ImageCache+InsertRemove.swift

extension ImageCache: ImageCacheType {
    func insertImage(_ image: UIImage?, for url: URL) {
        guard let image = image else { return removeImage(for: url) }
        let decodedImage = image.decodedImage()

        lock.lock(); defer { lock.unlock() }
        imageCache.setObject(decodedImage, forKey: url as AnyObject)
        decodedImageCache.setObject(image as AnyObject, forKey: url as AnyObject, cost: decodedImage.diskSize)
    }

    func removeImage(for url: URL) {
        lock.lock(); defer { lock.unlock() }
        imageCache.removeObject(forKey: url as AnyObject)
        decodedImageCache.removeObject(forKey: url as AnyObject)
    }
}

Hello, I was reading Reusable Image Cache in Swift
This code is adding the decoded image to the encoded cache and vise versa.

Also, you don't say what 'diskSize' is: presumably another convenience extension?

Other than that LGTM 👍