Downsampling large images for display at smaller size
有助于内存优化

    class func downsample(imageAt imageURL:URL, to pointSize:CGSize, scale:CGFloat) -> UIImage {
        let imageSourceOptions = [kCGImageSourceShouldCache:false] as CFDictionary
        let imageSource = CGImageSourceCreateWithURL(imageURL as CFURL, imageSourceOptions)!
        let maxDimensionInPixels = max(pointSize.width, pointSize.height) * scale
        let downsampleOptions = [
            kCGImageSourceCreateThumbnailFromImageAlways:true,
            kCGImageSourceShouldCacheImmediately: true,
            kCGImageSourceCreateThumbnailWithTransform: true,
            kCGImageSourceThumbnailMaxPixelSize: maxDimensionInPixels
        ] as CFDictionary
        let downsampledImage = CGImageSourceCreateThumbnailAtIndex(imageSource, 0, downsampleOptions)!
        return UIImage(cgImage: downsampledImage)
    }