In iOS 13, we gained the flexibility to simply ship and obtain information utilizing internet sockets by means of URLSession. With async/await, we gained the flexibility to fetch information from servers utilizing the await key phrase and we will iterate over asynchronous sequences utilizing async for loops.

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

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

for attempt await line in url.strains {
    // use line
}

Whereas that is actually cool and permits us to construct apps that ingest information in actual time if the server helps streaming our bodies, we can not use the strains property to arrange an online socket connection and hear for incoming messages and probably ship messages over the identical connection too.

On this publish, you’ll study every part you have to find out about constructing your individual mechanism to conveniently iterate over messages from an online socket asynchronously. We are going to leverage some current performance from URLSessionWebSocketTask and AsyncThrowingStream to construct our personal AsyncSequence that conveniently wraps our URLSessionWebSocketTask.

Word that the ensuing code has solely had comparatively restricted testing carried out so I can not assure that the supplied answer will probably be 100% appropriate for every part you throw at it. If you happen to discover any points with the ultimate code, be happy to contact me. Bonus factors for those who’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 rapidly evaluate easy methods to use an online socket with out async/await. The code particulars are outlined in this publish. You should definitely learn it if you wish to study 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 = attempt outcome.get()
            change message {
            case let .string(string):
                print(string)
            case let .information(information):
                print(information)
            @unknown default:
                print("unkown message obtained")
            }
        } catch {
            // deal with the error
            print(error)
        }
    }
}

setReceiveHandler()

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

This can be a nice instance of a scenario the place an async for loop resembling for attempt await message in socketConnection would make loads of sense. Sadly, this isn’t potential out of the field. Nevertheless, URLSessionWebSocketTask gives some type of help for async / await so we’re not completely 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 technique you noticed earlier.

This enables us to rewrite the instance above as an async technique as follows:

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

        change message {
        case let .string(string):
          print(string)
        case let .information(information):
          print(information)
        @unknown default:
          print("unkown message obtained")
        }
    } catch {
        print(error)
    }

    await setReceiveHandler()
}

This code works simply nice, besides we don’t actually have a method to cease the recursion right here. The code you noticed earlier really has the very same concern; 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 continues to be lively

This is able to look a bit as follows:

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

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

        change message {
        case let .string(string):
          print(string)
        case let .information(information):
          print(information)
        @unknown default:
          print("unkown message obtained")
        }

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

An open internet socket’s closed code is all the time stated to invalid to sign that the connection has not (but) been closed. We are able to leverage this to test that our connection continues to be lively earlier than ready for the following message to be obtained.

That is a lot 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 = attempt await socketConnection.obtain()

            change message {
            case let .string(string):
              print(string)
            case let .information(information):
              print(information)
            @unknown default:
              print("unkown message obtained")
            }
        } catch {
            print(error)
            isActive = false
        }
    }
}

To me, this model of the code is barely simpler to learn however which 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 proud of the place we’ve landed proper now. There’s loads of logic on this operate and I would favor to separate dealing with the incoming values from the calls to socketConnection.obtain() one way or the other. Ideally, I ought to have the ability to write the next:

do {
    for attempt await message in socketConnection {
        change message {
        case let .string(string):
            print(string)
        case let .information(information):
            print(information)
        @unknown default:
            print("unkown message obtained")
      }
} catch {
    // deal with error
}

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

To do that, we will leverage the ability 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 couple of methods for us to get the place we wish to be. The best means can be to write down a operate in an extension on URLSessionWebSocketTask that may 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
            Activity {
                var isAlive = true

                whereas isAlive && closeCode == .invalid {
                    do {
                        let worth = attempt 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 sort signature in all places.

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 may be one concern. If we determine to shut the net socket connection from the app’s facet by calling cancel(with:purpose:) on our socketConnection, our WebSocketStream doesn’t finish. As an alternative, will probably be caught ready for messages, and the decision website will probably be caught too.

Activity {
    attempt await Activity.sleep(for: .seconds(5))
    attempt await socketConnection.cancel(with: .goingAway, purpose: nil)
}

Activity {    
    do {
        for attempt await message in socketConnection.stream {
            // deal with incoming messages
        }
    } catch {
        // deal with error
    }

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

If every part 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 now not lively. Sadly, this isn’t the case, so we have to discover a higher strategy to mannequin our stream.

URLSessionWebSocketTask doesn’t present a means 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 by means of that object. This enables us to each finish the async stream we’re offering to callers and shut the net socket reference to one technique 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

            Activity {
                var isAlive = true

                whereas isAlive && job.closeCode == .invalid {
                    do {
                        let worth = attempt 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, purpose: nil)
        continuation?.end()
    }
}

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

When our SocketStream‘s deinit known as we be sure that we finish our stream. There’s additionally a cancel technique 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 inside 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)

Activity {  
    do {
        for attempt await message in stream {
            // deal with incoming messages
        }
    } catch {
        // deal with error
    }

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

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

Activity {
    attempt await Activity.sleep(for: .seconds(5))
    attempt await stream.cancel()
}

Activity {
    // iterate...
}

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

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

        Activity {
            var isAlive = true

            whereas isAlive && job.closeCode == .invalid {
                do {
                    let worth = attempt 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 technique we write earlier, the decision to obtain() won’t ever obtain an error nor a worth which signifies that will probably be caught without end.

Essentially the most dependable strategy to repair that is to return to the callback based mostly 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 = attempt outcome.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 inside logic.

In Abstract

Swift Concurrency gives many helpful options for writing higher code, and Apple rapidly adopted async / await for current APIs. Nevertheless, some APIs that may be helpful are lacking, resembling iterating over internet socket messages.

On this publish, you discovered easy methods 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 strategy 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 achieve out to me on Twitter.