JNI: Swift class hierarchies as Java class hierarchies

Samples/SwiftJavaExtractJNIKotlinSampleApp/Sources/KotlinSampleLibrary/ClassHierarchy.swift

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+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2026 Apple Inc. and the Swift.org project authors
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of Swift.org project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+
+/// Swift subclasses become Kotlin-visible subclasses.
+open class Animal {
+  public let name: String
+
+  public init(name: String) {
+    self.name = name
+  }
+
+  open func sound() -> String { "..." }
+
+  public func describe() -> String { "\(name) says \(sound())" }
+}
+
+public final class Dog: Animal {
+  public override init(name: String) {
+    super.init(name: name)
+  }
+
+  public override func sound() -> String { "woof" }
+
+  public func fetch() -> String { "stick" }
+}
+
+public enum Zoo {
+  public static func describe(_ animal: Animal) -> String { animal.describe() }
+
+  /// Returned as `Animal`; Kotlin receives a `Dog`.
+  public static func makeDog(name: String) -> Animal { Dog(name: name) }
+}

Samples/SwiftJavaExtractJNIKotlinSampleApp/Sources/KotlinSampleLibrary/ViewModels.swift

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+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2026 Apple Inc. and the Swift.org project authors
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of Swift.org project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+
+import Foundation
+import Observation
+
+/// The shape of a Skip Fuse view model: a main-actor, observable open base class; a subclass with
+/// callbacks taking structs, an input enum with payloads, and async loading.
+@MainActor
+@Observable
+open class BaseViewModel {
+  public var isLoading = false
+
+  public init() {}
+
+  open func setup() {}
+}
+
+@MainActor
+public final class MarketViewModel: BaseViewModel {
+  public struct Callbacks: Sendable {
+    let onOpen: @MainActor @Sendable (Market) -> Void
+    let onClose: @MainActor @Sendable () -> Void
+
+    public init(
+      onOpen: @escaping @MainActor @Sendable (Market) -> Void,
+      onClose: @escaping @MainActor @Sendable () -> Void
+    ) {
+      self.onOpen = onOpen
+      self.onClose = onClose
+    }
+  }
+
+  public enum Input: Sendable {
+    case tapped(Market)
+    case closeTapped
+  }
+
+  public private(set) var markets: [Market] = []
+  private let callbacks: Callbacks
+
+  public init(callbacks: Callbacks) {
+    self.callbacks = callbacks
+    super.init()
+  }
+
+  public override func setup() {
+    markets = [Market(id: "1")]
+  }
+
+  public func send(_ input: Input) {
+    switch input {
+    case .tapped(let market): callbacks.onOpen(market)
+    case .closeTapped: callbacks.onClose()
+    }
+  }
+
+  public func load() async {
+    isLoading = true
+    markets = [Market(id: "1"), Market(id: "2")]
+    isLoading = false
+  }
+}
+
+/// For the tests: runs the main run loop briefly, which drains the main queue as Android's main
+/// looper does, so main-actor work can run while the JVM main thread waits.
+public enum MainLoop {
+  @MainActor
+  public static func run(seconds: Double) {
+    _ = RunLoop.main.run(mode: .default, before: Date(timeIntervalSinceNow: seconds))
+  }
+}

Samples/SwiftJavaExtractJNIKotlinSampleApp/src/test/kotlin/com/example/kotlin/ClassHierarchyTest.kt

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+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2026 Apple Inc. and the Swift.org project authors
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of Swift.org project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+
+package com.example.kotlin
+
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertIs
+
+class ClassHierarchyTest {
+    @Test
+    fun subclassIsASuperclass() {
+        val dog = Dog.init("Rex")
+        val animal: Animal = dog
+        assertEquals("Rex says woof", animal.describe())
+        assertEquals("Rex", animal.name)
+        assertEquals("stick", dog.fetch())
+    }
+
+    @Test
+    fun subclassPassedAsSuperclass() {
+        assertEquals("Rex says woof", Zoo.describe(Dog.init("Rex")))
+    }
+
+    @Test
+    fun returnedInstanceKeepsItsDynamicClass() {
+        val animal = Zoo.makeDog("Rex")
+        assertIs<Dog>(animal)
+        assertEquals("stick", animal.fetch())
+    }
+}

