Kotlin Multiplatform: testing a shared module supporting iOS and Android

🇫🇷 Parlez-vous Français ? You can read a French Version of this post on our blog at Xebia France.

Conceiving your app as modular can be greatly beneficial to your codebase’s health. In particular, designing a separation between your view and business logic can decrease the coupling between your layers and improve maintainability as well as testability.

Tests are indeed your app’s best friends, as they can, at the same time, verify the code you wrote works as intended and act as a specification for the implemented functionality.

When it comes to Multiplatform, Kotlin provides an interesting solution for testing your logic: in this configuration, every written test is executed against each one of your targeted platform.

In other words, given you target are Android and iOS (via Kotlin/Native), your tests will be compiled and launched in both the JVM and the native runtime.

The setup of unit tests for Multiplatform can be tricky, though. Let’s see how to get a working configuration.

In order to do this, we’ll proceed from where we left in our last post.

As you might recall, our module did nothing more than producing two strings, bar-android and bar-ios for Android and iOS respectively. Thanks to test in Kotlin Multiplatform we’ll be able to verify that, no matter the targeted platform, the produced value starts with bar-.

Writing our Test

The full code for this article is available in the feature/multiplatform branch of our kotlin-ios-framework repository.

So, time to create a new file. First of all, let’s create a folder holding our test: myframework/src/test/kotlin/fr/xebia/myframework/. As you would expect, the path mirrors the one containing our implementation under test. In this directory we’ll create a new test class, TestFoo, whose contents are as follows:

As you can see, Kotlin comes with a testing and assertion framework, so no need to declare another dependency to your project.

Of course, for tests to be run, we need to edit our build.gradle files. Let’s start with the common project’s: it should declare two additional dependencies: kotlin-test-common and kotlin-test-annotations.

Please note that, in Kotlin Multiplatform, since tests can be only run against a specific target platform (such as Android’s JVM), kotlin-test-common will not suffice to be running your verification logic, but acts as a placeholder for an actual testing framework such as JUnit.

As a proof, we can already try compiling the tests, which action will produce a build error, as the platform target won’t know how to interpret annotations and assertions.

build.gradle for Android

Quite unsurprisingly, we’ll have to edit the build.gradle file for the Android project as well. Similarly to the build file included in the common project, the Android’s should declare additional dependencies, needed for testing: JUnit, kotlin-test-junit and kotlin-test. Those dependencies contain the actual frameworks that will be executing during the execution of your tests.

build.gradle for Kotlin/Native

Last but not least, let’s configure the gradle file for the Kotlin/Native project used on iOS, which setup is definitely trickier.

In the current version of Kotlin/Native, in order for tests to be launched, we’ll have to create a “dummy” program that will hold both our “main” and “test” code and that will be executed when running the “test” task with gradle.

To do so, let’s then add a new program in the build.gradle file contained in our myframework-ios folder:

Second, we need to ask gradle to run the program upon launch of the “test” task. To do so we’ll have to add a new task, at the bottom of the file:

When running gradle test, our program will compile, get executed and produce the following output:

Once again, you can grab the full code for this article from the feature/multiplatform branch of our kotlin-ios-framework repository.

Going Further

A nice feature of such configuration is that we can add specific tests for each platform. For instance, in case we want to add a more detailed test for our Kotlin/Native target, we’ll just need to create a new test folder in the myframework-ios subproject – without forgetting to add such folder as srcDir inside the build.gradle file.

Wrapping Up

As seen in this post, configuring a test environment for Multiplatform tests is just a matter of adding a few lines of code. Once we get the right setup, tests will be run on all the targeted platforms with little effort.

Sharing frameworks between iOS and Android with Kotlin Multiplatform

🇫🇷 Parlez-vous Français ? You can read a French Version of this post on our blog at Xebia France.

Today we’ll see how to use Kotlin code to write a cross platform module that can be shared between an iOS and an Android app.

Since the support for building iOS frameworks, introduced with Kotlin/Native 0.5 in December 2017, it has been possible to use the same code for creating libraries for both Android (.aar) and iOS (.framework). A couple of months later, Kotlin/Native 0.6 officially supports this use case, thanks to the support of Multiplatform, a new functionality focused on sharing code across platforms, introduced in the version 1.2 of the Kotlin language.

