Understanding Swift Concurrency’s AsyncStream – Donny Wals

In an earlier put up, I wrote about other ways you could bridge your present asynchronous code over to Swift’s new Concurrency system that leverages async / await. The mechanisms proven there work nice for code the place your code produces a single outcome that may be modeled as a single worth.

Nonetheless in some instances this isn’t attainable as a result of your present code will present a number of values over time. That is the case for issues like obtain progress, the person’s present location, and different related conditions.

Typically talking, these sorts of patterns could be modeled as AsyncSequence objects you could iterate over utilizing an asynchronous for loop. A primary instance of this may be the traces property on URL:

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

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

However what’s one of the best ways to construct your individual async sequences? Implementing the AsyncSequence protocol and constructing your on AsyncIterator sounds tedious and error-prone. Fortunately, there’s no motive so that you can be doing any of that.

On this put up, I’ll present you how one can leverage Swift’s AsyncStream to construct customized async sequences that produce values everytime you want them to.

Producing a easy async stream

An async stream will be produced in varied methods. The simplest solution to create an async stream is to make use of the AsyncStream(unfolding:) initializer. Its utilization seems to be a bit as follows:

let stream = AsyncStream(unfolding: {
    return Int.random(in: 0..<Int.max)
})

In fact, this instance isn’t significantly helpful by itself nevertheless it does present how easy the idea of AsyncStream(unfolding:) is. We use this model of AsyncStream each time we are able to produce and return return values for our async stream. The closure that’s handed to unfolding is async so because of this we are able to await asynchronous operations from inside our unfolding closure. Your unfolding closure will likely be known as each time you’re anticipated to start producing a worth in your stream. In observe because of this your closure will likely be known as, you carry out some work, you come a worth after which your closure known as. This repeats till the for loop is cancelled, the duty that accommodates your async for loop is cancelled, or till you come nil out of your unfolding closure.

The AsyncStream(unfolding:) solution to produce a stream of values is kind of handy nevertheless it’s significantly helpful in conditions the place:

• You need to carry out async work that must be awaited to provide parts
• You may have a must deal with again stress when bridging an API you personal

While you’re bridging an present API that’s primarily based on delegates or for APIs that leverage callbacks to speak outcomes, you most likely gained’t be capable of use AsyncStream(unfolding:). Whereas it’s the only and least error-prone solution to construct an async stream, it’s additionally the way in which that I’ve discovered to be most limiting and it doesn’t typically match properly with bridging present code over to Swift Concurrency.

Extra flexibility will be discovered within the continuation primarily based API for AsyncStream.

Producing an async stream with a continuation

When an asynchronous closure doesn’t fairly suit your use case for creating your individual async stream, a continuation primarily based method could be a significantly better resolution for you. With a continuation you’ve got the flexibility to assemble an async stream object and ship values over the async stream each time values change into out there.

We are able to do that by creating an AsyncStream utilizing the AsyncStream(construct:) initializer:

let stream2 = AsyncStream { cont in
    cont.yield(Int.random(in: 0..<Int.max))
}

The instance above creates an AsyncStream that produces a single integer worth. This worth is produced by calling yield on the continuation. Each time we’ve a worth to ship, we must always name yield on the continuation with the worth that we need to ship.

If we’re constructing an AsyncStream that wraps a delegate primarily based API, we are able to maintain on to our continuation within the delegate object and name yield each time a related delegate methodology known as.

For instance, we might name continuation.yield from inside a CLLocationManagerDelegate each time a brand new person location is made out there to us:

class AsyncLocationStream: NSObject, CLLocationManagerDelegate {
    lazy var stream: AsyncStream<CLLocation> = {
        AsyncStream { (continuation: AsyncStream<CLLocation>.Continuation) -> Void in
            self.continuation = continuation
        }
    }()
    var continuation: AsyncStream<CLLocation>.Continuation?

    func locationManager(_ supervisor: CLLocationManager, didUpdateLocations areas: [CLLocation]) {

        for location in areas {
            continuation?.yield(location)
        }
    }
}

The instance above is a really naive start line for creating an async stream of person areas. There are a few issues we don’t totally take note of similar to cancelling and beginning location statement or asking for location permissions.

At its core although, this instance is a good start line for experimenting with async streams.

