An Ergonomics Review of Using Kotlin from Swift

16 Aug 2020 - 14 minute read Updated: 16 Feb 2021

At Autodesk, my colleagues and I are more than a year and a half into our Kotlin multiplatform (KMP) shared library journey. That’s one Kotlin shared library, shared among our three mobile platforms that we support for the PlanGrid app (iOS, Android, and Windows).

Most new feature development for the PlanGrid app starts in the shared library now. It has become so much an extension of our main application that we’re planning a move to a mono-repo (iOS, Android, Windows, and the KMP shared library all in one place) later this year, which will help solve some of our scale issues (great problem to have all things considered).

Making it Work

Looking back, there were a few things that made Kotlin work well for us that have to do with a combination of developer experience and our team. Early on, the few of us working on the proof-of-concept and pitching KMP to the rest of the team focused on developer experience. We ensured that integrating our experiment wouldn’t get in the way of day-to-day work by distributing it to iOS as a binary framework via CocoaPods (already integrate other binary frameworks this way without issue).

While testing out KMP-based functionality alongside all of the other work going on, we made use of feature flags in case shit hit the fan in production. Once we had our foot in the door, had proved things worked well, and had come up with a plan for how the library was going to evolve, the next step was getting people to adopt the shared library for their teams’ features.

This again takes us back to developer experience. Lucky for us, KMP comes with iOS interop out-of-the-box. It’s so good that one of my colleagues thought they were using some kind of bridging layer that we must have written to make Kotlin feel Swift-y. When they command-clicked through to source, they were surprised to find the KMP-generated shared library header.

Having a Good Team Helps

Did I mention we have a Windows team? For that side of things, we got lucky. The level of support from KMP you get for Windows does not measure up to what you get for iOS. On the Windows side, my colleagues would like to have a C# library in the style of the KMP-generated Obj-C library with the nice, generated headers that we get on iOS. Instead, they get a library that uses C-interop— quite a different experience. Fortunate for us, a few folks on our Windows team were experienced and interested enough to write and maintain their own C# code generation on top of that. We hope to open source it later this year.

Looking at Interop

To recap, these elements made it possible for KMP to work well for us:

  1. Ease of integration on iOS and Android
  2. Ergonomic interop out-of-the-box for iOS
  3. Colleagues on our Windows team willing to take on the challenge of solving the interop issues there

If I could only pick one, it would be the interop on iOS that allowed this to become as successful as it has for us. It would be a much tougher sell, for example, had both the iOS and Windows teams needed to spend loads of effort on interop.

My favorite example of this great interop is one from my earlier post, where Phill and I wrote about the iOS interop:

// Kotlin
enum class LogLevel {

class Logger {
    companion object default {
        fun log(level: LogLevel = LogLevel.ERROR, message: String, completion: (Boolean) -> Unit) { }
// Swift
Logger.default.log(.error, "An error ocurred") {
    // Closure

This example highlights some idiomatic Kotlin that allows you to write idiomatic-looking Swift. It’s impressive. However, this example has a secret. It also highlights many of the areas where Kotlin/Native interop with iOS has room for improvement. With Kotlin 1.4.0 out the door, I hope now is a good time to raise these issues. Fixing them would take the sell to iOS teams to the next level, at least in terms of having excellent interop. I’ll discuss the issues in increasing in order of how long it took our team to notice and bump into them.

Exhaustive Enums

This is one of the earliest areas, for which I filed an issue in the Kotlin/Native GitHub. Enums are a great way to describe an input for an API, such as the above log method. However, we ran into problems with Kotlin enums as soon as we wanted to do an exhaustive switch (equivalent to exhaustive when in Kotlin) over them in Swift.

The problem lies in how enums are represented in Kotlin compared to Obj-C (remember Swift is irrelevant for comparison— interop is via Swift-y feeling Obj-C). Enums in Kotlin are reference types (a special class). In Obj-C, they’re integers. Attempting to switch over them from Swift would be like trying to switch over any other class instance that you defined in Swift, for example. The Swift compiler doesn’t know that there happens to be a finite number of instances of that enum class like Kotlin does.

