使用bloc时，Streambuilder快照不会退出等待状态

Question

我正在尝试复制此视频中显示的黑客新闻应用程序：https://youtu.be/fahC3ky_zW0。但是在我的StreamBuilder快照连接状态中，总是处于“等待”状态。因此，我的应用程序无法填充数据。

文件：hn_bloc.dart

class HackerNewsBloc {
  final _articlesSubject = BehaviorSubject<UnmodifiableListView<Article>>();
  var _articles = <Article>[];

  List<int> _ids = [
    23361987,
    23366546,
  ];

  HackerNewBlock() {
    _updateArticles().then((_) {
      _articlesSubject.add(UnmodifiableListView(_articles));
    });
  }

  Stream<UnmodifiableListView<Article>> get articles => _articlesSubject.stream;

  Future<Article> _getArticle(int id) async {
    final storyUrl = 'https://hacker-news.firebaseio.com/v0/item/$id.json';
    final storyRes = await http.get(storyUrl);
    return parseArticle(storyRes.body);
  }

  Future<Null> _updateArticles() async {
    final futureArticles = _ids.map((id) => _getArticle(id));
    final articles = await Future.wait(futureArticles);
    _articles = articles;
    return null;
  }
}

文件main.dart

import 'package:flutter/material.dart';
import 'package:url_launcher/url_launcher.dart';
import 'package:hackernews/src/article.dart';
import 'package:hackernews/src/hn_bloc.dart';
import 'dart:collection';
import 'dart:async';

void main() {
  final hnBloc = HackerNewsBloc();
  runApp(MyApp(
    bloc: hnBloc,
  ));
}

class MyApp extends StatelessWidget {
  final HackerNewsBloc bloc;

  MyApp({
    Key key,
    this.bloc,
  }) : super(key: key);

  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Hacker News',
      theme: ThemeData(
        primarySwatch: Colors.blue,
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: MyHomePage(
        title: 'Hacker News',
        bloc: bloc,
      ),
    );
  }
}

class MyHomePage extends StatefulWidget {
  final String title;
  final HackerNewsBloc bloc;
  MyHomePage({Key key, this.title, this.bloc}) : super(key: key);

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

class _MyHomePageState extends State<MyHomePage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: StreamBuilder<UnmodifiableListView<Article>>(
        stream: widget.bloc.articles,
        initialData: UnmodifiableListView<Article>([]),
        builder: (context, snapshot) {
          switch (snapshot.connectionState) {
            case ConnectionState.none:
              return Text('Press button to start.');
            case ConnectionState.active:
            case ConnectionState.waiting:
              return Text('Awaiting result...');
            case ConnectionState.done:
              if (snapshot.hasError) return Text('Error: ${snapshot.error}');
              return ListView(
                children: snapshot.data.map(_buildItem).toList(),
              );
            // You can reach your snapshot.data['url'] in here
          }
          return null; // unreachable
        },
      ),
    );
  }

  Widget _buildItem(Article article) {
    print(article);
    return Padding(
      key: Key(article.title),
      padding: const EdgeInsets.all(16.0),
      child: ExpansionTile(
        title: Text(
          article.title,
          style: TextStyle(fontSize: 24),
        ),
        children: <Widget>[
          Row(
            mainAxisAlignment: MainAxisAlignment.spaceEvenly,
            children: <Widget>[
              Text("${article.type}"),
              IconButton(
                icon: Icon(Icons.launch),
                onPressed: () async {
                  if (await canLaunch(article.url)) {
                    launch(article.url);
                  }
                },
              ),
            ],
          ),
        ],
      ),
    );
  }
}