[piercifani]

Stack 11/21 · base: jni/10-async-stream · previous: #14 · next: #16 (jni/12-optional-nested-arrays)

Problem

jextract skipped functions returning AsyncThrowingStream<T, Failure>, because the type isn't in its model of the standard library. #14 handled AsyncStream only.

Changes

Generator. AsyncThrowingStream is added to the stdlib model next to AsyncStream. A function returning AsyncThrowingStream<T, Failure> is translated exactly like one returning AsyncStream<T>: Java gets a SwiftAsyncStream<T>, and the Swift thunk calls _attachAsyncStream. Both Error and any Error failures are covered.

Runtime. _attachAsyncStream gains an AsyncThrowingStream overload, so the generated code is the same for both stream types. Both overloads share one task body. When the stream throws, the error is converted to a Java Throwable the same way a thrown Swift error is for any call: a jextracted error type becomes its own exception class, and any other error becomes SwiftErrorException.

Java. SwiftAsyncStream.Subscriber gets default void onError(Throwable error), called instead of onComplete when a throwing stream fails. Neither one is called after the subscription is cancelled; the Delivery that already guarantees one finish now covers the error path too.

Kotlin. SwiftKitKotlin's asFlow() closes the flow with that error, so collect throws the Swift error.

Kotlin in this PR

StreamsTest.kt with Streams.swift:

val error = assertFailsWith<ClientError> {
    PriceFeed.init().pricesUntilOffline(2).asFlow().collect { ids += it.id }
}
assertEquals(listOf("price-0", "price-1"), ids)
assertTrue(error.case is ClientError.Case.Offline)

The Swift stream yields two items and then finishes with throwing: ClientError.offline. Kotlin receives both items and then catches the typed Swift error, matching on its case.

Why Skip needs it

Skip bridges AsyncThrowingStream to a Kotlin Flow that fails with the error, the same way it bridges AsyncStream.

Testing

• Kotlin sample: ./gradlew :SwiftKitKotlin:build :Samples:SwiftJavaExtractJNIKotlinSampleApp:test passes (30 tests).
• JNIAsyncStreamTests: a new parameterized test for AsyncThrowingStream<Int64, Error> and <Int64, any Error> checks the Java method, the native signature and the Swift attach call.
• JNI sample: interruptedCountdown delivers its elements and then its error to onError, with the Swift description as the message.
• Full swift test --disable-experimental-prebuilts --disable-sandbox (654 tests) and ./gradlew :Samples:SwiftJavaExtractJNISampleApp:test (288 tests) pass.
• swift format lint --strict on changed files; git diff --check.

Not in this PR

• Other AsyncSequence types, and streams as parameters or properties.
• Typed Failure isn't reflected in Java: every stream is a SwiftAsyncStream<T>, and the error arrives as a Throwable.

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