加载大量本地图片，内存暴涨

项目中需要加载大量本地图片，导致内存暴涨 将UIImage(named: "xxx") 修改成：

    let wid = xxxx
    let hei = xxxx

    let filePath = Bundle.main.path(forResource: model.goods_id, ofType: "png")
    if filePath != nil {
            let data = NSData.init(contentsOfFile: filePath ?? "")
            if data != nil {
                let image = getImageFromData(imageAt: data!, to: CGSize(width: wid, height: hei), scale: 3) 
                goodsImageView.image = image
            } else {
                goodsImageView.image = nil
            }

    } else {
        goodsImageView.image = nil
    }

//pointSize 图片尺寸（点)
func getImageFromData(imageAt imageData: NSData, to pointSize: CGSize, scale: CGFloat) -> UIImage
    {
        let sourceOpt = [kCGImageSourceShouldCache : false] as CFDictionary
        //(data并未decode,所占内存没那么大),
        let source = CGImageSourceCreateWithData(imageData as CFData, sourceOpt)!
        let maxDimension = max(pointSize.width, pointSize.height) * scale //像素大小
        let downsampleOpt = [kCGImageSourceCreateThumbnailFromImageAlways : true,
                             kCGImageSourceShouldCacheImmediately : true,
                             kCGImageSourceCreateThumbnailWithTransform : true,
                             kCGImageSourceThumbnailMaxPixelSize : maxDimension] as CFDictionary
        let downsampleImage = CGImageSourceCreateThumbnailAtIndex(source, 0, downsampleOpt)!
        return UIImage(cgImage: downsampleImage)
    }

内存占用下降明显。 imageNamed: 加载本地图片会保留在全局缓存中

在长列表中，加载大量本地图片，这种方式加载图片虽然可以明显的减少内存的占用，但会造成滑动卡顿，需要做预加载