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Doug
33e7c1a819
Installed for Copilot with `gh skill install` and manually created a symlink for Claude to access all Copilot skills.
68 lines
2.1 KiB
Markdown
68 lines
2.1 KiB
Markdown
# Async streams
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## Prefer `makeStream(of:)` factory
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The modern way to create an `AsyncStream` is the static factory method, which returns both the stream and its continuation as a tuple. This avoids capturing the continuation in a closure.
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```swift
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// OLD: Closure-based, awkward to store the continuation.
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var continuation: AsyncStream<Event>.Continuation?
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let stream = AsyncStream<Event> { cont in
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continuation = cont
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}
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// NEW: Clean, no closure capture needed.
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let (stream, continuation) = AsyncStream.makeStream(of: Event.self)
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```
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This also works with `AsyncThrowingStream.makeStream(of:throwing:)`.
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## Continuation lifecycle
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A continuation must always be finished exactly once. Failing to finish it causes the consumer's `for await` loop to hang indefinitely. Finishing it twice is a programmer error (although `AsyncStream.Continuation` tolerates it, `CheckedContinuation` does not).
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Always finish in cleanup paths:
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```swift
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let (stream, continuation) = AsyncStream.makeStream(of: Event.self)
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let monitor = NetworkMonitor()
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monitor.onEvent = { event in
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continuation.yield(event)
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}
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monitor.onComplete = {
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continuation.finish()
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}
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// If the monitor can be deallocated before completing:
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continuation.onTermination = { _ in
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monitor.stop()
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}
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```
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## Buffering and back pressure
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`AsyncStream` has a default buffer of unlimited size. For high-throughput producers, this can cause unbounded memory growth. Specify a buffering policy:
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```swift
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let (stream, continuation) = AsyncStream.makeStream(
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of: SensorReading.self,
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bufferingPolicy: .bufferingNewest(100)
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)
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```
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Choose from:
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- `.bufferingNewest(n)` keeps the most recent `n` elements, dropping older ones.
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- `.bufferingOldest(n)` keeps the first `n` elements, dropping newer ones.
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- `.unbounded` is the default; use only when the consumer keeps up.
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## `for await` and cancellation
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A `for await` loop automatically stops when the task is cancelled or the stream finishes. You do not need to manually check cancellation inside the loop – but code *after* the loop does run, so handle cleanup there if needed.
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