In an earlier publish, I wrote about alternative ways that you would be able to bridge your current 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.
Nevertheless in some circumstances this isn’t potential as a result of your current code will present a number of values over time. That is the case for issues like obtain progress, the consumer’s present location, and different related conditions.
Typically talking, these sorts of patterns can be modeled as AsyncSequence objects that you would be able to iterate over utilizing an asynchronous for loop. A fundamental instance of this is able to 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 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 cause so that you can be doing any of that.
On this publish, 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 numerous 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)
})After all, this instance isn’t notably 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 will produce and return return values for our async stream. The shut that’s handed to unfolding is async so because of this we will await asynchronous operations from inside our unfolding closure. Your unfolding closure will probably be known as each time you’re anticipated to start producing a worth to your stream. In observe because of this your closure will probably be known as, you carry out some work, you come back 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 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 notably helpful in conditions the place:
- You need to carry out async work that must be awaited to supply components
- You might have a have to deal with again strain when bridging an API you personal
Once you’re bridging an current API that’s based mostly on delegates or for APIs that leverage callbacks to speak outcomes, you in all probability 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 way in which that I’ve discovered to be most limiting and it doesn’t usually match effectively with bridging current code over to Swift Concurrency.
Extra flexibility will 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 individual async stream, a continuation based mostly strategy is perhaps a significantly better answer for you. With a continuation you’ve the flexibility to assemble an async stream object and ship values over the async stream every 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 now we have 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 based mostly API, we will 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 consumer 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 consumer areas. There are a few issues we don’t absolutely bear in mind equivalent to cancelling and beginning location statement or asking for location permissions.
At its core although, this instance is a superb place to begin for experimenting with async streams.
Word that this strategy is not going to anticipate customers of your async stream to eat a worth absolutely earlier than you possibly can ship your subsequent worth down the stream. As an alternative, all values that you just ship will probably be buffered in your async stream by default which can or will 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 probably 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 might 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 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. Because of this 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 most recent location.
If our stream involves a pure shut, you possibly can name end() in your continuation to finish the stream of values.
In case your stream may fail with an error, you may as well 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 possibly can both name end(throwing:) in your continuation or you possibly can name yield(with:) utilizing a Consequence object that represents a failure.
Whereas the fundamentals of constructing an AsyncStream aren’t notably complicated, we do want to consider how we handle the lifecycles of the issues we create fastidiously. Particularly as a result of we’re not purported to make our continuations outlive our streams which is a very simple mistake to make whenever you’re bridging current delegate based mostly code.
Managing your stream’s lifecycle
There are basically 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 it’s essential to do on the similar time. For instance, you would 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 job = Process {
for await location in areas.stream {
print(location)
}
}
job.cancel()When one thing just like the above occurs, we are going to 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, lively, 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 is able to finally trigger our job to complete. That is typically talking not one thing it will finish your async stream so if you would like 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 method to be sure to 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 method, you possibly can carry out cleanup and never have a stream that’s sending values though no person is listening.
It’s additionally vital 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’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 flexibility to share their iterations with a number of loops.
In Abstract
On this publish, you discovered lots about async streams and how one can produce your individual 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 have to bridge current delegate or callback based mostly APIs.
After exploring unfolding for a bit, we took a have a look at the construct closure for async streams. You discovered that this strategy leverages a continuation object that may be known as to supply 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 stop sending values into an already accomplished stream.
When you’ve got any questions or feedback for me about this publish, please be happy to succeed in out on Twitter or on Mastodon.