Samples/SwiftJavaExtractJNIKotlinSampleApp/src/test/kotlin/com/example/kotlin/ViewModelTest.kt

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+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2026 Apple Inc. and the Swift.org project authors
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of Swift.org project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+
+package com.example.kotlin
+
+import org.junit.jupiter.api.condition.EnabledOnOs
+import org.junit.jupiter.api.condition.OS
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertFalse
+
+// A view model as Compose code uses it. Main-actor isolated, so macOS only (see MainActorTest).
+@EnabledOnOs(OS.MAC)
+class ViewModelTest {
+    @Test
+    fun viewModelLifecycle() {
+        val opened = mutableListOf<String>()
+        var closed = false
+        val vm = MarketViewModel.init(MarketViewModel.Callbacks.init({ market -> opened += market.id }, { closed = true }))
+        val base: BaseViewModel = vm
+        base.setup()
+        assertEquals(1, vm.markets.size)
+
+        // `load()` is main-actor isolated: pump the main loop instead of blocking the main thread on
+        // the future (on Android the main looper runs it while the caller suspends).
+        val loaded = vm.load()
+        val deadline = System.currentTimeMillis() + 10_000
+        while (!loaded.isDone && System.currentTimeMillis() < deadline) MainLoop.run(0.01)
+        loaded.get()
+        assertFalse(vm.isLoading)
+        assertEquals(2, vm.markets.size)
+
+        vm.send(MarketViewModel.Input.tapped(vm.markets[1]))
+        vm.send(MarketViewModel.Input.closeTapped())
+        assertEquals(listOf("2"), opened)
+        assertEquals(true, closed)
+    }
+}

Samples/SwiftJavaExtractJNISampleApp/Sources/MySwiftLibrary/ClassHierarchy.swift

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+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2026 Apple Inc. and the Swift.org project authors
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of Swift.org project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+
+
+/// Swift subclasses become Java subclasses.
+open class Shape {
+  public let name: String
+
+  public init(name: String) {
+    self.name = name
+  }
+
+  open func area() -> Double { 0 }
+
+  public func describe() -> String { "\(name): \(area())" }
+}
+
+public final class Square: Shape {
+  public let side: Double
+
+  public init(side: Double) {
+    self.side = side
+    super.init(name: "square")
+  }
+
+  public override func area() -> Double { side * side }
+}
+
+public func totalArea(_ shapes: [Shape]) -> Double {
+  shapes.reduce(0) { $0 + $1.area() }
+}
+
+/// Returned as `Shape`; Java receives a `Square`.
+public func makeSquareShape(side: Double) -> Shape {
+  Square(side: side)
+}

Samples/SwiftJavaExtractJNISampleApp/src/test/java/com/example/swift/ClassHierarchyTest.java

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+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2024 Apple Inc. and the Swift.org project authors
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of Swift.org project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+
+package com.example.swift;
+
+import org.junit.jupiter.api.Test;
+import org.swift.swiftkit.core.SwiftArena;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+public class ClassHierarchyTest {
+    @Test
+    void subclassIsASuperclass() {
+        try (var arena = SwiftArena.ofConfined()) {
+            Square square = Square.init(3, arena);
+            Shape shape = square;
+            assertEquals("square: 9.0", shape.describe());
+            assertEquals(3, square.getSide());
+            assertEquals(13, MySwiftLibrary.totalArea(new Shape[] { square, Square.init(2, arena) }));
+        }
+    }
+
+    @Test
+    void returnedInstanceHasItsDynamicClass() {
+        try (var arena = SwiftArena.ofConfined()) {
+            Shape shape = MySwiftLibrary.makeSquareShape(4, arena);
+            assertInstanceOf(Square.class, shape);
+            assertEquals(16, shape.area());
+        }
+    }
+}