Downsample images to reduce memory impact

ImageDownsampler.swift

enum ImageDownsampler {
    private static let pixelSize = 2_000

    /// - URL: https://christianselig.com/2020/09/phpickerviewcontroller-efficiently/
    static func downsample(imageAt url: URL) -> Data? {
        let sourceOptions: [CFString: Any] = [kCGImageSourceShouldCache: false]
        let downsampleOptions: [CFString: Any] = [
            kCGImageSourceCreateThumbnailFromImageAlways: true,
            kCGImageSourceCreateThumbnailWithTransform: true,
            kCGImageSourceThumbnailMaxPixelSize: pixelSize
        ]
        let destinationOptions: [CFString: Any] = [
            kCGImageDestinationLossyCompressionQuality: 0.95
        ]

        let data = NSMutableData()

        guard
            let source = CGImageSourceCreateWithURL(url as CFURL, sourceOptions as CFDictionary),
            let cgImage = CGImageSourceCreateThumbnailAtIndex(source, 0, downsampleOptions as CFDictionary),
            let imageDestination = CGImageDestinationCreateWithData(data, kUTTypeJPEG, 1, nil)
        else { return nil }

        CGImageDestinationAddImage(imageDestination, cgImage, destinationOptions as CFDictionary)
        CGImageDestinationFinalize(imageDestination)

        return data as Data
    }

    static func downsample(data sourceData: Data, quality: CGFloat) -> Data? {
        let sourceOptions: [CFString: Any] = [kCGImageSourceShouldCache: false]
        let downsampleOptions: [CFString: Any] = [
            kCGImageSourceCreateThumbnailFromImageAlways: true,
            kCGImageSourceCreateThumbnailWithTransform: true,
            kCGImageSourceThumbnailMaxPixelSize: pixelSize
        ]
        let destinationOptions: [CFString: Any] = [
            kCGImageDestinationLossyCompressionQuality: quality
        ]

        let targetData = NSMutableData()

        guard
            let source = CGImageSourceCreateWithData(sourceData as CFData, sourceOptions as CFDictionary),
            let cgImage = CGImageSourceCreateThumbnailAtIndex(source, 0, downsampleOptions as CFDictionary),
            let imageDestination = CGImageDestinationCreateWithData(targetData, kUTTypeJPEG, 1, nil)
        else { return nil }

        CGImageDestinationAddImage(imageDestination, cgImage, destinationOptions as CFDictionary)
        CGImageDestinationFinalize(imageDestination)

        return targetData as Data
    }

    /// - URL: https://developer.apple.com/videos/play/wwdc2018-219/?time=640
    static func downsample(imageAt data: Data, toSize size: CGSize, scale: CGFloat) -> UIImage? {
        let sourceOptions: [CFString: Any] = [kCGImageSourceShouldCache: false]
        let maxPixelSize: CGFloat = max(size.height, size.width) * scale
        let downsampleOptions: [CFString: Any] = [
            kCGImageSourceCreateThumbnailFromImageAlways: true,
            kCGImageSourceShouldCacheImmediately: true,
            kCGImageSourceCreateThumbnailWithTransform: true,
            kCGImageSourceThumbnailMaxPixelSize: maxPixelSize
        ]

        guard
            let imageSource = CGImageSourceCreateWithData(data as CFData, sourceOptions as CFDictionary),
            let downsampledImage = CGImageSourceCreateThumbnailAtIndex(
                imageSource,
                0,
                downsampleOptions as CFDictionary
            )
        else { return nil }

        return UIImage(cgImage: downsampledImage)
    }
}