Hello I'm a beginner to Swift Concurrency and have run into an issue with AsyncStream. I've run into a situation that causes an observing of a for loop to receiving a values from an AsyncStream.
At the bottom is the code that you can copy it into a Swift Playground and run.
The code is supposed to mock a system that has a service going through a filter to read and write to a connection.
Here is a log of the prints
🙈🫴 setupRTFAsyncWrites Start
⬅️ Pretend to write 0
➡️ Pretend to read 0
feed into filter 0
yield write data 1
🙈🫴 setupRTFAsyncWrites: write(1 bytes)
⬅️🙈🫴 Async Event: dataToDevice: 1 bytes
⬅️ Pretend to write 1
➡️ Pretend to read 1
feed into filter 1
yield write data 2
// here our for loop should have picked up the value sent down the continuation. But instead it just sits here.
Sample that can go into a playground
//: A UIKit based Playground for presenting user interface
import SwiftUI
import PlaygroundSupport
import Combine
import CommonCrypto
import Foundation
class TestConnection {
var didRead: ((Data) -> ()) = { _ in }
var count = 0
init() {
}
func write(data: Data) {
// pretend we sent this to the BT device
print("⬅️ Pretend to write \(count)")
Task {
try await Task.sleep(ms: 200)
print("➡️ Pretend to read \(self.count)")
self.count += 1
// pretend this is a response from the device
self.didRead(Data([0x00]))
}
}
}
enum TestEvent: Sendable {
/// ask some one to write this to the device
case write(Data)
/// the filter is done
case handshakeDone
}
class TestFilter {
var eventsStream: AsyncStream<TestEvent>
var continuation: AsyncStream<TestEvent>.Continuation
private var count = 0
init() {
(self.eventsStream, self.continuation) = AsyncStream<TestEvent>.makeStream(bufferingPolicy: .unbounded)
}
func feed(data: Data) {
print("\tfeed into filter \(count)")
count += 1
if count > 5 {
print("\t✅ handshake done")
self.continuation.yield(.handshakeDone)
return
}
Task {
// data delivered to us by a bluetooth device
// pretend it takes time to process this and then we return with a request to write back to the connection
try await Task.sleep(ms: 200)
print("\tyield write data \(self.count)")
// pretend this is a response from the device
let result = self.continuation.yield(.write(Data([0x11])))
}
}
/// gives the first request to fire to the device for the handshaking sequence
func start() -> Data {
return Data([0x00])
}
}
// Here we facilitate communication between the filter and the connection
class TestService {
private let filter: TestFilter
var task: Task<(), Never>?
let testConn: TestConnection
init(filter: TestFilter) {
self.filter = filter
self.testConn = TestConnection()
self.testConn.didRead = { [weak self] data in
self?.filter.feed(data: data)
}
self.task = Task { [weak self] () in
await self?.setupAsyncWrites()
}
}
func setupAsyncWrites() async {
print("🙈🫴 setupRTFAsyncWrites Start")
for await event in self.filter.eventsStream {
print("\t\t🙈🫴 setupRTFAsyncWrites: \(event)")
guard case .write(let data) = event else {
print("\t\t🙈🫴 NOT data to device: \(event)")
continue
}
print("\t\t⬅️🙈🫴 Async Event: dataToDevice: \(data)")
self.testConn.write(data: data)
} // for
// This shouldn't end
assertionFailure("This should not end")
}
public func handshake() async {
let data = self.filter.start()
self.testConn.write(data: data)
await self.waitForHandshakedone()
}
private func waitForHandshakedone() async {
for await event in self.filter.eventsStream {
if case .handshakeDone = event {
break
}
continue
}
}
}
Task {
let service = TestService(filter: TestFilter())
await service.handshake()
print("Done")
}
/*
This is what happens:
🙈🫴 setupRTFAsyncWrites Start
⬅️ Pretend to write 0
➡️ Pretend to read 0
feed into filter 0
yield write data 1
🙈🫴 setupRTFAsyncWrites: write(1 bytes)
⬅️🙈🫴 Async Event: dataToDevice: 1 bytes
⬅️ Pretend to write 1
➡️ Pretend to read 1
feed into filter 1
yield write data 2
// It just stops here, the `for` loop in setupAsyncWrites() should have picked up the event sent down the continuation after "yield write data 2"
// It should say
🙈🫴 setupRTFAsyncWrites: write(1 bytes)
⬅️🙈🫴 Async Event: dataToDevice: 1 bytes
*/
extension Task<Never, Never> {
public static func sleep(ms duration: UInt64) async throws {
try await Task.sleep(nanoseconds: 1_000_000 * duration)
}
}
dtanmasimo
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