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

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

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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 by means of URLSession. With async/await, we gained the power to fetch knowledge from servers utilizing the await key phrase and we will 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 strains property on URL:

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

for strive await line in url.strains {
    // 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 not use the strains property to arrange an internet socket connection and pay attention for incoming messages and doubtlessly ship messages over the identical connection too.

On this put up, you’ll be taught every little thing you have to learn about constructing your individual mechanism to conveniently iterate over messages from an internet socket asynchronously. We’ll leverage some current performance from URLSessionWebSocketTask and AsyncThrowingStream to construct our personal AsyncSequence that conveniently wraps our URLSessionWebSocketTask.

Notice that the ensuing code has solely had comparatively restricted testing executed so I can not assure that the supplied resolution will probably be 100% right for every little thing you throw at it. For those who discover any points with the ultimate code, be happy to contact me. Bonus factors in case you’re capable of present some concepts for a possible repair.

Utilizing an internet socket with out async / await

Earlier than we get began, let’s shortly assessment tips on how to use an internet socket with out async/await. The code particulars are outlined in this put up. Make sure 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 technique solely permits me to obtain a single incoming message, so I need 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 equivalent to for strive await message in socketConnection would make loads of sense. Sadly, this isn’t potential out of the field. Nonetheless, 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 technique you noticed earlier.

This enables us to rewrite the instance above as an async technique 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 advantageous, besides we don’t actually have a way to cease the recursion right here. The code you noticed earlier truly has the very same subject; 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 energetic

This might 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 examine that our connection remains to be energetic earlier than ready for the following 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 an alternative 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 which may not be the case for you. It’s functionally equal so you may select to make use of whichever choice fits you greatest.

Whereas this code works, I’m not fairly pleased with the place we’ve landed proper now. There’s loads of logic on this operate and I would like 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 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 will 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 aim, there are a number of methods for us to get the place we need to be. The simplest manner 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
            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 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 energetic 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 subject. If we determine to shut the net socket connection from the app’s aspect by calling cancel(with:motive:) on our socketConnection, our WebSocketStream doesn’t finish. As an alternative, will probably be caught ready for messages, and the decision web site will probably be caught too.

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

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

    print("this may 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 energetic. Sadly, this isn’t the case, so we have to discover a higher method to mannequin our stream.

URLSessionWebSocketTask doesn’t present a manner 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 appears like:

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

    personal var continuation: WebSocketStream.Continuation?
    personal let process: URLSessionWebSocketTask

    personal lazy var stream: WebSocketStream = {
        return WebSocketStream { continuation in
            self.continuation = continuation

            Process {
                var isAlive = true

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

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

    deinit {
        continuation?.end()
    }

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

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

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

When our SocketStream‘s deinit is known as we guarantee 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 after 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 appears as follows:

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

Process {  
    do {
        for strive 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 may run the next process in parallel with our iterating process:

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 difficulty right here due to the next little bit of code:

personal lazy var stream: WebSocketStream = {
    return WebSocketStream { continuation in
        self.continuation = continuation

        Process {
            var isAlive = true

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

The duty that we run our whereas loop in received’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 ceaselessly.

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

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

personal func waitForNextValue() {
    guard process.closeCode == .invalid else {
        continuation?.end()
        return
    }

    process.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 strategy we don’t have any lingering duties, and our name web site 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. Nonetheless, some APIs that may be helpful are lacking, equivalent to iterating over internet socket messages.

On this put up, you realized tips on 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 process lifecycle points. Then, you noticed a model that mixes a callback-based strategy with the async stream.

The result’s a simple method to iterate over incoming internet socket messages with async / await. When you have any questions, feedback, or enhancements for this put up, please do not hesitate to achieve out to me on Twitter.

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