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Using URLCache with download tasks (NSURLCache & NSURLSessionDownloadTask)
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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() | |
} | |
} |
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!
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
?
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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