Flutter: StreamBuilder inside StreamBuilder (web test).

main.dart

import 'dart:async';

import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({Key key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  const MyHomePage({Key key}) : super(key: key);

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  Stream<int> outerStream;
  Stream<int> innerStream;

  @override
  void initState() {
    super.initState();

    outerStream = Stream
        .fromIterable([1, 2])
        .asyncMap((int intEvent) => Future.delayed(Duration(seconds: 3), () => intEvent)).asBroadcastStream();

    innerStream = Stream
        .fromIterable([3, 4])
        .asyncMap((int intEvent) => Future.delayed(Duration(seconds: 4), () => intEvent)).asBroadcastStream();

    innerStream.listen((event) { });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
          child: StreamBuilder<int>(
              stream: outerStream,
              builder: (_, snapshot) {
                if (!snapshot.hasData) return CircularProgressIndicator();

                return Column(
                  mainAxisAlignment: MainAxisAlignment.center,
                  children: [
                    Text("outer: ${snapshot.data.toString()}"),

                    StreamBuilder<int>(
                      stream: innerStream,
                      builder: (_, innerSnapshot) {
                        if (!innerSnapshot.hasData) return CircularProgressIndicator(backgroundColor: Colors.red,);

                        return Text("inner: ${innerSnapshot.data.toString()}");
                      },
                    )
                  ],
                );
              }
          )
      ),
    );
  }
}