Vapor 3 Streaming Download

streaming-download.swift

import Vapor

public func routes(_ router: Router) throws {
    router.get("streaming") { req -> Future<Response> in
        // URL to test file that will be downloaded
        // https://speed.hetzner.de/100MB.bin
        // https://speed.hetzner.de/10GB.bin
        let testFile = URL(string: "https://speed.hetzner.de/100MB.bin")!

        // create an instance of our URL session delegate
        let downloader = Downloader(on: req)

        // create a URL session download task, delegating to our downloader
        let download = URLSession(configuration: .default, delegate: downloader, delegateQueue: nil)
            .downloadTask(with: testFile)

        // start the task
        download.resume()

        // will complete once the download has completed and we
        // have a path to the temporary file
        return downloader.filePromise.futureResult.flatMap { filePath in
            // use Request's FileIO convenience to stream the
            // temporary file to the connected client
            return try req.streamFile(at: filePath)
        }.map { res in
            // add a Content-Disposition header which will cause web
            // browsers to download the file
            // if the file is meant to be an image, add content type headers instead
            res.http.headers.replaceOrAdd(name: .contentDisposition, value: "attachment; filename=Download.test")
            return res
        }
    }
}

// URL session download delegate that moves the downloaded file
// to a temporary directory and completes a promise with the new file path
final class Downloader: NSObject, URLSessionDelegate, URLSessionDownloadDelegate {
    // promise that will be completed when the file is downloaded
    let filePromise: Promise<String>

    // creates a new download delegate, using the supplied worker for
    // promise creation
    init(on worker: Worker) {
        self.filePromise = worker.eventLoop.newPromise(of: String.self)
    }

    func urlSession(_ session: URLSession, downloadTask: URLSessionDownloadTask, didFinishDownloadingTo location: URL) {
        // create a new file name for identifying the download
        // we'll store the file in the /tmp/ directory so that it will be
        // cleared automatically on reboot
        // note: we may want to clear this file manually after the download
        // is served or at an interval to avoid ballooning disk usage
        let new = "/tmp/vapor-download-\(UUID())"
        // the location supplied to this method will be invalidated after
        // we return, so we must move it
        try! FileManager.default.moveItem(at: location, to: URL(string: "file://" + new)!)
        // complete the promise with the newly generated file name
        self.filePromise.succeed(result: new)
    }
}