Using URLCache with download tasks (NSURLCache & NSURLSessionDownloadTask)

URLCacheTest.swift

import Foundation
import os.log

class URLCacheTest {
    let logger = Logger(subsystem: "URLCacheTest", category: "main")

    // HTTP HEADERS:
    // Date: Wed, 04 Nov 2020 11:13:24 GMT
    // Server: Apache
    // Strict-Transport-Security: max-age=63072000; includeSubdomains; preload
    // X-Content-Type-Options: nosniff
    // X-Frame-Options: SAMEORIGIN
    // Last-Modified: Sun, 19 May 2002 14:49:00 GMT
    // Accept-Ranges: bytes
    // Content-Length: 20702285
    // Content-Type: application/pdf
    let flightPlanURL = URL(string: "https://www.hq.nasa.gov/alsj/a17/A17_FlightPlan.pdf")!

    // Custom URL cache with 1 GB disk storage
    lazy var cache: URLCache = {
        let cachesURL = FileManager.default.urls(for: .cachesDirectory, in: .userDomainMask)[0]
        let diskCacheURL = cachesURL.appendingPathComponent("DownloadCache")
        let cache = URLCache(memoryCapacity: 100_000_000, diskCapacity: 1_000_000_000, directory: diskCacheURL)
        logger.info("Cache path: \(diskCacheURL.path)")
        return cache
    }()

    // Custom URLSession that uses our cache
    lazy var session: URLSession = {
        let config = URLSessionConfiguration.default
        config.urlCache = cache
        return URLSession(configuration: config)
    }()

    init() {
        let documentURL = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask)[0]
        let targetURL = documentURL.appendingPathComponent(flightPlanURL.lastPathComponent)
        downloadFile(remoteURL: flightPlanURL, targetURL: targetURL)
    }

    func downloadFile(remoteURL: URL, targetURL: URL) {
        let request = URLRequest(url: remoteURL)
        let downloadTask = session.downloadTask(with: request) { url, response, error in
            self.logger.info("Download Task complete")

            // Store data in cache
            if let response = response, let url = url,
               self.cache.cachedResponse(for: request) == nil,
               let data = try? Data(contentsOf: url, options: [.mappedIfSafe]) {
                self.cache.storeCachedResponse(CachedURLResponse(response: response, data: data), for: request)
            }
                                                                
            // Move file to target location
            guard let tempURL = url else { return }
            _ = try? FileManager.default.replaceItemAt(targetURL, withItemAt: tempURL)
        }
        downloadTask.resume()
    }
}

https://pspdfkit.com/blog/2020/downloading-large-files-with-urlsession/

A small side note, using URLSession with background configuration will ignore the URLCache object, its a "minor" not well documented fact. I had it confirmed with a ATS a few years back as far as i know its still there, and very poorly documented

Indeed. I mentioned this in the blog post.

Thanks for this excellent article and accompanying gist!