AVAsset+Extension.swift

extension AVAsset {
    private func _generator(size: CGSize) -> AVAssetImageGenerator {
        let generator = AVAssetImageGenerator(asset: self)
        generator.maximumSize = size
        generator.appliesPreferredTrackTransform = true
        return generator
    }

    func extractUIImageAsync(size: CGSize) -> Single<UIImage?> {
        let generator = _generator(size: size)
        return .create { observer in
            generator.generateCGImagesAsynchronously(forTimes: [NSValue(time: .zero)]) { (_, image, _, _, error) in
                if let error = error {
                    observer(.error(error))
                    return
                }
                observer(.success(image.map(UIImage.init(cgImage:))))
            }
            return Disposables.create {
                generator.cancelAllCGImageGeneration()
            }
        }
    }
}