Modern Concurrency: Getting Started

Oct 18 2022 · Swift 5.5, iOS 15, Xcode 13.4

Part 2: Asynchronous Sequences

13. Downloading Chunks

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Previous episode: 12. Displaying a Progress View Next episode: 14. Canceling Tasks

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00:02Make sure the course server is running and continue with your project from the previous episode or open the starter project for this episode.

00:21You’ve coded the Silver download option, which fetches the complete file in one go and presents an onscreen preview.

00:39You’ll do this by reading the file as an asynchronous sequence of bytes from the server. This is similar to what you did in episodes 4 and 5, but now you’ll implement some helper methods to work with chunks, or batches, of bytes. This lets you update the progress bar one chunk at a time, as you’re receiving the file’s contents.

return try await downloadWithProgress(fileName: file.name, name: file.name, size: file.size)

private func downloadWithProgress(fileName: String, name: String, size: Int, offset: Int? = nil) async throws -> Data {
  guard let url = URL(string: "http://localhost:8080/files/download?\(fileName)") else {
    throw "Could not create the URL."
  }
  await addDownload(name: name)

  let result: (downloadStream: URLSession.AsyncBytes, response: URLResponse)
  if let offset = offset {
    // code for Cloud 9 plan
  }
  else {
    result = try await URLSession.shared.bytes(from: url)
    guard (result.response as? HTTPURLResponse)?.statusCode == 200 else {
      throw "The server responded with an error."
    }
  }

  // Add code here, replacing placeholder return statement
  return Data()
}

else {
  result = try await URLSession.shared.bytes(from: url)
  guard (result.response as? HTTPURLResponse)?.statusCode == 200 else {
    throw "The server responded with an error."
  }
}

var asyncDownloadIterator = result.downloadStream.makeAsyncIterator()

//    if let offset = offset {
//      // Add code for Cloud 9 plan
//    }
//    else {
      result = try await URLSession.shared.bytes(from: url)
      guard (result.response as? HTTPURLResponse)?.statusCode == 200 else {
        throw "The server responded with an error."
      }
//    }

03:21You won’t update the progress bar after every byte. Instead, you’ll process a batch of bytes at a time and update the progress bar after each batch.

let accumulator = ByteAccumulator(name: name, size: size)
while !stopDownloads,  // you'll set this in the next episode
 !accumulator.checkCompleted() {  // accumulator can still collect more bytes

}

while !accumulator.isBatchCompleted,  // nested while runs until this batch is full
  let byte = try await asyncDownloadIterator.next() {  // or the byte sequence completes
  accumulator.append(byte)
}

05:04After a batch completes, it’s time to update the download progress bar.

await updateDownload(name: name, progress: accumulator.progress)

Task.detached(priority: .medium) {
  await self.updateDownload(name: name, progress: accumulator.progress)  
  // need self. in a closure
}

let progress = accumulator.progress

let progress = accumulator.progress
Task.detached(priority: .medium) {
  await self.updateDownload(name: name, progress: progress)  // delete accumulator.
}

let progress = accumulator.progress
Task.detached(priority: .medium) {
  await self.updateDownload(name: name, progress: progress)
}
🟩print(accumulator.description)

07:42And finally, replace the dummy return value:

return accumulator.data

isDownloadActive = true
Task {
  do {
    fileData = try await model.download🟩WithProgress🟥(file: file)
  } catch { }
  isDownloadActive = false
}