Understanding Swift Concurrency’s AsyncStream – Donny Wals

In an earlier put up, I wrote about other ways you can 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 circumstances 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 comparable conditions.

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

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

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

However what’s one of the simplest ways to construct your personal async sequences? Implementing the AsyncSequence protocol and constructing your on AsyncIterator sounds tedious and error-prone. Fortunately, there’s no purpose 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 could be produced in varied methods. The best method to create an async stream is to make use of the AsyncStream(unfolding:) initializer. Its utilization appears 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 but it surely does present how easy the idea of AsyncStream(unfolding:) is. We use this model of AsyncStream every time we are able to produce and return return values for our async stream. The shut that’s handed to unfolding is async so which means we are able to await asynchronous operations from inside our unfolding closure. Your unfolding closure might be referred to as each time you’re anticipated to start producing a price in your stream. In observe which means your closure might be referred to as, you carry out some work, you come back a price after which your closure known as. This repeats till the for loop is cancelled, the duty that incorporates your async for loop is cancelled, or till you come back nil out of your unfolding closure.

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

• You need to carry out async work that must be awaited to provide components
• You’ve got a must deal with again strain when bridging an API you personal

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

Extra flexibility could be discovered within the continuation based mostly API for AsyncStream.

Producing an async stream with a continuation

When an asynchronous closure doesn’t fairly suit your use case for creating your personal async stream, a continuation based mostly strategy is perhaps a a lot better answer for you. With a continuation you have got the power to assemble an async stream object and ship values over the async stream every time values turn out to be out there.

We will 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 got a price 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 based mostly API, we are able to maintain on to our continuation within the delegate object and name yield every time a related delegate technique known as.

For instance, we might name continuation.yield from inside a CLLocationManagerDelegate every 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 place to begin for creating an async stream of person areas. There are a few issues we don’t absolutely consider resembling cancelling and beginning location remark or asking for location permissions.

At its core although, this instance is a good place to begin for experimenting with async streams.

Word that this strategy won’t look forward to shoppers of your async stream to eat a price absolutely earlier than you may ship your subsequent worth down the stream. As a substitute, all values that you just ship might be buffered in your async stream by default which can or will not be what you need.

In sensible phrases which means while you ship values down your stream quicker than the consuming for loop can course of these values, you’ll find yourself with a buffer stuffed with values that might be delivered to the consuming for loop with a delay. This is perhaps precisely what you want, but when the values you ship are considerably time delicate and ephemeral it could probably 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 recognized location to keep away from processing stale information. We will 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 that we’ll solely buffer a single worth if the consuming for loop isn’t processing gadgets quick sufficient, and we’ll discard older values in favor of maintaining solely the most recent 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 customers 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 complicated, we do want to consider how we handle the lifecycles of the issues we create rigorously. Particularly as a result of we’re not speculated to make our continuations outlive our streams which is an easy mistake to make while you’re bridging present delegate based mostly 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 could be produced. You’ll name end in your continuation and you may present any cleanup that you have to do on the similar time. For instance, you might set the continuation that you just’re holding on to to nil to be sure you can’t by accident use it anymore.

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

let areas = AsyncLocationStream()

let job = Process {
    for await location in areas.stream {
        print(location)
    }
}

job.cancel()

When one thing just like the above occurs, we’ll need to be sure that we don’t name yield on our continuation anymore until we begin a brand new stream with a brand new, energetic, continuation.

We will detect and reply to the top 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 job to complete. That is usually talking not one thing it will finish your async stream so if you’d like breaking out of your loop to finish your stream, you will have to take this into consideration your self.

Personally, I’ve discovered that the simplest method to be sure you do some cleanup is to have some technique 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 though no person is listening.

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

In Abstract

On this put up, you discovered so much about async streams and how one can produce your personal async sequences. First, you noticed the unfolding strategy of constructing an async stream and also you discovered that this strategy is comparatively simple however won’t be very helpful for those that must bridge present delegate or callback based mostly APIs.

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

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

When you have any questions or feedback for me about this put up, please be happy to achieve out on Twitter or on Mastodon.