Use business code for Android and iOS written in Kotlin in a multiplatform Xamarin application?

Hartmut Neubauer 1

I have some code that is written in Java and Kotlin for Android; the Java part can be translated into Kotlin using the Android Studio. Most of this code is business; that means, independent on any hardware or platform specifics; some Android specific classes (like "Bitmap") can be replaced by abstract or general self-defined classes.

As already known, Kotlin business code can be used in multiplatform applications for Android and iOS. Description here : https://kotlinlang.org/docs/multiplatform-mobile-integrate-in-existing-app.html .
Xamarin is used for multiplatform apps, too.

On the other hand, there is a way to include Kotlin code in Xamarin projects. For this purpose, the Xamarin.Kotlin.StdLib is used : https://libraries.io/nuget/Xamarin.Kotlin.StdLib .

My question: Is it possible to develop a Xamarin project (maybe with Xamarin Forms) that includes the Kotlin business code and will work in both Android and iOS environments?

Wenyan Zhang (Shanghai Wicresoft Co,.Ltd.) 26,221 Reputation points Microsoft Vendor

2022-06-06T10:10:12.107+00:00

I'm not sure what's your main purpose, may I ask have you followed the tutorial and make your Kotlin code work on iOS? I check this tutorial doc, the iOS part is for native iOS, I'm not sure if it is included in your Kotlin library and binding this library in Xamarin.
Hartmut Neubauer 1 Reputation point

2022-06-08T07:17:30.5+00:00
Hi WenyanZhang,
thank you for the answer. As you see I have mentioned two tutorials. One is for developing multiplatform apps for Android and iOS using Kotlin - but without Xamarin. The second is to import Kotlin code via Xamarin.Kotlin into Xamarin and to develop apps for Android with this - but it is not clear whether it is possible to use this for iOS, too, and how to do it.
My purpose is:

write some code in Kotlin via Android Studio that does not contain any Android specific classes, for example, string comparison or simple calculation,

maybe, build some .aar or .jar from this code,

import it to Xamarin and use it for a multiplatform app that works in Android as well as in iOS.
Wenyan Zhang (Shanghai Wicresoft Co,.Ltd.) 26,221 Reputation points Microsoft Vendor

2022-06-08T09:56:52.123+00:00

If your code doesn't mention any iOS/Android specific class, and you can get the .aar or .jar , you could try Binding an .AAR and Binding a .JAR.
Wenyan Zhang (Shanghai Wicresoft Co,.Ltd.) 26,221 Reputation points Microsoft Vendor

2022-06-10T01:24:50.137+00:00

We can't bind an Android aar or jar into an iOS app. If you can use Kotlin to build an iOS Framework, then you are able to create an iOS binding to call this Framework. Follow the Kotlin docs https://kotlinlang.org/docs/multiplatform-mobile-integrate-in-existing-app.html#connect-the-framework-to-your-ios-project until "Connect the framework to your iOS project" and then switch to Xamarin to create a binding and include it in the Xamarin app.
About Binding iOS Libraries, see https://learn.microsoft.com/en-us/xamarin/ios/platform/binding-objective-c/
https://learn.microsoft.com/en-us/xamarin/ios/platform/binding-swift/
Hartmut Neubauer 1 Reputation point

2022-06-10T14:33:12.547+00:00

Hi Wenyan Zhang,
with the latest Android Studio Chipmunk 2021.2.1 Patch 1, I have built an AAR with the new feature:
"New -> Kotlin Multiplatform Library". For the AAR I have only taken the multiplatform parts, without any Android or iOS specific code, and removed "expect" and "actual" code.
Now I am trying to bind this multiplatform AAR into a Xamarin project so that it can be imported by C# code. Is it true that the Xamarin binding project must include the AAR as well as the Xamarin.Kotlin.Stdlib nuget? Or do I understand your last answer so that I should create two bindings (with the same MP AAR), one for Android and one for iOS?
Wenyan Zhang (Shanghai Wicresoft Co,.Ltd.) 26,221 Reputation points Microsoft Vendor

2022-06-13T01:46:19.077+00:00

Is it true that the Xamarin binding project must include the AAR as well as the Xamarin.Kotlin.Stdlib nuget?

