StreamBuilder 부분 질문 드립니다.

강사님 강의 대로 bloc부문의 StreamBuilder 부분을 처리 하고 있는데요.

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

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: Scaffold(
        appBar: AppBar(
          title: Text('카운터'),
        ),
        body: Counter(),
      ),
    );
  }
}

class Counter extends StatefulWidget {
  @override
  _CounterState createState() => _CounterState();
}

class _CounterState extends State<Counter> {
  final counterSubject = BehaviorSubject<int>();

  int counter = 0;

  @override
  Widget build(BuildContext context) {
    return Center(
      child: Column(
        mainAxisAlignment: MainAxisAlignment.center,
        children: <Widget>[
          RaisedButton(
              onPressed: () {
                counterSubject.add(++counter);
              },
              child: Text('add')),
          StreamBuilder<int>(
              stream: counterSubject.stream,
              initialData: 0,
              builder: (context, snapshot) {
                if (snapshot.hasData) {
                  return Text('{$snapshot.data}',
                      style: TextStyle(fontSize: 30));
                }
              })
        ],
      ),
    );
  }
}

위와 같이 하였으며 빌드를 하면

{AsyncSnpshot<int>(Connectio State.waiting, 0, null).data} 라고 보입니다.

add버튼을 누르면 {AsyncSnpshot<int>(Connectio State.waiting, 1(<--이부분이 계속 증가 됨), null).data}

어느부분이 잘못 되어 있는지요?