Streams

Overview

A C++ function becomes a Dart Stream<T> when it has an export marker (BRIDGE_SYNC / BRIDGE_ASYNC / BRIDGE_NORMAL) and takes a required dcb::StreamSink<T> parameter. The export marker is the gate: a function with a StreamSink parameter but no export marker is not generated (the generator warns and skips it). Plain void stream functions typically use BRIDGE_NORMAL; the sink parameter overrides the marker’s usual scheduling and the generated Dart API returns Stream<T>.

C++	Dart
void f(dcb::StreamSink<T> sink, ...)	Stream<T> f(...)
R f(args..., std::optional<dcb::StreamSink<T>> progress) (BRIDGE_SYNC)	R f(..., {StreamController<T>? progress})
Lazy<R> f(args..., std::optional<dcb::StreamSink<T>> progress)	Future<R> f(..., {StreamController<T>? progress})

The function may return immediately: the sink can be kept and used later from any thread. This page uses the codegen_demo fixture as a reference.

Required stream

C++ header

BRIDGE_NORMAL
void tick_stream(dcb::StreamSink<std::int32_t> sink, std::int32_t count = 5,
                 std::int32_t interval_ms = 10);

C++ implementation

The function is an ordinary void function. It usually returns immediately and the real work continues on another thread (here the runtime thread pool); sink.add() is thread-safe.

void tick_stream(dcb::StreamSink<std::int32_t> sink, std::int32_t count,
                 std::int32_t interval_ms) {
  asio::post(dcb::Runtime::instance().pool(),
             [sink = std::move(sink), count, interval_ms]() mutable {
               for (std::int32_t i = 0; i < count; ++i) {
                 sink.add(i);
                 if (interval_ms > 0) {
                   std::this_thread::sleep_for(std::chrono::milliseconds(interval_ms));
                 }
               }
               sink.end();
             });
}

Generated Dart

Stream<int> tickStream({int count = 5, int intervalMs = 10}) => ...;

Dart usage

// Collect all events:
final values = await tickStream(count: 5, intervalMs: 10).toList();
// values == [0, 1, 2, 3, 4]

// Or listen and cancel:
final sub = tickStream(count: 100, intervalMs: 10).listen((v) { ... });
await sub.cancel();

Optional stream

std::optional<dcb::StreamSink<T>> changes the Dart signature from “returns a stream” to “accepts a stream”: the caller passes a StreamController<T>? input parameter. When null is passed, C++ receives std::nullopt, no stream events flow, and the function still returns its normal result. It works on BRIDGE_SYNC, BRIDGE_ASYNC, and BRIDGE_NORMAL functions. For BRIDGE_SYNC, events posted by C++ during the blocking FFI call are queued on the reply port and delivered to the controller right after the call returns.

C++ header

BRIDGE_ASYNC
async_simple::coro::Lazy<std::string> download_with_progress(
    std::string url, std::optional<dcb::StreamSink<std::int32_t>> progress);

C++ implementation

The coroutine runs on the io thread: emit events as part of the asynchronous work (between co_await points) and never block the thread.

async_simple::coro::Lazy<std::string> download_with_progress(
    std::string url, std::optional<dcb::StreamSink<std::int32_t>> progress) {
  for (std::int32_t i = 1; i <= 5; ++i) {
    if (progress) {
      progress->add(i * 20); // 20, 40, 60, 80, 100
      // In real code events come from async work; a cancellable sleep
      // simulates the async interval here.
      co_await async_simple::coro::sleep(std::chrono::milliseconds(10));
    }
  }
  co_return std::string("downloaded: ") + url;
}

Generated Dart

Future<String> downloadWithProgress({
  required String url,
  StreamController<int>? progress,
}) => ...;

Dart usage

// With progress events:
final controller = StreamController<int>();
controller.stream.listen(progressValues.add);
final result = await downloadWithProgress(
  url: 'https://example.com/file.zip',
  progress: controller,
);
await controller.close(); // close the controller yourself when you are done

// Without progress:
final result = await downloadWithProgress(url: 'test.txt');

The same request carries both the stream events and the final result: streamData / streamEnd frames for the stream and a final responseOk frame for the returned value.

Sync variant

BRIDGE_SYNC functions use the same std::optional<dcb::StreamSink<T>> parameter; the generated Dart API is synchronous:

BRIDGE_SYNC
std::string sync_download_with_progress(
    std::string url, std::optional<dcb::StreamSink<std::int32_t>> progress);

C++ implementation

The sync function must not emit events inside the blocking FFI call — spawn a coroutine that sends them asynchronously, then return the result immediately:

namespace {
// Free coroutine function: parameters are copied into the coroutine frame,
// avoiding the dangling-capture problem of a coroutine lambda.
async_simple::coro::Lazy<> emit_progress(dcb::StreamSink<std::int32_t> sink) {
  for (std::int32_t i = 1; i <= 5; ++i) {
    sink.add(i * 20);  // 20, 40, 60, 80, 100
  }
  co_return;
}
}  // namespace

std::string sync_download_with_progress(
    std::string url, std::optional<dcb::StreamSink<std::int32_t>> progress) {
  if (progress) {
    dcb::spawn_detached(emit_progress(std::move(*progress)));
  }
  return std::string("downloaded: ") + url;
}

The events are queued on the reply port while the FFI call is still in flight and delivered to the controller right after the call returns. dcb::spawn_detached is the fire-and-forget launcher (dcb::spawn(lazy).start([](auto&&) {})); do not use a coroutine lambda here — its captures would dangle once the temporary lambda object is destroyed.

final controller = StreamController<int>();
controller.stream.listen(progressValues.add);
final result = syncDownloadWithProgress(
  url: 'https://example.com/file.zip',
  progress: controller,
);
// result is available immediately; progress events arrive right after.
await Future<void>.delayed(const Duration(milliseconds: 20));
expect(progressValues, [20, 40, 60, 80, 100]);
await controller.close();

Errors

• sink.error(message) sends an error event to the Dart stream and closes it — this is the supported way to signal a stream-level error.
• For a required stream, the business function usually returns immediately; do not rely on throwing from the dispatch path — use sink.error() instead.
• For an optional stream, a C++ exception fails the returned Future, but the bridge does not close the StreamController automatically. Call progress->error(...) in C++ or controller.close() in Dart to finish the stream.

Cancellation

• Dart sub.cancel() sends dcb_stream_close; the native stream is marked closed.
• The C++ side is not interrupted: it keeps running, and later add() / end() calls are silently dropped.
• A new call creates a fresh stream with a new stream id, so cancel-then-resubscribe works (covered by the codegen_demo stress tests).

Threads and lifetime

• add() / end() / error() are thread-safe and can be called from the io thread, a pool thread, or a foreign event-loop thread (see uv_stream / worker_stream in codegen_demo, which bridge an mpsc channel into the sink on the io thread).
• The sink holds a std::shared_ptr<Session> internally, so it may outlive the dispatch frame and even a session close; after the session is disposed, late calls are silently dropped by the generation check instead of touching freed memory.

Opaque class member streams

Opaque classes can also expose streams as member functions:

class BRIDGE_OPAQUE Counter {
 public:
  void tickStream(dcb::StreamSink<std::int32_t> sink, std::int32_t count = 5,
                  std::int32_t intervalMs = 10);
};

final counter = Counter.int32T(initialValue: 3);
final values = await counter.tickStream(count: 3, intervalMs: 10).toList();
// values == [3, 3, 3]

Further reading

• Choosing a Marker
• Basic Runtime
• Exceptions and Error Handling
• Type Mapping