URLRequestLowDataMode.swift

func downloadHighResImage() {
    var request = URLRequest(url: URL(string: "highQualityImageURL")!)

    // Trigger error in `session.dataTask` if user has low data mode enabled.
    request.allowsConstrainedNetworkAccess = false

    URLSession.shared.dataTask(with: request) { data, response, error in

        if let error = error as? URLError,
           error.networkUnavailableReason == .constrained {

            // Download low res image which uses less data
            downloadLowResImage()

        }

        // Logic...

    }.resume()
}