Partially downloading data in Swift

Question

I'm trying to develop a download accelerator in Swift. It should get the file's size and divide it to n parts. Then it should download them at once by running multiple threads, and then merge the parts.

I read C# - Creating a Download Accelerator, unfortunately it doesn't help me.

I can do the multiple thread part easily by

DispatchQueue.main.async {
    // The new thread
}

but the other part is harder. I usually download a file like this:

try Data(contentsOf: URL(string: assetsUrl!)!)

or I can do the thing that is explained in this answer

class Downloader {
    class func load(url: URL, to localUrl: URL, completion: @escaping () -> ()) {
        let sessionConfig = URLSessionConfiguration.default
        let session = URLSession(configuration: sessionConfig)
        let request = try! URLRequest(url: url, method: .get)

        let task = session.downloadTask(with: request) { (tempLocalUrl, response, error) in
            if let tempLocalUrl = tempLocalUrl, error == nil {
                // Success
                if let statusCode = (response as? HTTPURLResponse)?.statusCode {
                    print("Success: \(statusCode)")
                }

                do {
                    try FileManager.default.copyItem(at: tempLocalUrl, to: localUrl)
                    completion()
                } catch (let writeError) {
                    print("error writing file \(localUrl) : \(writeError)")
                }

            } else {
                print("Failure: %@", error?.localizedDescription);
            }
        }
        task.resume()
    }
}

But this is not C - it's very simplistic and doesn't accept many arguments. How can I make it get "first 200_000 bytes" from the server?

Answer 1

First of all, the server needs to implement HTTP range requests. If it doesn't, and you don't control the server, then you will not be able to do this.

If the server supports HTTP range requests, then you need to specify the range with request headers, as explained here: https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests

The essentials are that you first send a HEAD request to figure out whether the server supports HTTP range requests. This is determined by whether the response includes the Accept-Ranges header, with a non-zero value.

If the server supports HTTP range requests, then you can make a request for the resource, with the Range header set for example to a value of bytes=0-1023 (depends which format the Accept-Ranges header specified, in this case bytes)