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!

Hmm doesn’t this defeat the main benefit of a download task, which is to avoid loading the entire file contents into memory?

The data is memory mapped, it is not actually loaded into memory. This approach works even for multi-GB-files.

Oooh whoops missed the .mappedIfSafe. Thanks, @steipete!