ARQuicklook - Fatal error: Unexpectedly found nil while unwrapping an Optional value

Question

我正在关注 this tutorial 如何制作 AR Quicklook 应用。只有几个步骤，看起来很简单。但是在最后一步我得到了一个致命错误，因为 let 变量被强制展开。我尝试将其设为可选，但出现不同的错误，例如：

Optional chain has no effect, expression already produces 'URL?'

如果我删除可选的，我会在下一行收到此警告：

'URL?' is not convertible to 'QLPreviewItem'; did you mean to use 'as!' to force downcast?

如果我强行解开这一行，应用程序就会崩溃。我不知道如何解决这个问题。我什至看过官方视频 here 并且在大约 14:30 分钟时他们也有相同的代码，他们强制展开那条线。

@IBOutlet var collectionView: UICollectionView!
let models = ["A", "B", "C", "D", "E"]

var thumbnails = [UIImage]()
var thumbnailIndex = 0

override func viewDidLoad() {
    super.viewDidLoad()
    for model in models {
        if let thumbnail = UIImage(named: "\(model).jpg") {
            thumbnails.append(thumbnail)
        }
    }

    collectionView.dataSource = self
    collectionView.delegate = self
    collectionView.reloadData()
}

func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
    return models.count
}

func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
    let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "LibraryCell", for: indexPath) as? LibraryCollectionViewCell

    if let cell = cell {
        cell.modelThumbnail.image = thumbnails[indexPath.item]
        let title = models[indexPath.item]
        cell.modelTitle.text = title.capitalized
    }

    return cell!
}

func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
    thumbnailIndex = indexPath.item

    let previewController = QLPreviewController()
    previewController.dataSource = self
    previewController.delegate = self
    present(previewController, animated: true)
}

func numberOfPreviewItems(in controller: QLPreviewController) -> Int {
    return 1
}

func previewController(_ controller: QLPreviewController, previewItemAt index: Int) -> QLPreviewItem {
    let url = Bundle.main.url(forResource: models[thumbnailIndex], withExtension: "usdz")!
    return url as QLPreviewItem
}

Answer 1

这个

let url = Bundle.main.url(forResource: models[thumbnailIndex], withExtension: "usdz")!

如果主捆绑包中不存在该项目，或者存在但未检查目标成员资格，则只能return零，因此请验证所有这些资源是否存在

A.usdz,B.usdz,C.usdz,D.usdz,E.usdz

教程制作者在此处显示

ARQuicklook - Fatal error: Unexpectedly found nil while unwrapping an Optional value

ARQuicklook - Fatal error: Unexpectedly found nil while unwrapping an Optional value

xcode

quicklook

ios

swift