Modern Concurrency: Beyond the Basics
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Notes: 02. AsyncStream
- For more about push-based vs pull-based AsyncStreams, see AsyncSequence & AsyncStream Tutorial for iOS.
- This video uses Xcode 14’s
Task.sleep(until:clock:). If you use Xcode 13, replace this withTask.sleep(nanoseconds: 1_000_000_000).
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In this episode, you’ll learn about AsyncStream, an easier way to create a custom AsyncSequence.
Custom AsyncSequence from episode 4 of Getting Started
In the preceding course, you created this simple typewriter with a custom AsyncSequence that “types” a phrase, adding a character every second.
struct Typewriter: AsyncSequence {
typealias Element = String
let phrase: String
func makeAsyncIterator() -> TypewriterIterator {
return TypewriterIterator(phrase)
}
}
struct TypewriterIterator: AsyncIteratorProtocol {
typealias Element = String
let phrase: String
var index: String.Index
init(_ phrase: String) {
self.phrase = phrase
self.index = phrase.startIndex
}
mutating func next() async throws -> String? {
guard index < phrase.endIndex else { return nil }
try await Task.sleep(until: .now + .seconds(1),
clock: .continuous)
defer { index = phrase.index(after: index) }
return String(phrase[phrase.startIndex...index])
}
}
Task {
for try await item in Typewriter(phrase: "Hello, world!") {
print(item)
}
print("AsyncSequence Done")
}
He
Hel
Hell
Hello
Hello,
Hello,
Hello, w
Hello, wo
Hello, wor
Hello, worl
Hello, world
Hello, world!
AsyncSequence Done
AsyncStream Typewriter
In the next section, the starter has already set up test phrase and index properties:
let phrase = "Hello, world!"
var index = phrase.startIndex
let stream_pull = AsyncStream<String> {
}
let stream_pull = AsyncStream<String> {
🟩
guard index < phrase.endIndex else { return nil }
try await Task.sleep(until: .now + .seconds(1),
clock: .continuous)
defer { index = phrase.index(after: index) }
return String(phrase[phrase.startIndex...index])
🟥
}
let stream_pull = 🟩AsyncThrowingStream<String, Error🟥> {
do {
try await Task.sleep(until: .now + .seconds(1),
clock: .continuous)
} catch {
return nil
}
Task {
for try await item in stream_pull {
print(item)
}
print("Pull AsyncStream Done")
}
AsyncStream: pull or push?
Option-click AsyncStream then Open in Developer Documentation. Scroll down to Topics.
// These two lines are already above the pull-based stream code
let phrase = "Hello, world!"
var index = phrase.startIndex
let stream_push = AsyncStream<String> { continuation in // closure receives a continuation
Task { // wrap any asynchronous code in a Task
}
}
while index < phrase.endIndex { // while index hasn't reached the end of phrase
}
while index < phrase.endIndex {
do {
try await Task.sleep(until: .now + .seconds(1), clock: .continuous)
} catch {
// delete return nil
}
}
continuation.yield(String(phrase[phrase.startIndex...index])) // copy pull-based return value
index = phrase.index(after: index) // copy index increment from pull-based defer
continuation.finish()
// These two lines are already above the pull-based stream code
var phrase = "Hello, world!"
var index = phrase.startIndex
let stream_push = AsyncStream<String> { continuation in
Task {
while index < phrase.endIndex {
do {
try await Task.sleep(until: .now + .seconds(1),
clock: .continuous)
continuation.yield(String(phrase[phrase.startIndex...index]))
index = phrase.index(after: index)
} catch {
continuation.finish()
}
}
continuation.finish()
}
}
🟩Task {
for try await item in stream_push { // change pull to push
print(item)
}
print("Push AsyncStream Done") // change pull to push
}🟥