Iterating over internet socket messages with async / await in Swift – Donny Wals

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Printed on: January 24, 2023

In iOS 13, we gained the power to simply ship and obtain knowledge utilizing internet sockets via URLSession. With async/await, we gained the power to fetch knowledge from servers utilizing the await key phrase and we are able to iterate over asynchronous sequences utilizing async for loops.

We are able to even learn knowledge from a URL one line at a time by calling the traces property on URL:

let url = URL(string: "https://donnywals.com")!

for strive await line in url.traces {
    // use line
}

Whereas that is actually cool and permits us to construct apps that ingest knowledge in actual time if the server helps streaming our bodies, we can’t use the traces property to arrange an online socket connection and pay attention for incoming messages and doubtlessly ship messages over the identical connection too.

On this publish, you’ll be taught every little thing you’ll want to find out about constructing your personal mechanism to conveniently iterate over messages from an online socket asynchronously. We’ll leverage some current performance from URLSessionWebSocketTask and AsyncThrowingStream to construct our personal AsyncSequence that conveniently wraps our URLSessionWebSocketTask.

Be aware that the ensuing code has solely had comparatively restricted testing completed so I can’t assure that the supplied answer will likely be 100% appropriate for every little thing you throw at it. When you discover any points with the ultimate code, be at liberty to contact me. Bonus factors in case you’re in a position to present some concepts for a possible repair.

Utilizing an online socket with out async / await

Earlier than we get began, let’s shortly evaluation learn how to use an online socket with out async/await. The code particulars are outlined in this publish. Remember to learn it if you wish to be taught extra about utilizing internet sockets in your apps.


let url = URL(string: "ws://127.0.0.1:8080")!
let socketConnection = URLSession.shared.webSocketTask(with: url)
socketConnection.resume()

func setReceiveHandler() {
    socketConnection.obtain { lead to
        defer { self.setReceiveHandler() }

        do {
            let message = strive end result.get()
            swap message {
            case let .string(string):
                print(string)
            case let .knowledge(knowledge):
                print(knowledge)
            @unknown default:
                print("unkown message acquired")
            }
        } catch {
            // deal with the error
            print(error)
        }
    }
}

setReceiveHandler()

Discover how, to obtain messages from the socket, I need to name obtain with a completion handler. This methodology solely permits me to obtain a single incoming message, so I need to re-set my handler after receiving a message to mechanically start listening for the subsequent message.

It is a nice instance of a scenario the place an async for loop akin to for strive await message in socketConnection would make lots of sense. Sadly, this isn’t potential out of the field. Nevertheless, URLSessionWebSocketTask offers some type of help for async / await so we’re not solely out of luck.

A primary implementation of internet sockets with async / await

Whereas URLSessionWebSocketTask doesn’t expose an AsyncSequence that emits incoming messages out of the field, it does include an async model of the obtain methodology you noticed earlier.

This permits us to rewrite the instance above as an async methodology as follows:

func setReceiveHandler() async {
    do {
        let message = strive await socketConnection.obtain()

        swap message {
        case let .string(string):
          print(string)
        case let .knowledge(knowledge):
          print(knowledge)
        @unknown default:
          print("unkown message acquired")
        }
    } catch {
        print(error)
    }

    await setReceiveHandler()
}

This code works simply tremendous, besides we don’t actually have a method to cease the recursion right here. The code you noticed earlier truly has the very same problem; there’s no situation to cease listening for internet socket messages even when the net socket connection has already been closed.

We may enhance our code by solely recursing if:

  1. We didn’t encounter any errors
  2. The socket connection remains to be lively

This may look a bit as follows:

func setReceiveHandler() async {
    guard socketConnection.closeCode == .invalid else {
        return
    }

    do {
        let message = strive await socketConnection.obtain()

        swap message {
        case let .string(string):
          print(string)
        case let .knowledge(knowledge):
          print(knowledge)
        @unknown default:
          print("unkown message acquired")
        }

        await setReceiveHandler()
    } catch {
        print(error)
    }
}

An open internet socket’s closed code is at all times mentioned to invalid to sign that the connection has not (but) been closed. We are able to leverage this to test that our connection remains to be lively earlier than ready for the subsequent message to be acquired.

