier: "vc", for: indexPath) as! VertItemCell
c.fillData(rowNum: indexPath.row, numItems: counts[indexPath.item % counts.count], bk: colors[indexPath.item % colors.count])
return c
}
func scrollViewDidScroll(_ scrollView: UIScrollView) {
print(type(of: self), "Vertical Scrolling", scrollView.contentOffset)
}
}
It looks like this when running:
and this is some sample Debug Console output:
TestingVC Vertical Scrolling (0.0, 100.5)
TestingVC Vertical Scrolling (0.0, 107.5)
TestingVC Vertical Scrolling (0.0, 115.5)
VertItemCell Horizontal Scrolling - Row: 3 contentOffset: (314.5, 0.0)
VertItemCell Horizontal Scrolling - Row: 3 contentOffset: (315.0, 0.0)
VertItemCell Horizontal Scrolling - Row: 4 contentOffset: (1.0, 0.0)
VertItemCell Horizontal Scrolling - Row: 4 contentOffset: (22.5, 0.0)
VertItemCell Horizontal Scrolling - Row: 5 contentOffset: (308.0, 0.0)
VertItemCell Horizontal Scrolling - Row: 5 contentOffset: (307.5, 0.0)
VertItemCell Horizontal Scrolling - Row: 6 contentOffset: (12.5, 0.0)
VertItemCell Horizontal Scrolling - Row: 6 contentOffset: (32.5, 0.0)