Your Xamarin binding project needs to include the AAR, Xamarin.Kotlin.Stdlib NuGet package is not necessary.

do I understand your last answer so that I should create two bindings (with the same MP AAR), one for Android and one for iOS?

Yes, you should create two bindings. But may be not the same MP AAR. For the iOS binding, it should be your iOS Framework.
Hartmut Neubauer 1 Reputation point

2022-06-23T16:21:13.91+00:00

Hi Wenyan Zhang,
thank you for your help until now.
Now i succeeded to use some Kotlin code and to include it into an iOS app using the kotlinlang tutorial; I also used XCode for this purpose.
The next step would be the building of the iOS library that would be binded to the Xamarin project later. Is it then correct to use the following tutorial to build a framework:
https://learn.microsoft.com/en-us/xamarin/ios/platform/binding-swift/walkthrough
? I tried it - i also included the Gigya.framework - but I did not succeed to verify whether the Objective-C header file name is enabled because the "Swift Compiler -General" tab does not appear in the XCode Build Settings.
Or should I use / build another target ?
Wenyan Zhang (Shanghai Wicresoft Co,.Ltd.) 26,221 Reputation points Microsoft Vendor

2022-06-24T08:01:37.777+00:00

Yes, you could follow the totorial.
Now you are meeting this scenario:
If you have access to the code, you can decorate the required Swift member(s) with the @objc attribute and apply a few additional rules to let the Xcode build tools know that these members should be exposed to the Objective-C world and the header.

The tutorial is binding for the Gigya Swift SDK, you can replace this SDK by yours. You mean the step8, right? You could try to find it under PROJECT
Hartmut Neubauer 1 Reputation point

2022-06-30T09:57:27.213+00:00

Hi,
I have made some progress on this issue. But there are new questions.
First I have used XCode; XCode successfully built a framework from the code what was originally written in Kotlin. The name of the framework is "shared". The directory tree is:
shared->build->xcode-frameworks->Debug->iphonesimulator15.5->shared.framework. This folder contains a "Headers" folder with shared.h, further "Info.plist", a "Modules" folder and the "shared" binary. "iphonesimulator15.5" also contains a "shared.framework.dSYM". I build the same for Release, so there is the same tree structure there. ==>
Hartmut Neubauer 1 Reputation point

2022-06-30T09:58:01.887+00:00

As the next step I downloaded "Objective sharpie" and followed the steps. I succeeded to build an "ApiDefinitions.cs" from my framework. This file obviously contains all types and interfaces of the "shared.h" that, again, includes classes of the original Kotlin project. So I tried to create a new C# project with VisualStudio with this file. I added the "shared.framework" inside the Release folder to the "Native References". I wanted to see how many compiler errors there are, that means, which symbols are marked in red. I see that the "using shared" is marked, furthermore there are many red "NativeHandle" items that are returnd from Constructors (and I don't see any references to this type even in the web); and some other marked items. Of course there are some "Verify" issues that must be resolved manually, that is documented, and that is not the question.
I tried the last updates from VS 2019 Community, VS 2022 Comm, Sharpie 3.5.60.
Do I something wrong? Or should I build a "fat" framework?
Wenyan Zhang (Shanghai Wicresoft Co,.Ltd.) 26,221 Reputation points Microsoft Vendor

2022-07-01T09:53:11.277+00:00

Would you mind sharing the detailed error messages?
Hartmut Neubauer 1 Reputation point

2022-07-01T13:55:54.23+00:00
For example, in both files there are first a couple of number type definitions, following a definition of type "SharedNumber". As an example: "SharedInt".

shared.h (Objective-C)

attribute((swift_name("KotlinInt")))
@interface SharedInt : SharedNumber

(instancetype)initWithInt:(int)value;