That is significantly better already as a result of we respect closed sockets and failures a lot nicer now, however we may enhance the readability of this code a tiny bit by leveraging a whereas loop as a substitute of recursively calling the setReceiveHandler operate:

func setReceiveHandler() async {
    var isActive = true

    whereas isActive && socketConnection.closeCode == .invalid {
        do {
            let message = strive await socketConnection.obtain()

            swap message {
            case let .string(string):
              print(string)
            case let .knowledge(knowledge):
              print(knowledge)
            @unknown default:
              print("unkown message acquired")
            }
        } catch {
            print(error)
            isActive = false
        }
    }
}

To me, this model of the code is barely simpler to learn however that may not be the case for you. It’s functionally equal so you’ll be able to select to make use of whichever choice fits you finest.

Whereas this code works, I’m not fairly pleased with the place we’ve landed proper now. There’s lots of logic on this operate and I would like to separate dealing with the incoming values from the calls to socketConnection.obtain() in some way. Ideally, I ought to be capable to write the next:

do {
    for strive await message in socketConnection {
        swap message {
        case let .string(string):
            print(string)
        case let .knowledge(knowledge):
            print(knowledge)
        @unknown default:
            print("unkown message acquired")
      }
} catch {
    // deal with error
}

That is a lot, a lot nicer from a call-site perspective and it will permit us to place the ugly bits elsewhere.

To do that, we are able to leverage the facility of AsyncStream which permits us to construct a customized async sequence of values.

Utilizing AsyncStream to emit internet socket messages

Given our finish objective, there are a number of methods for us to get the place we wish to be. The simplest approach can be to write down a operate in an extension on URLSessionWebSocketTask that might encapsulate the whereas loop you noticed earlier. This implementation would look as follows:

typealias WebSocketStream = AsyncThrowingStream<URLSessionWebSocketTask.Message, Error>

public extension URLSessionWebSocketTask {    
    var stream: WebSocketStream {
        return WebSocketStream { continuation in
            Process {
                var isAlive = true

                whereas isAlive && closeCode == .invalid {
                    do {
                        let worth = strive await obtain()
                        continuation.yield(worth)
                    } catch {
                        continuation.end(throwing: error)
                        isAlive = false
                    }
                }
            }
        }
    }
}

To make the code just a little bit simpler to learn, I’ve outlined a typealias for my AsyncThrowingStream so we don’t have to have a look at the identical lengthy kind signature far and wide.

The code above creates an occasion of AsyncThrowingStream that asynchronously awaits new values from the net socket so long as the net socket is taken into account lively and hasn’t been closed. To emit incoming messages and potential errors, the continuation’s yield and end strategies are used. These strategies will both emit a brand new worth (yield) or finish the stream of values with an error (end).

This code works nice in lots of conditions, however there’s one problem. If we resolve to shut the net socket connection from the app’s aspect by calling cancel(with:cause:) on our socketConnection, our WebSocketStream doesn’t finish. As an alternative, will probably be caught ready for messages, and the decision website will likely be caught too.

Process {
    strive await Process.sleep(for: .seconds(5))
    strive await socketConnection.cancel(with: .goingAway, cause: nil)
}

Process {    
    do {
        for strive await message in socketConnection.stream2 {
            // deal with incoming messages
        }
    } catch {
        // deal with error
    }

    print("this might by no means be printed")
}

If every little thing works as anticipated, our internet socket connection will shut after 5 seconds. At that time, our for loop ought to finish and our print assertion ought to execute, for the reason that asynchronous stream is not lively. Sadly, this isn’t the case, so we have to discover a higher approach to mannequin our stream.

URLSessionWebSocketTask doesn’t present a approach for us to detect cancellation. So, I’ve discovered that it’s best to make use of an object that wraps the URLSessionWebSocketTask, and to cancel the duty via that object. This permits us to each finish the async stream we’re offering to callers and shut the net socket reference to one methodology name.

