Modern Concurrency: Getting Started

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

Part 1: Asynchronous Code

05. Using AsyncSequence in Views

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Previous episode: 04. Asynchronous Sequences Next episode: 06. Additional Error Handling

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Async sequence

In episode 3, you used Swift concurrency to fetch this list of available stock symbols.

.padding
// TODO...
🟩
.task {
  do {
    try await model.startTicker(selectedSymbols)
  } catch {
    lastErrorMessage = error.localizedDescription
  }
}

let (stream, response) = try await liveURLSession.bytes(from: url)

guard (response as? HTTPURLResponse)?.statusCode == 200 else {
  throw "The server responded with an error."
}

for try await line in stream.lines {
  print(line)
}

for try await line in stream.lines {
  print(line)
🟩
let sortedSymbols = try JSONDecoder()
  .decode([Stock].self, from: Data(line.utf8))
  .sorted(by: { $0.name < $1.name })
tickerSymbols = sortedSymbols
🟥
}

Update UI on main thread

It works! But there are warnings interspersed and also purple warnings next to tickerSymbols = [] and tickerSymbols = sortedSymbols.

await MainActor.run {
  tickerSymbols = []
}

await MainActor.run {
  tickerSymbols = sortedSymbols
}

await MainActor.run {
  tickerSymbols = sortedSymbols
  🟩print("Updated: \(Date())")🟥
}

Task hierarchy

First, a word about structured concurrency: click back to TickerView.

Handle cancellation errors

Keep an eye on the Xcode console while you tap the Back button.

} catch {
🟩
  if let error = error as? URLError, 
    error.code == .cancelled {
    return
  }
🟥
  lastErrorMessage = error.localizedDescription
}