(instancetype)numberWithInt:(int)value;
@Eslam Nader ;

ApiDefinitions.cs (C#)

// @interface SharedInt : SharedNumber
[BaseType(typeof(SharedNumber))]
interface SharedInt
{
// -(instancetype _Nonnull)initWithInt:(int)value;
[Export("initWithInt:")]
NativeHandle Constructor(int value);

// +(instancetype _Nonnull)numberWithInt:(int)value; [Static] [Export("numberWithInt:")] SharedInt NumberWithInt(int value); }

There the name "NativeHandle" is red underlined, and the error message is "CS0246 The type or namespace name 'NativeHandle' could not be found (are you missing a using directive or an assembly reference?)"
I suppose that Sharpie thinks that NativeHandle is a kind of abstract handle as return type of constructors in context of native programming. VS or any add-on should know this type.

Another example in the next post.
Hartmut Neubauer 1 Reputation point

2022-07-01T14:19:06.163+00:00
Another example is the definition of the type "SharedResult":

Objective C (shared.h):

attribute ((swift_name("Result")))
@interface SharedResult<**covariant T> : SharedBase

(instancetype)init __attribute**((swift_name("init()"))) attribute((objc_designated_initializer));

(instancetype)new attribute((availability(swift, unavailable, message="use object initializers instead")));

(NSString *)description attribute((swift_name("description()")));
@Eslam Nader ;

C# (ApiDefinition.cs)

// audit-objc-generics: @interface SharedResult<__covariant T> : SharedBase [BaseType(typeof(SharedBase))] interface SharedResult { // +(instancetype _Nonnull)new __attribute__((availability(swift, unavailable))); [Unavailable(PlatformName.Swift)] [Static] [Export("new")] SharedResult New(); // -(NSString * _Nonnull)description __attribute__((swift_name("description()"))); [Export("description")] [Verify(MethodToProperty)] string Description { get; } }

https://learn.microsoft.com/en-us/dotnet/api/objcruntime.platformname?view=xamarin-ios-sdk-12Here, the Swift term in "Unavailable(PlatformName.Swift)" is underlined red. Here the https://learn.microsoft.com/en-us/dotnet/api/objcruntime.platformname?view=xamarin-ios-sdk-12" site tells me that PlatformName is an enum containing the elements iOX, MacOSC, None, TvOS, WatchOS, but not Swift.
Hartmut Neubauer 1 Reputation point

2022-07-04T09:42:54.19+00:00

UPDATE 2022-07-04

When I compile this solution using Visual Studio Community 2019 for Mac, the first error is: "bgen: Could not compile the API bindings. (BI0002).
Wenyan Zhang (Shanghai Wicresoft Co,.Ltd.) 26,221 Reputation points Microsoft Vendor

2022-07-07T11:02:57.667+00:00

After discussing with the team, the only supporting Frameworks available are Android & .Net 6. Therefore, the answers is no, it cannot work for iOS at this time. It can work for Xamarin Android in a single project. There is a note referencing this limitation would be removed in .Net 6 Build Items - Xamarin | Microsoft Learn. And there is no built-in support for Kotlin in iOS.
Hartmut Neubauer 1 Reputation point

2022-07-11T08:57:14.613+00:00

Hi,

so I understand your statement that currently the import of Kotlin programming code compiled for iOS into a Xamarin project is not yet possible. But it will be possible in .NET 6? When do you think there will be a release?

Here my findings until now. I succeeded to modify the file "ApiDefinitions.cs" until there were no longer compiler errors and only one warning. However, when i bind this iOS native Xamarin project with ApiDefinitions.cs with a Xamarin iOS project, I get some compiler errors, for example "warning: dylib (.../shared.a) was built for newer iOS Simulator version (9.0) than being linked (8.0)"; and some Native linking failed errors ("undefined Objective_C class", but their names are defined in ApiDefinitions.cs, however, as interfaces).