Right here’s what that object seems to be like:

class SocketStream: AsyncSequence {
    typealias AsyncIterator = WebSocketStream.Iterator
    typealias Component = URLSessionWebSocketTask.Message

    non-public var continuation: WebSocketStream.Continuation?
    non-public let job: URLSessionWebSocketTask

    non-public lazy var stream: WebSocketStream = {
        return WebSocketStream { continuation in
            self.continuation = continuation

            Process {
                var isAlive = true

                whereas isAlive && job.closeCode == .invalid {
                    do {
                        let worth = strive await job.obtain()
                        continuation.yield(worth)
                    } catch {
                        continuation.end(throwing: error)
                        isAlive = false
                    }
                }
            }
        }
    }()

    init(job: URLSessionWebSocketTask) {
        self.job = job
        job.resume()
    }

    deinit {
        continuation?.end()
    }

    func makeAsyncIterator() -> AsyncIterator {
        return stream.makeAsyncIterator()
    }

    func cancel() async throws {
        job.cancel(with: .goingAway, cause: nil)
        continuation?.end()
    }
}

There’s a bunch of code right here, but it surely’s not too dangerous. The primary few traces are all about establishing some kind aliases and properties for comfort. The lazy var stream is actually the very same code that you just’ve already within the URLSessionWebSocketTask extension from earlier than.

When our SocketStream‘s deinit is known as we ensure that we finish our stream. There’s additionally a cancel methodology that closes the socket connection in addition to the stream. As a result of SocketStream conforms to AsyncSequence we should present an Iterator object that’s used once we attempt to iterate over our SocketStreams. We merely ask our inner stream object to make an iterator and use that as our return worth.

Utilizing the code above seems to be as follows:

let url = URL(string: "ws://127.0.0.1:8080")!
let socketConnection = URLSession.shared.webSocketTask(with: url)
let stream = SocketStream(job: socketConnection)

Process {  
    do {
        for strive await message in stream {
            // deal with incoming messages
        }
    } catch {
        // deal with error
    }

    print("this will likely be printed as soon as the stream ends")
}

To cancel our stream after 5 seconds identical to earlier than, you’ll be able to run the next job in parallel with our iterating job:

Process {
    strive await Process.sleep(for: .seconds(5))
    strive await stream.cancel()
}

Process {
    // iterate...
}

Whereas that is fairly cool, we do have a little bit of a problem right here due to the next little bit of code:

non-public lazy var stream: WebSocketStream = {
    return WebSocketStream { continuation in
        self.continuation = continuation

        Process {
            var isAlive = true

            whereas isAlive && job.closeCode == .invalid {
                do {
                    let worth = strive await job.obtain()
                    continuation.yield(worth)
                } catch {
                    continuation.end(throwing: error)
                    isAlive = false
                }
            }
        }
    }
}()

The duty that we run our whereas loop in gained’t finish until we finish our stream from inside our catch block. If we manually shut the net socket connection utilizing the cancel methodology we write earlier, the decision to obtain() won’t ever obtain an error nor a worth which implies that will probably be caught perpetually.

Essentially the most dependable approach to repair that is to return to the callback primarily based model of obtain to drive your async stream:

non-public lazy var stream: WebSocketStream = {
    return WebSocketStream { continuation in
        self.continuation = continuation
        waitForNextValue()
    }
}()

non-public func waitForNextValue() {
    guard job.closeCode == .invalid else {
        continuation?.end()
        return
    }

    job.obtain(completionHandler: { [weak self] lead to
        guard let continuation = self?.continuation else {
            return
        }

        do {
            let message = strive end result.get()
            continuation.yield(message)
            self?.waitForNextValue()
        } catch {
            continuation.end(throwing: error)
        }
    })
}

With this method we don’t have any lingering duties, and our name website is as clear and concise as ever; we’ve solely modified a few of our inner logic.

In Abstract

Swift Concurrency offers many helpful options for writing higher code, and Apple shortly adopted async / await for current APIs. Nevertheless, some APIs that might be helpful are lacking, akin to iterating over internet socket messages.

On this publish, you discovered learn how to use async streams to create an async sequence that emits internet socket messages. You first noticed a completely async / await model that was neat, however had reminiscence and job lifecycle points. Then, you noticed a model that mixes a callback-based method with the async stream.

The result’s a straightforward approach to iterate over incoming internet socket messages with async / await. When you have any questions, feedback, or enhancements for this publish, please do not hesitate to succeed in out to me on Twitter.

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