StreamBuilder未返回结果的ListBuilder

import 'dart:async';
import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(title: Text('StreamBuilder Example')),
        body: StreamBuilderExample(),
      ),
    );
  }
}

class StreamBuilderExample extends StatefulWidget {
  @override
  _StreamBuilderExampleState createState() => _StreamBuilderExampleState();
}

class _StreamBuilderExampleState extends State<StreamBuilderExample> {
  final StreamController<int> _controller = StreamController<int>();
  Timer _timer;

  @override
  void initState() {
    super.initState();
    _timer = Timer.periodic(Duration(seconds: 1), (Timer t) => _controller.add(t.tick));
  }

  @override
  void dispose() {
    _timer.cancel();
    _controller.close();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return StreamBuilder(
      stream: _controller.stream,
      builder: (BuildContext context, AsyncSnapshot snapshot) {
        if (snapshot.hasData) {
          return ListView.builder(
            itemCount: snapshot.data.length,
            itemBuilder: (context, index) {
              return ListTile(
                title: Text('Number: ${snapshot.data[index]}'),
              );
            },
          );
        } else {
          return Center(child: CircularProgressIndicator());
        }
      },
    );
  }
}