Be aware that this method won’t anticipate shoppers of your async stream to devour a worth totally earlier than you may ship your subsequent worth down the stream. As a substitute, all values that you just ship will likely be buffered in your async stream by default which can or might not be what you need.

In sensible phrases because of this whenever you ship values down your stream sooner than the consuming for loop can course of these values, you’ll find yourself with a buffer crammed with values that will likely be delivered to the consuming for loop with a delay. This could be precisely what you want, but when the values you ship are considerably time delicate and ephemeral it might doubtlessly make sense to drop values if the consuming for loop isn’t able to obtain values.

We might resolve that we by no means need to maintain on to greater than 1 location and that we solely need to buffer the final identified location to keep away from processing stale knowledge. We are able to do that by setting a buffering coverage on our async stream:

lazy var stream: AsyncStream<CLLocation> = {
    AsyncStream(bufferingPolicy: .bufferingNewest(1)) { (continuation: AsyncStream<CLLocation>.Continuation) -> Void in
        self.continuation = continuation
    }
}()

This code passes a bufferingPolicy of .bufferingNewest(1) to our AsyncStream. Which means we are going to solely buffer a single worth if the consuming for loop isn’t processing gadgets quick sufficient, and we are going to discard older values in favor of maintaining solely the newest location.

If our stream involves a pure shut, you may name end() in your continuation to finish the stream of values.

In case your stream may fail with an error, you can too select to create an AsyncThrowingStream as an alternative of an AsyncStream. The important thing distinction is that buyers of a throwing stream should await new values utilizing strive await as an alternative simply await. To make your stream throw an error you may both name end(throwing:) in your continuation or you may name yield(with:) utilizing a End result object that represents a failure.

Whereas the fundamentals of constructing an AsyncStream aren’t significantly advanced, we do want to consider how we handle the lifecycles of the issues we create rigorously. Particularly as a result of we’re not purported to make our continuations outlive our streams which is an easy mistake to make whenever you’re bridging present delegate primarily based code.

Managing your stream’s lifecycle

There are primarily two methods for an async stream to finish. First, the stream may naturally finish producing values as a result of no additional values will be produced. You’ll name end in your continuation and you’ll present any cleanup that you could do on the identical time. For instance, you could possibly set the continuation that you just’re holding on to to nil to be sure to can’t by chance use it anymore.

Alternatively, your stream can finish as a result of the duty that’s used to run your async stream is cancelled. Take into account the next:

let areas = AsyncLocationStream()

let activity = Job {
    for await location in areas.stream {
        print(location)
    }
}

activity.cancel()

When one thing just like the above occurs, we are going to need to make it possible for we don’t name yield on our continuation anymore except we begin a brand new stream with a brand new, energetic, continuation.

We are able to detect and reply to the tip of our stream by setting an onTermination handler on our continuation:

self.continuation?.onTermination = { lead to
    print(outcome)
    self.continuation = nil
}

Ideally we set this handler instantly once we first create our async stream.

Along with the stream being cancelled or in any other case going out of scope, we might break out of our loop which can finally trigger our activity to complete. That is typically talking not one thing this can finish your async stream so in order for you breaking out of your loop to finish your stream, you have to to take this into consideration your self.

Personally, I’ve discovered that the best solution to be sure to do some cleanup is to have some methodology in your stream producing object to cancel the stream as an alternative of simply breaking out of an async for loop. That manner, you may carry out cleanup and never have a stream that’s sending values regardless that no one is listening.

It’s additionally necessary to keep in mind that the sample I confirmed earlier will solely work if one client makes use of your location stream object. You can not have a number of for loops iterating over a single stream in Swift Concurrency as a result of by default, async sequences lack the flexibility to share their iterations with a number of loops.

In Abstract

On this put up, you discovered lots about async streams and how one can produce your individual async sequences. First, you noticed the unfolding method of constructing an async stream and also you discovered that this method is comparatively simple however may not be very helpful for those who must bridge present delegate or callback primarily based APIs.

After exploring unfolding for a bit, we took a take a look at the construct closure for async streams. You discovered that this method leverages a continuation object that may be known as to provide values if and when wanted.

You noticed a really rudimentary instance of an object that will bridge a CLLocationManager into async await, and also you discovered a however about accurately managing your continuations to forestall sending values into an already accomplished stream.

In case you have any questions or feedback for me about this put up, please be at liberty to achieve out on Twitter or on Mastodon.