Modern Concurrency: Beyond the Basics

Your challenge is to rewrite countdown(to:) using the buffered push-based version of AsyncStream.

Use Timer instead of Task.sleep to wait 1 second between messages. The starter project already has code for a Timer.scheduledTimer that fires every second.

Inside the scheduledTimer closure, yield the correct message when countdown is 3, 2, 1 and 0. Remember to decrement countdown after yielding 3, 2 and 1.

When countdown is 0, return the user’s message, invalidate the timer and finish the continuation.

Welcome back! Hopefully you had success with this task. Here’s how I did it.

In BlabberModel, countdown(to:) already has starter code to create a Timer that fires each second, instead of the pull-based AsyncStream code you wrote in episode 3:

let counter = AsyncStream<String> { continuation in
  var countdown = 3
  Timer.scheduledTimer(
    withTimeInterval: 1.0,
    repeats: true
  ) { timer in

  }
}

Inside the timer closure, I first handled the countdown digits:

continuation.yield("\(countdown) ...")
countdown -= 1

On every timer tick, I called yield(_:) on the continuation to produce the countdown value, then decreased countdown.

Next, I needed to add code for when countdown reaches 0.

I did this before continuation.yield...

) { timer in
  🟩guard countdown > 0 else {
    timer.invalidate()
    continuation.yield("🎉 " + message)
    continuation.finish()
    return
  }
  🟥continuation.yield("\(countdown) ...")
  countdown -= 1
}

When countdown reaches 0: I stop the timer, yield the user’s message, and call continuation.finish() to complete the sequence.

Actually, there’s a shortcut to produce the last value and complete the sequence at the same time:

continuation.yield(with: .success("🎉 " + message))

To show it works:

The next episode is about unit testing.