returns image data by fetching locally or remote, caching the result so subsequent fetches are local

ImageCacher.swift

    func getImageDataWithUrlString(imageUrlString: String) -> NSData? {
        let url = URL(string: imageUrlString)
        if let filename: String = url?.lastPathComponent {
            let imagesDirUrl = try! FileManager.default.url(for: .cachesDirectory, in: .localDomainMask, appropriateFor: nil, create: true)
            let fileUrl = imagesDirUrl.appendingPathComponent(filename)
            
            do {
                // get a cached copy
                let _ = try fileUrl.checkResourceIsReachable()
                let imageData = NSData(contentsOf: fileUrl)
                return imageData
            }
            catch {
                // need to download
                let imageData = NSData(contentsOf: url!)
                imageData?.write(to: fileUrl, atomically: true)
                return imageData
            }
        }
        else {
            print("error getting last path component")
            return nil
        }
    }