Today’s goal will be to create two modules, one for iOS and another one for Android, sharing a common interface but providing slightly different implementations, both written in Kotlin.

Multiplatform supports a DSL bringing two new kinds of modules:

  • “Common”: they contain code that is not specific to any platform, and unimplemented (“apxpect”) placeholder declarations that need to rely on a platform-specific implementation
  • “Platform”: they contain platform-specific code that implement the placeholder declarations in the common module. Platform modules can also contain other platform-dependent code.
  • “Regular”: they regroup those modules which are neither “Common” nor “Platform”.

A good example for this need is File I/O: as expected, any implementation of file access or networking is beyond the scope of Kotlin Standard Library. So, in case our commons module wants to use the file system, it’ll have to rely on two different underlying implementations which are platform dependent. For instance, to access files on Android, we would be using the Java.io.File while on iOS we would rather use NSFileManager.

So, let’s begin where we left in our previous post about the creation of iOS frameworks with Kotlin/Native.

Our module didn’t do anything except returning a string value. So, we’ll modify this behaviour by making our function return a different value depending on the system our module runs on. Our two implementations will be packaged under two different forms: an Android Archive (.aar) for Android and a Framework for iOS.

In order to do this we have to slightly change our project structure by adding two new folders corresponding to the Platform modules (myframework-ios and myframework-android), and create a directory tree containing “src/main/kotlin/fr/xebia/myframework” in both of them.

The resulting file tree should now be as follows:


The common project


Now, we have to make sure Gradle recognises myrframework-android as part of our project. To do this, we need to edit our settings.gradle file and, in the second line, add :myframework-android and :myframework-ios.

The complete file contents are:

We can now run

to double check your new configuration does not output any error.


Time to modify our build tasks: in our main build.gradle, we’ll have to change the kotlin version to 1.2.30. This is needed in order to use the Multiplatform feature on Android.

The new, complete build.gradle is as follows:

Back to the code

Let’s get back to our Foo class. We’d like to provide an extremely basic implementation, in which the bar() function will return a different string, depending on the OS. On Android, the returned string will be "bar-android", while, on iOS, "bar-ios".

Now, it’s the time to introduce a new keyword: expect. By the means of this we can tell the compiler that this declaration is nothing more than a placeholder – but that we’ll be expecting a concrete, actual, implementation from one of our platform-dependent projects.

So, we’ll modify the Foo.kt file inside the myframework subproject as follows:


Also, we need to declare that this project supports Multiplatform: in order to do this, we’ll have to modify the build.gradle at myframework/build.gradle as follows:

The actual implementation

So, it’s now time to get to the platform-specific implementations, starting with Android.

We’ll now add a new build.gradle just inside the  myframework-android, holding the configuration for this subproject.

build.gradle (Android)

In the new build gradle we’re going to declare that myframework-android provides an implementation that is expected by another project, namely myframework. We do this by using the expectedBy scope.

Also, and foremost, let’s not forget we’re building an Android library here, so we’ll have to add all the settings required by such context, under the ‘android’ property.

Here’s how the build.gradle file looks like for the myframework-android subproject:


Since we’re building an .aar, we also need for an AndroidManifest.xml, which is required by the targets.

In this project, we’ll add, inside the myframework-android/src/main folder, a simple manifest file.


It is the time to provide the platform-specific implementation. In this extremely basic implementation, the bar() function will return a different string, depending on the OS. On Android, the returned string will be "bar-android".

In order to do this, we have to make sure our “actual” implementation:

  • Belongs to the same package as the base (or “expect”) implementation
  • Uses the keywords “actual”, both before class (and optionally before its constructor) and before the actual function implementation.

The result is as follows:



Creating the iOS counterpart doesn’t differ much from what we learned in our previous article.

First of all, we have to add a new build.gradle file containing the konan (i.e., the Kotlin/Native compiler) definitions. As mentioned, the build.gradle file should look quite familiar now, with the exception of the enableMultiplatform true directive, enabling the Multiplatform support in this subproject.

As we did in the Android counterpart, the expectedBy directive under dependencies tells gradle that our project contains the “actual” implementation of myframework.



The code is once again a matter of few lines. We’ll be adding a Foo.kt class inside myframework-ios/src/main/kotlin/fr/xebia/myframework

In this case, the “actual” implementation of the Foo class will return the string bar-ios. Once again, in order for the class to match the “expected” one, we must remember to declare the package this file lives in.

