[piercifani]

Stack 10/21 · base: jni/09-object-identity · previous: #13 · next: #15 (jni/11-async-throwing-stream)

Problem

jextract couldn't extract a function returning AsyncStream<T>: the type isn't in its model of the standard library, so the declaration was skipped (unknown(... AsyncStream<Item>)).

Changes

A Java type for Swift streams. SwiftKitCore gets SwiftAsyncStream<T>:

• subscribe(Subscriber<T>) starts a Swift task that iterates the stream and calls onNext for each element and onComplete when the stream finishes. No Java thread is blocked.
• The returned Subscription.cancel() cancels that task. Cancellation reaches the stream's onTermination, so an endless producer can stop.
• map(...) returns a view with converted elements. An AsyncStream has a single consumer, so a stream (and its mapped views) can be subscribed to once; a second subscription throws IllegalStateException.
• An internal Delivery owns the Swift task handle and releases it exactly once, whether the stream finishes or is cancelled first, including when it finishes before subscribe returns.

I chose this over java.util.concurrent.Flow.Publisher (Android API 30+) and over a blocking Iterator (a thread per stream).

Kotlin Flow: a new swiftkit-kotlin artifact. SwiftKitKotlin adds SwiftAsyncStream<T>.asFlow(), a callbackFlow whose cancellation cancels the Swift task; a slow collector makes the Swift task wait for buffer space. It's a separate artifact so SwiftKitCore stays a plain Java library without a kotlinx-coroutines dependency, as CompletableFuture results rely on kotlinx's await() without SwiftKitCore depending on it. It targets Java 11, like SwiftKitCore.

Runtime. SwiftJavaRuntimeSupport's _attachAsyncStream(_:to:environment:convert:) stores the stream and an element converter behind a handle on the Java object. Subscribing consumes the handle and starts the task, which converts and delivers each element with the current thread's JNIEnv.

Generator.

• AsyncStream is added to the stdlib model (as Swift.AsyncStream, next to Set).
• A function returning AsyncStream<T> is translated as returning T, then converted: Java passes a SwiftAsyncStream out-parameter (like future$ for async functions) and returns stream$.map(element$ -> <the conversion a T result would get>). The Swift thunk attaches the stream, with a converter that boxes primitives exactly as async results are boxed.
• async functions returning a stream, and elements that need several Java values (tuples), are skipped with a reported reason.

Kotlin in this PR

StreamsTest.kt with Streams.swift:

• asFlow() comes from SwiftKitKotlin (import org.swift.swiftkit.kotlin.asFlow); the sample depends on it instead of kotlinx-coroutines directly.
• feed.prices(3).asFlow().map { it.id }.toList() collects a stream of structs.
• feed.live().asFlow().take(2) cancels an endless Swift stream, whose onTermination then runs.

Why Skip needs it

Skip bridges AsyncStream to Kotlin Flow, which Compose collects with collectAsState. A production Skip Fuse app (16 Skip modules) returns 18 streams from Swift to Kotlin this way.

Testing

• Kotlin sample: ./gradlew :SwiftKitKotlin:build :Samples:SwiftJavaExtractJNIKotlinSampleApp:test passes at this point in the stack (29 tests); swiftkit-kotlin classes are Java 11 bytecode.
• New JNIAsyncStreamTests (4 tests): the Java method and native signature, the Swift attach with its element converter, jextracted elements mapped to their Java class, and unsupported streams skipped with a reason.
• JNI sample: countdown (primitives) and shapes (jextracted Squares) collected in Java, plus the single-subscription rule.
• Full swift test --disable-experimental-prebuilts --disable-sandbox and ./gradlew :Samples:SwiftJavaExtractJNISampleApp:test pass.
• swift format lint --strict on changed files; git diff --check.

Not in this PR

• AsyncThrowingStream and other AsyncSequence types, and streams as parameters or properties.
• The reverse direction: a Java implementation of a Swift protocol returning AsyncStream.
• A stream that is returned but never subscribed to is not released.

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