Modern Concurrency: Getting Started

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

Part 2: Asynchronous Sequences

16. Concurrent Downloads

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Previous episode: 15. Using Combine Next episode: 17. Conclusion

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

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."
  }
}

if let offset = offset {
  🟩
  let urlRequest = URLRequest(url: url, offset: offset, length: size)
  🟥
}

extension URLRequest {
  init(url: URL, offset: Int, length: Int) {
    self.init(url: url)
    addValue("bytes=\(offset)-\(offset + length - 1)", forHTTPHeaderField: "Range")
  }
}

0-19999 [offset 0, length 20000]
20000-39999 [offset 20000, length 20000]
40000-59999 [offset 40000, length 20000]
60000-77344 [offset 0, length 17345]

if let offset = offset {
  let urlRequest = URLRequest(url: url, offset: offset, length: size)
  🟩result = try await URLSession.shared.bytes(for: urlRequest)🟥
}

if let offset = offset {
  let urlRequest = URLRequest(url: url, offset: offset, length: size)
  result = try await URLSession.shared.bytes(for: urlRequest)
  🟩
  guard (result.response as? HTTPURLResponse)?.statusCode == 206 else {
    throw "The server responded with an error."
  }
  🟥
}

func multiDownloadWithProgress(file: DownloadFile) async throws -> Data {
  func partInfo(index: Int, of count: Int) -> (offset: Int, size: Int, name: String) {
    let standardPartSize = Int((Double(file.size) / Double(count)).rounded(.up))
    let partOffset = index * standardPartSize
    let partSize = min(standardPartSize, file.size - partOffset)
    let partName = "\(file.name) (part \(index + 1))"
    return (offset: partOffset, size: partSize, name: partName)
  }
  let total = 4
  let parts = (0..<total).map { partInfo(index: $0, of: total) }
  
  // Add code here, replacing placeholder return statement
  return Data()
}

downloadWithProgress(fileName: String, name: String, size: Int, offset: Int? = nil)

Call multiDownloadWithProgress(file:)

Start with part0: Define a promise with async let:

async let part0 =
  downloadWithProgress(fileName: file.name, name: parts[0].name, size: parts[0].size, offset: parts[0].offset)

async let part0 =
downloadWithProgress(fileName: file.name, name: parts[0].name, size: parts[0].size, offset: parts[0].offset)
🟩 
async let part1 =
downloadWithProgress(fileName: file.name, name: parts[1].name, size: parts[1].size, offset: parts[1].offset)
async let part2 =
downloadWithProgress(fileName: file.name, name: parts[2].name, size: parts[2].size, offset: parts[2].offset)
async let part3 =
downloadWithProgress(fileName: file.name, name: parts[3].name, size: parts[3].size, offset: parts[3].offset)
🟥 

try await [part0, part1, part2, part3]

try await [part0, part1, part2, part3]
  🟩.reduce(Data(), +)🟥

🟩return 🟥try await [part0, part1, part2, part3]
  .reduce(Data(), +)
❌return Data()

isDownloadActive = true
Task {  // delete downloadTask =
  do {
    fileData = try await model.🟩multiDownloadWithProgress🟥(file: file) 
  } catch { }
  isDownloadActive = false
  // delete timerTask...
}