New to Dart|Flutter.

I can’t determine why the Flutter StreamBuilder doesn’t work with the gRPC async watch method even though I’ve confirmed that:

1. the gRPC watch method works (using a standalone Dart app)
2. I can use StreamBuilder with very a similiar async pingpong method.

The known working Dart gRPC client:

import "package:grpc/grpc.dart";
import "package:ackalui/generated/health.pbgrpc.dart";

class Client {
    final address = "[REDACTED]";
    final port = 443;

    late HealthClient _stub;

    Client() {
        _createChannel();
    }

    _createChannel() {
        final channel = ClientChannel(
            address,
            port: port,
            options: ChannelOptions(
                credentials: ChannelCredentials.secure(),
            ),
        );
        _stub = HealthClient(channel);
    }

    Stream<String> watch() async* {
        try {
            final rqst = HealthCheckRequest();
            var resp = await _stub.watch(rqst);
            print("[Client:watch] await");
            await for (var item in resp) {
                print("[Client:watch] yielding");
                yield item.status.toString();
            }
        } on GrpcError catch (e) {
            print("gRPC error: ${e.code} -- ${e.message}");
            // rethrow;
            yield* Stream.error(e);
        } catch (e) {
            print("Unexpected error: ${e}");
            // rethrow;
            yield* Stream.error(e);
        }
    }
}

NOTE I’ve tried rethrow and yield* Stream.error(e) on the catch‘es with no evident difference in behavior.

I’m able to run a Dart client that streams watch gRPC responses from a server.

Future<void> main(List<String> args) async {
  // Test gRPC Watch
  final client = Client();
  var resp = await client.watch();
  await for (var status in resp) {
    print(status);
  }
}

Logs:

[Client:watch] await
[Client:watch] yielding
UNKNOWN
[Client:watch] yielding
SERVING
[Client:watch] yielding
NOT_SERVING
[Client:watch] yielding
SERVING
[Client:watch] yielding
UNKNOWN
...

But the Flutter Widget that I want to stream the status values hits the final else and returns CircularProgressIndicator and the gRPC watch method does not proceed beyond the logged [Client:watch] await step.

StreamBuilder<String>(
  stream: _client.watch(),
  builder: (context, snapshot) {
    print("builder");
    if (snapshot.hasError) {
      print("[builder] error");
      return Text('Error: ${snapshot.error}');
    } else if (snapshot.connectionState == ConnectionState.waiting) {
      print("[builder] waiting");
      return const CircularProgressIndicator();
    } else { 
      print("[builder] else");
      return Text(
        snapshot.data ?? '[empty]',
        style: Theme.of(context).textTheme.headlineMedium,
      );
    }
  },
)

initialized with:

class _MyHomePageState extends State<MyHomePage> {
  late Client _client;
  late TestClient _testClient;

  @override initState() {
    super.initState();
    _client = Client();
    _testClient = TestClient();
  }

  @override
  Widget build(BuildContext context) {
    ...
   }
}

Logs:

builder
waiting
[Client:watch] await

NOTE Unlike the (pure) Dart example, it’s not proceeding to [Client:watch] yielding

I have a working Flutter app that streams (similarly yield‘ed String‘s):

StreamBuilder<String>(
  stream: _testClient.pingpong(1000),
  builder: (context, snapshot) {
    print("builder");
    if (snapshot.hasError) {
      print("[builder] error");
      return Text('Error: ${snapshot.error}');
    } else if (snapshot.connectionState == ConnectionState.waiting) {
      print("[builder] waiting");
      return const CircularProgressIndicator();
    } else { 
      print("[builder] else");
      return Text(
        "Value: ${snapshot.data}",
        style: Theme.of(context).textTheme.headlineMedium,
      );
    }
  },
)