具有CGImageSourceCreateThumbnailAtIndex的DownSample使内存急剧增加

Question

我使用CGImageSourceCreateThumbnailAtIndex对一个大小为23622××11811的巨大图像进行下采样。我将kCGImageSourceThumbnailMaxPixelSize键设置为4683。然后，图像将被调整为4683 * 2341，解码后将占用大约43877376.0字节。但是当我运行CGImageSourceCreateThumbnailAtIndex时，内存增加了约230MB。这是我的代码。有什么不对？

    private func resizeImageIfNeed(_ url: URL, complete: @escaping (UIImage?, URL) -> Void) {
        let destSize = ImageDownsampleUtils.destSize(of: url)
        DispatchQueue.global().async {[weak self] in
            let sourceOptions = [kCGImageSourceShouldCache: false] as CFDictionary
            guard let source = CGImageSourceCreateWithURL(url as CFURL, sourceOptions) else {
                complete(nil, url)
                return
            }
            let downsampleOptions = [kCGImageSourceCreateThumbnailFromImageAlways: true,
                                     kCGImageSourceThumbnailMaxPixelSize: destSize,
                                     kCGImageSourceShouldCacheImmediately: true,
                                     kCGImageSourceCreateThumbnailWithTransform: true] as CFDictionary
            if let downsampledImage = CGImageSourceCreateThumbnailAtIndex(source, 0, downsampleOptions) {
                let img = UIImage(cgImage: downsampledImage)
                complete(img, url)
            } else {
                complete(nil, url)
            }
        }
    }

----------更新---------------

我发现使用iOS 13时，CGImageSourceCreateThumbnailAtIndex没有内存高峰。我不知道这是否是iOS 12的错误。