Wrapping Kotlin Flow with Swift Combine Publisher in a Kotlin Multiplatform project03 Jan 2021 Share on:
One area of “friction”, when developing an iOS client that uses Kotlin Multiplatform code, is the consumption of APIs from
shared code that expose a Kotlin Flow. This has typically required providing APIs for starting to collect the flow (and passing data back to iOS
client through provided closure/lambda) and then for cancelling the flow (through associated job). A related question then is how this process
can be integrated in to particular reactive frameworks being used on iOS. One example of such a framework is
Russell Wolf wrote an article (and published associated code )
last year that described how to manage process of integrating with that framework. I had started to use Swift’s
Combine framework in a number of projects
and this article describes exploration to use Russell’s approach with that.
The example I’m going to use is the polling for the position of the International Space Station (ISS) that’s performed in shared code in the
PeopleInSpace project. The following were the methods previously exposed
PeopleInSpaceRepository in the shared code.
These were invoked by following functions in our Swift View Model. Updates to the position are passed back to Swift code through provided closure and which
In our SwiftUI code then we trigger starting and stopping observation of the position data in
onDisappear callbacks respectively.
and then show the current position in UI then using the
issPosition value published from the view model.
Replacing with Swift Combine Publisher
Swift’s Combine framework, announced at WWDC 2019, is described as:
a declarative Swift API for processing values over time. These values can represent many kinds of asynchronous events. Combine declares publishers to expose values that can change over time, and subscribers to receive those values from the publishers.
Publisher maps pretty closely to Kotlin Flows and as such I thought it would be interesting to see what would be involved
in creating one that encapsulates access to the Kotlin Flows exposed from shared Kotlin Multiplatform code.
To start off with I added following
KotlinNativeFlowWrapper class to shared code. This is pretty much same code that Russell used in his
In this project the flow is running on the main thread and then invoking “main-safe” suspend functions provided by
Ktor. If the flow does need to run on
a background thread then you’ll need to update this (as per Russell’s article) to invoke
freeze() in appropriate places. (there’s more information about
the Kotlin/Native concurrency model that makes this necessary in this article)
We can then use that to wrap
pollISSPosition()(this is the function that returns the
flow and that’s currently consumed directly in Android client).
With that in place we can now create
IssPositionPublisher, our Combine Publisher. This in turn exposes
subscribe() using the flow wrapper returned by
repository.iosPollISSPosition() (and also using the
iosScope coroutine scope also
PeopleInSpaceRepository. We store the job returned and use that then to cancel the flow when the associated Combine Subscription is cancelled.
Now in our Swift view model we can call following which will update
issPosition (this is same variable we used in original code) whenever there’s an update.
As with flows a benefit of having our stream of data represented by a
Publisher like this is that we can start to say combine multiple sources or apply
flatMap. For example we can update our view model to include following so that formatted version of the information is exposed
to the UI.
Note that the code shown here has been pushed to PeopleInSpace repo.
At this point this is primarily a proof of concept. A more robust version would firstly need of course to build out proper error handling (e.g. mapping from
any errors returned in
onThrow to something exposed from the Publisher). It also feels like there could be possibility of automatically generating code from
the original flow function in shared code (using perhaps some custom annotation)…generating both Kotlin wrapper function and also the Swift Publisher. Much of
what’s described here also applies to “normal” suspend functions (as original article discussed in RxSwift case).
Featured in Kotlin Weekly Issue #232
Wrapping Kotlin Flow with Swift Combine Publisher in a Kotlin Multiplatform project https://t.co/0T72z3w1PB— John O'Reilly #BlackLivesMatter (@joreilly) January 3, 2021
Was curious if similar approach described below could be used with Swift Combine Publisher. Code in article has been pushed to PeopleInSpace repo (https://t.co/RjvKxaMjMG) https://t.co/hLpEEogXvo