UICollectionView reloadData only works one time in two

This is a very very weird bug that happens when you use PDFView in the UICollectionViewCell. I confirmed this in following environment -

1. Xcode 12.5
2. iPhone SE 2020 (iOS 14.6)

UICollectionView.reloadData() calls are not reliably working when PDFView is added as a subview inside UICollectionViewCell.contentView.

What else can we try?

Surprisingly UICollectionView.insertItems(at:) works where UICollectionView.reloadData() doesn't for this case. There's working code sample provided at the end of this answer for anyone else trying to reproduce/confirm the issue.

Why this might be happening?

Honestly no idea. UICollectionView.reloadData() is supposed to guarantee that UI is in sync with your dataSource. Let's look at the stack traces of reloadData() (when it works in this case) & insertItems(at:).

ReloadData_StackTrace

InsertItemsAtIndexPaths_StackTrace

Conclusion

1. reloadData() relies on layoutSubviews() to perform the UI refresh. This is inherited from UIView like - UIView.layoutSubviews() > UIScrollView > UICollectionView. It's a very well known UI event and can easily be intercepted by any subclass of UIView. PDFView: UIView can also do that. Why does it happen inconsistently? Only someone who can disassemble & inspect the PDFKit.framework may know about this. This is clearly a bug in PDFView.layoutSubviews() implementation that interferes with it's superview's layoutSubviews() implementation.

2. insertItems(at:) adds new instance(s) of cell(s) at specified indexPath(s) and clearly does not rely on layoutSubviews() and hence works reliably in this case.

Sample Code

import MobileCoreServices
import PDFKit
import UIKit

class ViewController: UIViewController {
    
    // MARK: - Instance Variables
    private lazy var flowLayout: UICollectionViewFlowLayout = {
        let sectionInset = UIEdgeInsets(top: 20, left: 20, bottom: 20, right: 20)
        
        let layout = UICollectionViewFlowLayout()
        layout.scrollDirection = .vertical
        layout.sectionInset = sectionInset
        layout.itemSize = CGSize(width: 150, height: 150)
        layout.minimumInteritemSpacing = 20
        layout.minimumLineSpacing = 20
        
        return layout
    }()
    
    private lazy var pdfsCollectionView: UICollectionView = {
        let cv = UICollectionView(frame: self.view.bounds, collectionViewLayout: flowLayout)
        cv.autoresizingMask = [.flexibleWidth, .flexibleHeight]
        cv.backgroundColor = .red
        
        cv.dataSource = self
        cv.delegate = self
        return cv
    }()
    
    private lazy var pickFileButton: UIButton = {
        let button = UIButton(frame: CGRect(x: 300, y: 610, width: 60, height: 40)) // hard-coded for iPhone SE
        button.setTitle("Pick", for: .normal)
        button.setTitleColor(.white, for: .normal)
        button.backgroundColor = .purple
        
        button.addTarget(self, action: #selector(pickFile), for: .touchUpInside)
        return button
    }()
    
    private var urls: [URL] = []
    
    
    // MARK: - View Life Cycle
    override func viewDidLoad() {
        super.viewDidLoad()
        
        self.view.addSubview(pdfsCollectionView)
        pdfsCollectionView.register(
            PDFCollectionViewCell.self,
            forCellWithReuseIdentifier: PDFCollectionViewCell.cellIdentifier
        )
        
        self.view.addSubview(pickFileButton)
    }
    
    
    // MARK: - Helpers
    @objc private func pickFile() {
        let documentPicker = UIDocumentPickerViewController(documentTypes: [kUTTypePDF as String], in: .import)
        documentPicker.delegate = self
        documentPicker.modalPresentationStyle = .formSheet
        self.present(documentPicker, animated: true, completion: nil)
    }
    
