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()
    }
}

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!

Do you think you could avoid the automatic cache purging for certain more critical resources by specifying the directory to be the document directory instead of Caches?

HI @steipete, thanks for great article 👏 I'm wondering about on thing. In line 48 you check if doesn't have entry for given request, but what if we already had old version of response cached and we want to substitute it with the recent one? Is it guaranteed that URLSession removes old data before invoking completion?

It’s been 5 years since this - I would not assume any of this is still accurate. This needs a re-test. I don’t do iOS work these days, so someone else could pick this up.