KTViewModelBuilder

A macro to use inside a Kotlin multiplatform project for wrapping a Kotlin ViewModel into a SwiftUI ObservableObject, based on the SKIE library.

The goal of this macro is to increase the iOS development experience by removing the complexity of using a Kotlin ViewModel inside an iOS application written in Swift and also respecting the Lifecycle of easily.

For example, instead of using KotlinInt/KotlinInt?, we're using Int/Int?, it can work with Float/Double/...

Note: the macro creates a unidirectional binding as usually used on Kotlin MVVM, so use can't directly SwiftUI @Binding.

Example

Kotlin ViewModel

A Kotlin ViewModel shared between Android and iOS with Observable content and methods.

public class ExampleViewModel : ViewModel() {

    private val _stringData = MutableStateFlow("Some Data")
    public val stringData: StateFlow<String> = _stringData

    private val _intNullableData = MutableStateFlow<Int?>(null)
    public val intNullableData: StateFlow<Int?> = _intNullableData

    private val _randomValue = MutableStateFlow(0)
    public val randomValue: StateFlow<Int> = _randomValue

    private val _entityData = MutableStateFlow<MyData?>(MyData())
    public val entityData: StateFlow<MyData?> = _entityData

    public fun randomizeValue() {
        _randomValue.value = (0..100).random()
    }
}

SwiftUI ViewModel

The macro generate a SwiftUI ViewModel from the content of the Kotlin ExampleViewModel class.

@ktViewModel(ofType: ExampleViewModel.self,
                 publishing:
                    (\.stringData, String.self),
                 (\.intNullableData, Int?.self),
                 (\.randomValue, Double.self),
                 (\.entityData, MyData?.self)
)
class SharedExampleViewModel : ObservableObject {}

Generated content

class SharedExampleViewModel : ObservableObject {
    private let viewModelStore = ViewModelStore()
    
    @Published private(set) var stringData: String
    
    @Published private(set) var intNullableData: Int?
    
    @Published private(set) var randomValue: Double
    
    @Published private(set) var entityData: MyData?
    
    init(_ viewModel: ExampleViewModel) {
        self.viewModelStore.put(key: "ExampleViewModelKey", viewModel: viewModel)
        self.stringData = viewModel.stringData.value
        print("INIT stringData : " + String(describing: viewModel.stringData.value))
        self.intNullableData = viewModel.intNullableData.value?.intValue
        print("INIT intNullableData : " + String(describing: viewModel.intNullableData.value))
        self.randomValue = viewModel.randomValue.value.doubleValue
        print("INIT randomValue : " + String(describing: viewModel.randomValue.value))
        self.entityData = viewModel.entityData.value
        print("INIT entityData : " + String(describing: viewModel.entityData.value))
    }
    
    var instance: ExampleViewModel {
        self.viewModelStore.get(key: "ExampleViewModelKey") as! ExampleViewModel
    }
    
    func start() async {
        await withTaskGroup(of: (Void).self) {
            $0.addTask { @MainActor [weak self] in
                for await value in self!.instance.stringData where self != nil {
                    if value != self?.stringData {
                        #if DEBUG
                        print("UPDATING stringData : " + String(describing: value))
                        #endif
                        self?.stringData = value
                    }
                }
            }
            $0.addTask { @MainActor [weak self] in
                for await value in self!.instance.intNullableData where self != nil {
                    if value?.intValue != self?.intNullableData {
                        #if DEBUG
                        print("UPDATING intNullableData : " + String(describing: value))
                        #endif
                        self?.intNullableData = value?.intValue
                    }
                }
            }
            $0.addTask { @MainActor [weak self] in
                for await value in self!.instance.randomValue where self != nil {
                    if value.doubleValue != self?.randomValue {
                        #if DEBUG
                        print("UPDATING randomValue : " + String(describing: value))
                        #endif
                        self?.randomValue = value.doubleValue
                    }
                }
            }
            $0.addTask { @MainActor [weak self] in
                for await value in self!.instance.entityData where self != nil {
                    if value != self?.entityData {
                        #if DEBUG
                        print("UPDATING entityData : " + String(describing: value))
                        #endif
                        self?.entityData = value
                    }
                }
            }
        }
    }
    
    deinit {
        self.viewModelStore.clear()
    }
}

SwiftUi View

The properties of the ViewModel can be directly used from the viewModel property and the method from viewModel.instance.

The binding must be triggered from a SwiftUI task modifier; it will start the Observability of the declared properties.

struct ExampleScreen: View {
    
    // Initialize the ViewModel, binding and lifecycle
    @StateObject var viewModel = SharedExampleViewModel(ExampleViewModel())
    
    var body: some View {
        VStack {
            Text("STRING VALUE \(viewModel.stringData)")
            Text("NULL VALUE \(String(describing: viewModel.intNullableData))")
            Text("RANDOM VALUE \(viewModel.randomValue)")
            Button {
                viewModel.instance.randomizeValue()
            } label: {
                Text("randomizeValue")
            }
        }.task {
            // start the ViewModel lifecycle and keep it alive until the view disappear
            await viewModel.start()
        }
    }
}

Requirement

As this solution is based on Kotlin multiplatform and SKIE, some requirements need to be met.

Import SKIE library

Please follow the installation step of the library.

Add and export the kotlin ViewModel to Swift

• Add in your .toml or .gradle the following dependency

androidx-lifecycle-viewmodel = { module = "androidx.lifecycle:lifecycle-viewmodel", version.ref = "androidx_lifecycle_version" }

• Then follow this configuration or equivalent

listOf(
    iosX64(),
    iosArm64(),
    iosSimulatorArm64()
).forEach {
    it.binaries.framework {
        baseName = "shared"
        isStatic = true
        export(libs.androidx.lifecycle.viewmodel) // !! export the library for the iOS target, so it can be accessible from swift code !!
    }
}

sourceSets {
    commonMain.dependencies {
        api(libs.androidx.lifecycle.viewmodel) // the library itself
    }
}

Add the macro to your xcode project.

Finally, add this package to your application swift package dependencies.

https://github.com/frankois944/KTViewModelBuilder

Conclusion

That's all you need.

A Sample is available in this repository, which has a shared library and an iOS/Android app.