There is a workaround though that can improve the ergonomics a bit. You can define a matching C-style enum (with a matching name), and redefine the ordinal property as having that type. At Autodesk, we do this with a bit of fragile (albeit has worked in our codebase with only one minor tweak since its creation) code generation that adds these C-style enums to the Obj-C framework header as Obj-C extensions on all of our enum types. With that, we get exhaustive switch for all of our Kotlin enums, by doing a switch over the ordinal property.

In the above sample code, you can see there’s no problem passing enums around. You wouldn’t have any idea about how Kotlin enums are represented, until you go to switch on one.

Companion Objects

In my first article, I mentioned how, as an iOS developer, I found the companion object a bit strange at first, but I get it now. As my iOS colleagues have become better acquainted with them, we have begun to see them more often in our shared library. The problem comes if you don’t know that you can name a companion object, like in the above example. Unnamed ones are more common, in my experience. Here is what the Swift code would look like, if we hadn’t named the above companion object “default”:

// Swift
Logger.Companion().log(.error, "An error ocurred") {
    // Closure

As someone who writes a lot of Swift, this looks funny at first. Are we creating a new Logger.Companion? If so, where can I see what this does in Kotlin? The ergonomics of the unnamed companion object is another issue that I filed early on. To answer the question, you aren’t creating a new Logger.Companion.

The fix here isn’t straightforward. You can convince your team to prefer naming a companion object, but that doesn’t always make sense, if say your goal is to use a companion object to namespace a public constant. Solutions discussed in the ticket would be breaking changes. That said, I think improving the ergonomics here would be an easy way to prevent less enthusiastic iOS developers from having this easy (and small) thing to point at. At Autodesk, we just acknowledge this quirk of the Obj-C export and move on. But, that’s easy for us to do now, as we have a critical mass.

Default Arguments

Although we bumped into this one later on, a fix for this would make the biggest difference for our team right now because our dependence on KMP has grown. If you notice in the above Kotlin, the log function has a default argument. However, you cannot use that default argument from Swift. As a log parameter, this might not be that important. In other contexts though, this could be a default integer parameter, for example. In that case, it can be near impossible to know what argument you should pass in the “default” case. To avoid bugs, your best bet is to go back to the Kotlin source and find the answer.

Though, I’ve heard folks say: but Swift supports default arguments, why shouldn’t this work? This is often the first place folks begin to internalize that this doesn’t matter. The interop is via Obj-C, so all that matters is what Obj-C supports. You can repeat this same answer for a bevy of similar complaints. Interface extensions? Swift supports the equivalent protocol extensions. Obj-C does not. Optional primitive types? Obj-C doesn’t have those either. None of this is a knock on Kotlin/Native’s interop, which again is great! Obj-C interop was the correct and stable choice at the time, as that was before Swift had a stable ABI. I made comments about this in my previous interop article, so let’s go back to the issue at hand.

Default arguments have a solution in Kotlin/JVM interop, which is used in hand-written Obj-C as well: generated overloads. Generating overloads for C and Obj-C KMP libraries would be a huge improvement to Kotlin/Native’s export facilities. I hope with Kotlin 1.4.0 out the door, time can be made for this one.

Enums - Missing Exports

I want to go back to enums for a moment. I mentioned that I would review the issues in the order that my team ran into them. After getting more comfortable using Kotlin enums in our code, we began to come up with cases where we wanted to enumerate Kotlin enums. However, the needed values() function is not exported to Obj-C by default. If you need this, you’re left to define it yourself in your library and have it call the equivalent function. The workaround is fine, but it gets in the way, when the library you’re using is one that’s already packaged up and distributed. A fix requires another PR and waiting on another CI deploy.