However, despite these errors, I sometimes succeeded to compile the source and debug the test app with the Simulator. But the application of a "New()" operator on a class defined in the ApiDefinitions file returned a null reference. So I tried to add a "Create" method of the Companion class of this class doing the same as "New" should do, but since then I did not succeed to compile this project without the errors mentioned above. ->
Hartmut Neubauer 1 Reputation point

2022-07-11T08:58:55.007+00:00

So it would be great to support this feature in one of the next releases. Maybe in three steps:
(1) create and compile a Kotlin Multiplatform Mobile (KMM) project in Android Studio,
(2) import the business code into iOS with XCode to create a native library or framework, (until here it works)
(3) bind this with MS VS into a Xamarin project (maybe Xam Forms).

Thank you.

3 answers

Wenyan Zhang (Shanghai Wicresoft Co,.Ltd.) 26,221 Reputation points Microsoft Vendor

2022-07-13T01:58:13.58+00:00

Hello @Hartmut Neubauer ,

After discussing with the team, the only supporting Frameworks available are Android & .Net 6. Therefore, it cannot work for iOS at this time. It can work for Xamarin Android in a single project. And there is no built-in support for Kotlin in iOS.

Only a single LibraryProjectZip can be included in a Xamarin.Android binding project. This limitation will be removed in .NET 6. See : Build Items - Xamarin | Microsoft Learn.

(1) create and compile a Kotlin Multiplatform Mobile (KMM) project in Android Studio

For the Android Studio issue, you can send the feedback to them, see: Feedback - Android Studio Project Site

(2) import the business code into iOS with XCode to create a native library or framework, (until here it works)

This needs to be discussed with Apple, you can create a new thread on Apple forums and get help from Apple at https://developer.apple.com/forums/

(3) bind this with MS VS into a Xamarin project (maybe Xam Forms).

It's recommend to submit feed back through the VS2022 Feedback channel, be sure to provide detailed information of what you want to achieve. About how to report the feedback to the product team, see : Report a problem with Visual Studio - Visual Studio (Windows) | Microsoft Learn

Best Regards,
Wenyan Zhang

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Hartmut Neubauer 1 Reputation point

2022-07-19T07:08:28.637+00:00

Hello @Wenyan Zhang (Shanghai Wicresoft Co,.Ltd.) ,

yesterday I have in fact succeeded to write a little test application with Xamarin (Xamarin Forms) that runs on iOS and uses my original Kotlin code. I ran it at least on an iOS simulator running on a Mac mini. It is complicated to describe it and there are many obstacles, but it is possible.

I already had succeeded to complete the first two steps as described earlier:
(1) create and compile a Kotlin Multiplatform Mobile (KMM) project in Android Studio,
(2) import the business code into iOS with XCode to create a static native library and/or framework. It is also possible to use the tool "lipo" to create a "fat" library that runs on a couple of architectures (arm64, x86_64 and so on).
(3) The next step is to apply "Objective Sharpie" to create the "ApiDefinitions.cs".
(4) Afterwards, create a solution within Visual Studio 2019 or 2022 and a project containing this API definitions file that also uses the framework and/or static library created in (2). Compiling may first yield a couple of errors, so the ApiDefinitions.cs has to be adapted manually. The changes that are necessary to do this may be discussed.
(5) The solution should contain at least three more projects: "MyApp", "MyApp.Android", "MyApp.iOS". (Android is not discussed here) The MyApp.iOS should depend on "MyApp" and the project containing the ApiDefinitons and library/framework. Still then errors may happen, and further adaptations of ApiDefinitions.cs might be necessary.
(6) Finally, the app can be written. in "MyApp" project, interfaces may be defined that correspond on the classes and functions defined in the library and originally written in the Kotlin project. These interfaces may facilitate the usage of those classes.

Perhaps you can discuss these outcomings in your team. If there is interest, I also can supply some more details.
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Hartmut Neubauer 1 Reputation point

2022-08-08T12:30:47.967+00:00

I have written some instructions here:

https://stackoverflow.com/a/73276611/2576374
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