}

extension ViewController: UICollectionViewDataSource, UICollectionViewDelegate {
    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: PDFCollectionViewCell.cellIdentifier, for: indexPath) as! PDFCollectionViewCell
        cell.pdfView.document = PDFDocument(url: urls[indexPath.row])
        cell.contentView.backgroundColor = .yellow
        return cell
    }
    
    func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return urls.count
    }
}

extension ViewController: UIDocumentPickerDelegate {
    func documentPickerWasCancelled(_ controller: UIDocumentPickerViewController) {
        controller.dismiss(animated: true, completion: nil)
    }
    
    func documentPicker(_ controller: UIDocumentPickerViewController, didPickDocumentsAt urls: [URL]) {
        
        // CAUTION:
        // These urls are in the temporary directory - `.../tmp/<CFBundleIdentifier>-Inbox/<File_Name>.pdf`
        // You should move/copy these files to your app's document directory
        
        controller.dismiss(animated: true, completion: {
            DispatchQueue.main.async {
                let count = self.urls.count
                var indexPaths: [IndexPath] = []
                for i in 0..<urls.count {
                    indexPaths.append(IndexPath(item: count+i, section: 0))
                }
                
                self.urls.append(contentsOf: urls)
                
                // Does not work reliably
                /*
                self.pdfsCollectionView.reloadData()
                */
                
                // Works reliably
                self.pdfsCollectionView.insertItems(at: indexPaths)
            }
        })
    }
}


class PDFCollectionViewCell: UICollectionViewCell {
    static let cellIdentifier = "PDFCollectionViewCell"
    
    lazy var pdfView: PDFView = {
        let view = PDFView(frame: self.contentView.bounds)
        view.displayMode = .singlePage
        view.autoScales = true
        view.displayDirection = .vertical
        view.isUserInteractionEnabled = false
        
        self.contentView.addSubview(view)
        view.autoresizingMask = [.flexibleWidth, .flexibleHeight]
        view.backgroundColor = .yellow
        
        return view
    }()
}

This is being caused by the PDFViews in your cells, for reasons I'm not sure about. PDFView may be too heavyweight for use in a collection view cell, or it may have some issue when reloading or re-adding a document. I reproduced your issue without managing to fix it, though you may want to bear these points in mind:

• You're using a deprecated initalizer for the document picker
• The "inbox" url you're given is not guaranteed to be persistent, indeed you can see files appearing and disappearing in there as you select more documents, so you probably want to look into moving selected files into a more permanent home

Addressing these things made no difference to the strange reloading issue. What I did instead was import QuickLookThumbnailing, add an image view to the cell instead of a PDF view, and add this code:

class PDFCollectionViewCell: UICollectionViewCell {
    static let identifier = "PDFCollectionViewCell"
    
    @IBOutlet var imageView: UIImageView!
    private var request: QLThumbnailGenerator.Request?
        
    func load(_ url: URL) {
        let req = QLThumbnailGenerator.Request.init(fileAt: url, size: bounds.size, scale: UIScreen.main.scale, representationTypes: .all)
        
        QLThumbnailGenerator.shared.generateRepresentations(for: req) {
            [weak self]
            rep, type, error in
            DispatchQueue.main.async {
                self?.imageView.image = rep?.uiImage
            }
        }
        request = req
    }
    
    override func prepareForReuse() {
        if let request = request {
            QLThumbnailGenerator.shared.cancel(request)
            self.request = nil
        }
        imageView.image = nil
    }
}

This asynchronously renders thumbnails for whatever URL you pass it, increasing the quality as it goes.

Somehow collectionView caches for milliseconds reloadingData so it wont reload twice as you call reloadData() twice, also based on apple developer documentation:

You should not call this method in the middle of animation blocks where items are being inserted or deleted. Insertions and deletions automatically cause the collection’s data to be updated appropriately.

I have some workaround to achieve this:

Invalidate collectionView layout:

As far as I know there is remove cache method inside collectionViewLayout prepare

collectionView!.reloadData()
collectionView!.collectionViewLayout.invalidateLayout()
collectionView!.layoutSubviews()