New in 1.4.30: Enum<T>.values()

As of Kotlin 1.4.30, your Kotlin enums now export a values() class method, which maps directly to the Enum<T>.values() Kotlin method. One thing to note though is that this direct mapping exposes the unfortunate (for Obj-C/Swift) Kotlin.Array. If you’re using Kotlin/Native, you may know that Kotlin.List is the type that Kotlin/Native exposes as a native NSArray/Swift.Array. With Kotlin.Array, you end up with a special KotlinArray type, which exposes an iterator. To make of use of this in your Swift code, you’ll want to write something like this:

extension Array where Element: AnyObject {
    /// Allows conversion from `KotlinArray` to `Array`
    public init(_ array: KotlinArray<Element>) {
        let iterator = array.iterator()
        while iterator.hasNext() {
            self.append( as! Element)

This will let you bridge KotlinArray to Swift.Array like so: Array(MyEnum.values()). Not bad! I also tried adding some kind of Swift extension to enable bridging like MyEnum.values().toArray(), but I ran into Obj-C to Swift generics compatibility issues. If you find a way to make that work though, please let me know on mastodon!

Translation to Obj-C Primitives

The final one I want to talk about is the most minor, but we do come across it on occasion. In the above example, we are hiding the fact that the completion closure has one parameter. It’s a Kotlin boolean, but it is translated to Obj-C as a KotlinBoolean class. This is bound to happen if the type is optional. Again, Obj-C doesn’t support optional primitives. In this case however, it’s not. It should be a BOOL instead, which will translate to a Swift Bool.

In most cases, Kotlin/Native does the right thing, but there are occasions like this one that come across a bit clunky. I admit though, I don’t have the expertise to understand why this happened in this case. Again, the interop you get for iOS is great. Fixing polish-type issues like this would take the interop to the next level.

Interface Method Collisions

Okay I lied. There’s one more quirk, but I felt that it didn’t fit as well with some of the above issues. It is also maybe a bit niche, and it depends on how you architect your code, as to whether or not you’ll bump into this one. In our codebase, we create an interface for each of our repository classes. Let’s say you have a repository like this one:

// Swift
interface FishRepository {
   fun fetchById(id: String): Fish?

Over time, other people on my team will also add repositories with fetchById methods with the same name. Why not? fetchById is a reasonable way to name that method. However, every time you add such a method, methods with the same name generated for Obj-C will get an underscore added to the end to disambiguate them from others. But why you ask? Again, the explanation has to do with Kotlin vs. Obj-C. Read this issue for the full details. At the time of this writing, one of our fetchById methods is up to seven underscores in Obj-C 😂.

If you have a look at the GitHub issue, there doesn’t appear to be an easy solution from the Kotlin/Native side. What we’ve started doing is just naming our methods better. In this case, renaming the method to fetchFishById should prevent this from happening (until we add another fishy repository). That said, it would be great if Kotlin/Native could emit a warning about such issues. Then, we can catch these during development before the Obj-C export happens, and a random API gets an extra underscore (making the overall change breaking). Kotlin 1.4.0 has new native-specific frontend checkers. A new one that helps us out here would be most welcome.

❤️ Kotlin/Native

Kotlin/Native’s interop for iOS is great, and it has allowed us to scale a shared codebase with little effort on the iOS side. That’s a huge deal, when you consider that most of the mobile engineers that work on the PlanGrid app had to learn a new language for this to work. Again, I hope folks (including those at JB) don’t read this as a knock on Kotlin/Native. Now that Kotlin 1.4.0 is out the door, I hope these issues can be addressed to take Kotlin/Native’s interop to the next level 🚀.

For those evaluating Kotlin/Native, of course you’ll want to know what issues lie ahead as you scale up. This lays out all of the source-level issues that we’ve run into. Despite these issues, it has been well worth it.

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Find a mistake or inaccuracy? Please open an issue or PR on github.