How to create NSCollectionView programatically?

Code Block 
        view.register(HostingCellView.self, forItemWithIdentifier: Cell.reuseIdentifier)
        view.register(HostingSupplementaryView.self, forSupplementaryViewOfKind: Self.headerIdentifier, withIdentifier: Header.reuseIdentifier)
        view.register(HostingSupplementaryView.self, forSupplementaryViewOfKind: Self.footerIdentifier, withIdentifier: Footer.reuseIdentifier)

Code Block 
import SwiftUI
import AppKit
final class HostingCellView: NSCollectionViewItem {
    func setView<Content>(_ newValue: Content) where Content: View {
        for view in self.view.subviews {
            view.removeFromSuperview()
        }
        let view = NSHostingView(rootView: newValue)
        view.autoresizingMask = [.width, .height]
        self.view.addSubview(view)
    }
}
struct Cell: View {
    static let reuseIdentifier = NSUserInterfaceItemIdentifier("Cell")
    var body: some View {
        Text("Cell")
    }
}
final class HostingSupplementaryView: NSView, NSCollectionViewElement {
    override init(frame frameRect: NSRect) {
        super.init(frame: frameRect)
    }
    @objc required dynamic init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
    func setView<Content>(_ newValue: Content) where Content: View {
        for view in self.subviews {
            view.removeFromSuperview()
        }
        let view = NSHostingView(rootView: newValue)
        view.autoresizingMask = [.width, .height]
        self.addSubview(view)
    }
}
struct Header: View {
    static let reuseIdentifier = NSUserInterfaceItemIdentifier("Header")
    var body: some View {
        Text("Header")
    }
}
struct Footer: View {
    static let reuseIdentifier = NSUserInterfaceItemIdentifier("Footer")
    var body: some View {
        Text("Footer")
    }
}
struct Collection: NSViewRepresentable {
    static let headerIdentifier = "ViewHeader"
    static let footerIdentifier = "ViewFooter"
    typealias NSViewType = NSCollectionView
    func makeNSView(context: Context) -> NSCollectionView {
        let view = NSCollectionView()
        view.delegate = context.coordinator
        view.register(HostingCellView.self, forItemWithIdentifier: Cell.reuseIdentifier)
        view.register(HostingSupplementaryView.self, forSupplementaryViewOfKind: Self.headerIdentifier, withIdentifier: Header.reuseIdentifier)
        view.register(HostingSupplementaryView.self, forSupplementaryViewOfKind: Self.footerIdentifier, withIdentifier: Footer.reuseIdentifier)
        view.collectionViewLayout = NSCollectionViewCompositionalLayout { (sectionIndex: Int, layoutEnvironment: NSCollectionLayoutEnvironment) -> NSCollectionLayoutSection in
            let itemSize = NSCollectionLayoutSize(widthDimension: .fractionalWidth(1.0), heightDimension: .fractionalHeight(1.0))
            let item = NSCollectionLayoutItem(layoutSize: itemSize)
            let group = NSCollectionLayoutGroup.vertical(layoutSize: itemSize, subitem: item, count: 1)
            let section = NSCollectionLayoutSection(group: group)
            section.orthogonalScrollingBehavior = .continuous
            let supplementarySize = NSCollectionLayoutSize(widthDimension: .fractionalWidth(1.0), heightDimension: .estimated(44))
            let header = NSCollectionLayoutBoundarySupplementaryItem(layoutSize: supplementarySize, elementKind: Self.headerIdentifier, alignment: .topLeading)
            let footer = NSCollectionLayoutBoundarySupplementaryItem(layoutSize: supplementarySize, elementKind: Self.footerIdentifier, alignment: .bottomTrailing)
            section.boundarySupplementaryItems = [header, footer]
            return section
        }
        let dataSource = NSCollectionViewDiffableDataSource<Int, Int>(collectionView: view) { (view, indexPath, sectionIndex) -> NSCollectionViewItem? in
            guard let item = view.makeItem(withIdentifier: Cell.reuseIdentifier, for: indexPath) as? HostingCellView else {
                fatalError()
            }
            item.setView(Cell())
            return item
        }
        dataSource.supplementaryViewProvider = { (view: NSCollectionView, kind: String, indexPath: IndexPath) -> (NSView & NSCollectionViewElement)? in
            switch kind {
            case Self.headerIdentifier:
                guard let supplementary = view.makeSupplementaryView(ofKind: kind, withIdentifier: Header.reuseIdentifier, for: indexPath) as? HostingSupplementaryView else {
                    fatalError()
                }
                supplementary.setView(Header())
                return supplementary
            case Self.footerIdentifier:
                guard let supplementary = view.makeSupplementaryView(ofKind: kind, withIdentifier: Footer.reuseIdentifier, for: indexPath) as? HostingSupplementaryView else {
                    fatalError()
                }
                supplementary.setView(Footer())
                return supplementary
            default:
                return nil
            }
        }
        context.coordinator.dataSource = dataSource
        return view
    }
    func updateNSView(_ nsView: NSCollectionView, context: Context) {
        var snapshot = NSDiffableDataSourceSnapshot<Int, Int>()
        snapshot.appendSections([0])
//        snapshot.appendItems(Array<Int>(0..<10), toSection: 0)
        context.coordinator.dataSource?.apply(snapshot, animatingDifferences: true)
    }
    func makeCoordinator() -> Coordinator {
        return Coordinator(self)
    }
    class Coordinator: NSObject {
        let view: Collection
        var dataSource: NSCollectionViewDiffableDataSource<Int, Int>? = nil
        init(_ view: Collection) {
            self.view = view
        }
    }
}
extension Collection.Coordinator: NSCollectionViewDelegate {}

Were you able to solve this problem? I'm encountering it as well and I hit the runtime error whether I register an itemClass or a nib. This is a big problem if I try to implement an NSCollectionView in a framework since AppKit seems to try to load the nib from the app bundle and not the framework bundle, and there's no way to configure it to use the framework bundle. My workaround was to add the nib to the target project instead of defining it in the target framework.

I am an AppKit noob with a UIKit background who just stumbled on the same issue. However, this unrelated StackOverflow question gives me some insight: you can override loadView of a NSCollectionViewItem and simply create any view programmatically:

class TestViewItem: NSCollectionViewItem {
    override func loadView() {
        self.view = NSView()
    }
}

I suppose this is the place where the default implementation from AppKit looks for NIB files.

Seeing the same behavior. Not sure what's going on, but I was able to work around it by making the reuse identifier the same as the nib name. E.g.:

    static let reuseIdentifier = NSUserInterfaceItemIdentifier("ListItem") // was "list-item-reuse-identifier"

I suspect I'm now picking up some fallback behavior.

-Steve

Setting collectionViewLayout somehow clear the cell class registrations, causing makeItem(withIdentifier:for:) to dequeue the cell from nib unexpectedly.

This seems like a bug to me, so I fire a feedback FB14769347.

As a workaround, set collectionViewLayout BEFORE calling your register(_:for:) methods.