And here is the full implementation:



And that’s all. By running

we should now be able to get all the iOS (e.g. konanCompile) and Android tasks.

We can build both iOS and Android at the same time by running, from our project folder.

The artefacts will be created in the myframework-ios/build and myframework-android/build folders.

You can get a complete example of the code above from the feature/multiplatform branch of this repo.

Wrapping Up

As we’ve seen, the workflow introduced by Kotlin multiplatform is pretty straightforward and has been quickly implemented in Kotlin/Native. I can say I am quite surprised to see so many features getting implemented into Kotlin and Kotlin/Native and I’m really looking forward to seeing the reception of mobile developers in respect to such tools.


Once again, I particularly wish to thank my fellow colleagues at Xebia, Bastien Bonnet, Sergio Dos Santos and Benjamin Lacroix, for thoroughly reviewing this article.

Creating an iOS framework with… Kotlin

🇫🇷 Parlez-vous Français ? You can read a French Version of this post on our blog at Xebia France.
Kotlin/Native is a new Kotlin dialect which supports compiling code in an executable form that can be run on the targeted system, without needing for a JVM. The Kotlin/Native compiler, named Konan, interfaces with the LLVM compiler infrastructure in order to support a number of targets, such as Linux, macOS, iOS, Web Assembly and others.
First announced in April, Kotlin/Native has now reached v0.6, published in February 2018. Since its version 0.5 Kotlin/Native introduces a new target, which can particularly be useful for iOS projects, that is iOS frameworks.
In this article, we’ll see how to create an iOS framework using Kotlin code. Our library will actually do nothing too fancy, but it’ll be a great starting point for future evolutions.
Please note that to complete this tutorial you will need a machine running macOS and Xcode 9.2.

How Konan works

As mentioned above, Konan interfaces with the LLVM backend compiler in order to support a number of targets, such as Linux, macOS, iOS, Web Assembly and others. The LLVM compiler takes as input an intermediate representation (IR, or LLVM-IR) and generates machine code (or, better, object code) for a number of supported CPU architectures (such as ARM, x86 and many others).
In other words, Konan will compile Kotlin code to LLVM-IR and LLVM will take care of the rest, by compiling LLVM-IR down to ARM (or other CPUs).
Kotlin and LLVM

Before we start: Gradle

In order to complete this tutorial, you’ll need to install Gradle first. If you’ve never stumbled on it, Gradle is the main build tool in the JVM environment and it is used by the vast majority of Android projects. With Gradle, we’ll be able to define dependencies, and define tasks that will take care of compiling our project. Also, and foremost, gradle is easily extendable via plugins, and we’ll actually use one of them to help us creating the tasks we need to create our framework.
The easiest way to install Gradle is via Homebrew:

Project Setup

We then create a new project folder, for instance “MyProject”.
We can now open the shell inside “MyProject” and run

This operation will create the basic configuration for our project.

Now, we’ll need to add the following resources to “MyProject”:
  • An empty gradle.properties file
  • A folder, named “myframework”, which will contain code and properties for our library
  • Inside the “myframework” folder, we’ll be adding:
    • A tree of folders as follows: src/main/kotlin/fr/xebia/myframework
    • Another empty build.gradle file
The result should now be as follows:

This structure follows the conventions of most Java and Kotlin projects and allows us to prepare our project to host other subprojects in the future in case we need it (and we sure will in a further post).

Configuring the environment

Now we’ll be setting the basic properties of our environment.
The settings.gradle file should reference reference the modules of the project we’re building. In our case, it’s “myframework”, so we’ll add to the file the following line:

The root build.gradle file will contain the main configuration used throughout our project and subproject. In particular, we’ll set the kotlin_version to be used (1.2.21) and the version of the compiler (0.6). We also set which repositories will be used by our build script, i.e. jCenter.

Configuring the “myframework” subproject

Let’s open the build.gradle contained in the “myframework” subproject. This file instructs Konan on what and how it will need to handle the compilation.
In particular, via the apply plugin directive, we’ll define the plugin to be used in our context: ‘konan’, which is of course, the plugin used for easily interfacing with the Kotlin/Native compiler.
Also, we’ll instruct our script about its repository: i.e., maven, and its dependency, the kotlin-native-gradle-plugin.

