下采样图像

Question

我的图片加载代码如下：

final class ImageStore {
    typealias _ImageDictionary = [String: CGImage]
    fileprivate var images: _ImageDictionary = [:]
    
    fileprivate static var scale = 2
    
    static var shared = ImageStore()
    
    func image(name: String) -> Image {
        let index = _guaranteeImage(name: name)
        
        return Image(images.values[index], scale: CGFloat(ImageStore.scale), label: Text(verbatim: name))
    }
    
    static func loadImage(name: String) -> CGImage {
        guard
            let url = Bundle.main.url(forResource: name, withExtension: "jpg"),
            let imageSource = CGImageSourceCreateWithURL(url as NSURL, nil),
            let image = CGImageSourceCreateImageAtIndex(imageSource, 0, nil)
        else {
            fatalError("Couldn't load image \(name).jpg from main bundle.")
        }
        return image
    }
    
    fileprivate func _guaranteeImage(name: String) -> _ImageDictionary.Index {
        if let index = images.index(forKey: name) { return index }
        
        images[name] = ImageStore.loadImage(name: name)
        return images.index(forKey: name)!
    }
}

我有 300 张图片，它们的平均大小为 500 kb（3000 像素 X 3000 像素）。

我用 LazyVGrid 展示给他们看。加载大约 250 张图片后，内存使用量高达 1GB，应用程序崩溃。

我发现每个像素在 RAM 中占用 4 个字节（红色 1 个，绿色 1 个，蓝色 1 个，alpha 1 个），这导致每个图像占用 36 MB 内存。 (3000x3000x4)。见here。

以上链接表明下采样有助于减少内存占用。

这是下采样代码：

func downsample(imageAt imageURL: URL,
                to pointSize: CGSize,
                scale: CGFloat = UIScreen.main.scale) -> UIImage? {

    // Create an CGImageSource that represent an image
    let imageSourceOptions = [kCGImageSourceShouldCache: false] as CFDictionary
    guard let imageSource = CGImageSourceCreateWithURL(imageURL as CFURL, imageSourceOptions) else {
        return nil
    }
    
    // Calculate the desired dimension
    let maxDimensionInPixels = max(pointSize.width, pointSize.height) * scale
    
    // Perform downsampling
    let downsampleOptions = [
        kCGImageSourceCreateThumbnailFromImageAlways: true,
        kCGImageSourceShouldCacheImmediately: true,
        kCGImageSourceCreateThumbnailWithTransform: true,
        kCGImageSourceThumbnailMaxPixelSize: maxDimensionInPixels
    ] as CFDictionary
    guard let downsampledImage = CGImageSourceCreateThumbnailAtIndex(imageSource, 0, downsampleOptions) else {
        return nil
    }
    
    // Return the downsampled image as UIImage
    return UIImage(cgImage: downsampledImage)
}

我将上面的 2 个代码合并如下：

final class ImageStore {
    typealias _ImageDictionary = [String: CGImage]
    fileprivate var images: _ImageDictionary = [:]
    
    fileprivate static var scale = 2
    
    static var shared = ImageStore()

    // Perform downsampling
    let downsampleOptions = [
        kCGImageSourceCreateThumbnailFromImageAlways: true,
        kCGImageSourceShouldCacheImmediately: true,
        kCGImageSourceCreateThumbnailWithTransform: true,
        kCGImageSourceThumbnailMaxPixelSize: 950
    ] as CFDictionary //Added this block
    
    
    func image(name: String) -> Image {
        let index = _guaranteeImage(name: name)
        
        return Image(images.values[index], scale: CGFloat(ImageStore.scale), label: Text(verbatim: name))
    }
    
    static func loadImage(name: String) -> CGImage {
        guard
            let url = Bundle.main.url(forResource: name, withExtension: "jpg"),
            let imageSource = CGImageSourceCreateWithURL(url as NSURL, nil),
            let image = CGImageSourceCreateThumbnailAtIndex(imageSource, 0, downsampleOptions) //Added this line
        else {
            fatalError("Couldn't load image \(name).jpg from main bundle.")
        }
        return image
    }
    
    fileprivate func _guaranteeImage(name: String) -> _ImageDictionary.Index {
        if let index = images.index(forKey: name) { return index }
        
        images[name] = ImageStore.loadImage(name: name)
        return images.index(forKey: name)!
    }
}

但我收到此错误： Instance member 'downsampleOptions' cannot be used on type 'ImageStore'

如何对图像进行下采样？（换句话说，我需要图片库的缩略图）