ThumbnailSequence.swift

struct ThumbnailSequence: AsyncSequence {
    typealias AsyncIterator = Iterator
    typealias Element = UIImage
    
    var imageNames: [String]
    let maxSize: CGSize
    
    func makeAsyncIterator() -> Iterator {
        return Iterator(imageNames: imageNames, maxSize: maxSize)
    }
    
    struct Iterator: AsyncIteratorProtocol {
        typealias Element = UIImage
        var imageNames: [String]
        let maxSize: CGSize

        mutating func next() async -> UIImage? {
            guard !imageNames.isEmpty else { return nil }
            guard let image = UIImage(named: imageNames.removeFirst()) else { return nil }
            let ratio = image.size.height / maxSize.height // simplified scaling
            return await image.byPreparingThumbnail(ofSize: CGSize(width: image.size.width / ratio, height: maxSize.height))
        }
    }
}