Xcode Swift UICollectionView问题：numberOfItemsInSection已调用但cellForItemAt IndexPath未执行

Hey there, I know how frustrating it is to spend hours stuck on a collection view issue where part of the data source works but cells never get created. Let’s walk through the most likely fixes for your scenario:

1. Confirm messages.count is actually greater than 0 #

Even though numberOfItemsInSection is being called, if messages.count returns 0, the system has no reason to trigger cellForItemAt. Add a print statement to verify the actual count:

override func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
    print("Actual message count: \(messages.count)") // Add this line to check
    print("Item wurde erstellt")
    return messages.count
}

If the count is 0, you’ll need to debug why your messages array isn’t being populated correctly before the collection view loads.

2. Validate your collection view layout configuration #

Since you’re using a code-only UICollectionViewController, the default flow layout might have invalid item sizes (like width/height set to 0) that prevent cells from being rendered (and thus cellForItemAt from firing). Test with a fixed, visible item size in viewDidLoad:

override func viewDidLoad() {
    super.viewDidLoad()
    // Double-check delegate/data source assignments (you mentioned this, but it never hurts to confirm)
    collectionView.delegate = self
    collectionView.dataSource = self
    
    // Override default flow layout settings
    guard let flowLayout = collectionView.collectionViewLayout as? UICollectionViewFlowLayout else {
        print("Collection view isn't using a flow layout!")
        return
    }
    flowLayout.itemSize = CGSize(width: view.bounds.width - 40, height: 60) // Fixed visible size
    flowLayout.minimumLineSpacing = 8
    flowLayout.sectionInset = UIEdgeInsets(top: 16, left: 20, bottom: 16, right: 20)
}

If cells start appearing after this, your original dynamic sizing logic was likely causing items to have zero dimensions.

3. Double-check cell registration #

Make sure you’ve registered your ChatMessageCollectionViewCell class with the collection view using your cellId in viewDidLoad:

override func viewDidLoad() {
    super.viewDidLoad()
    collectionView.register(ChatMessageCollectionViewCell.self, forCellWithReuseIdentifier: cellId)
    // ... other setup code
}

While missing registration usually causes a crash, typos in the reuse identifier string can lead to unexpected behavior—worth ruling out.

4. Verify estimateFrameForText isn’t returning invalid values #

Though this shouldn’t prevent cellForItemAt from running, if your estimateFrameForText function returns a width of 0, your cell’s bubble view might collapse. Add a print to confirm it returns a valid value:

func estimateFrameForText(text: String) -> CGRect {
    let size = CGSize(width: 200, height: .infinity)
    let options = NSStringDrawingOptions.usesFontLeading.union(.usesLineFragmentOrigin)
    let rect = text.boundingRect(with: size, options: options, attributes: [.font: UIFont.systemFont(ofSize: 16)], context: nil)
    print("Estimated text width: \(rect.width)") // Add this to check
    return rect
}

Give these steps a try—more often than not, the issue is either an empty data array or a misconfigured layout that the system interprets as needing no cells to render.

内容的提问来源于stack exchange，提问作者user15816962