Finally, we’ll add to the same build.gradle file the directives for creating the iOS framework. We’ll be targeting iPhone and the iPhone Simulator, which are based on two different architectures, namely ARM for the iPhone and x86 for the Simulator. We’re also setting, in the framework parameters, the output file name of our task: ‘MyKotlinFramework’.

Here, we’ll also indicate which folder contains our source code, in our case src/main/kotlin.
Not much else to see here, apart from enabling the debugging mode, via the enableDebug true line.

Adding some source code

Now it’s finally the time to add some Kotlin code.

As you clearly understand, this code doesn’t do much, but it’s enough to try out the basic functionalities of Kotlin/Native.


That’s pretty much all. We just need to compile our project by running grade.
Before doing this, let’s just run

This should output a number of tasks preconfigured by Gradle. Among them, the compileKonan task, compileKonanMyKotlinFrameworkIphone, compileKonanMyKotlinFrameworkIphone_sim are automatically created by the konan plugin applied in the build.gradle file: as the names say, they will compile code and produce frameworks for iPhone and iPhone simulator.

We can now run

and wait for the project to compile. Please note that Gradle will also take care of automatically downloading the Kotlin/Native compiler for us – even though this’ll take a while.

When the compilation is finished (it may take a little while at the time of writing), verify the contents of the folder myframework/build/konan/bin/iphone_sim: they should contain three files:
  • MyKotlinFramework.framework
  • MyKotlinFramework.framework.dSYM
  • MyKotlinFramework.kt.bc
What are those three files?
The first one is, of course the framework we’ve just produced. The second file, having the dSYM extension, contains the information needed to debug our framework with an lldb debugger, such as the one provided by Xcode. Last, the .kt.bc file is the bitcode file of our program. In other words, it is the binary form of the LLVM Intermediate Representation of our code.

Integrating in an iOS application

In order to try how the framework runs in our iOS app, let’s add it to the “Embedded Binaries” section of our target.
Embedded Binaries Section
Then, in our Swift code, we’ll just need to import the library and use the bar() method of our Foo class. As you’ll see, please note that Konan prefixes all your classes with the trigram of your framework output name. In our case, MyKotlinFramework. In our case, the Foo class will become visible in Objective-C and Swift as MKFFoo.

The complete code of this tutorial is available at this GitHub repo.

Where to go from here?

Needless to say, this is just the beginning. Thanks to Kotlin/Native 0.6 it is now easy to share code between iOS and Android, by using the Multiplatform DSL. This will allow for a more effective reuse of your business logic between your mobile apps, no matter the platform.


Huge thanks to my colleagues at Xebia France for reviewing this article, in particular Sergio Dos Santos, Benjamin Lacroix, Michaël Ohayon and Julien Datour.

Kotlin/Native 0.5 and iOS

Kotlin Native interoperability is rapidly evolving and the latest iteration, version 0.5, brings support for calling Kotlin code from Swift and Objective-C.

Also, iPhone Simulator is now supported out of the box. Just run

to compile the Kotlin bridge for the iOS Simulator platform libs and runtime. The command will take about 1 hour to complete on a recent MacBook Pro.

Also, the samples have been adapted and you will now be able to try out the features straight on your simulator.

And that’s pretty neat.

Running a Kotlin/Native iOS app in the Simulator

Update: since Kotlin/Native 0.5, the following article is now outdated. I’ll be keeping its content for reference.

Kotlin Native 0.4 is out since the beginning of November and, with it, some nice sample iOS demo apps to be run into your favourite iOS device. That sounds amazing, even for some hardcore Swift fans like me. Yet, there’s a catch: the sample apps can only run on a real iOS device but not in the Simulator. Sure, that’s just an unsignificant drawback compared to what the Kotlin Native team achieved so far. Still, I’m a huge fan of the iOS Simulator, as it’s by far the fastest way to do some quick testing of an app while doing a pretty good job in mimicking the behaviour of an app on a real device.

So, as a way to understand the Kotlin Native project, I decided to try supporting the Simulator on a Kotln Native iOS app.

In order to do this, I started from the UIKit project sample found in the samples dir of the main repo and tried to make it run in the sim.

Continue reading Running a Kotlin/Native iOS app in the Simulator