一、判断当前用户是否已经登录，如果登录，则直接进入首页；如果没有登录，则直接进入登录页；
关键点在于：不要在首页已经出现以后，在判断数据，然后显示登录页；
伪代码：

class _StartAppState extends State<MyHome>{
  var loginState;
  @override
  void initState(){
    super.initState();
    _validateLogin();
  }
  @override
  Widget build(BuildContext context){
    if(loginState == 0){
      return login();
    }else{
      return home();
    }
  }

  Future _validateLogin() async{
    Future<dynamic> future = Future(()async{
      SharedPreferences prefs = await SharedPreferences.getInstance();
      String loginToken = prefs.get("loginToken");
      return loginToken;
    });
    future.then((val){
      if(val == null){
        setState(() {
          loginState = 0;
        });
      }else{
        setState(() {
          loginState = 1;
        });
      }
    }).catchError((_){
      print("catchError");
    });
  }
}

分析：在页面初始化(initState)的时候，读取本地存储的用户信息；在build显示页面的时候，判断显示提前注册的路由；

二、登录/注册页面

class login extends StatefulWidget {
  @override
  State<StatefulWidget> createState() {
    // TODO: implement createState
    return page();
  }
}

class page extends State<StatefulWidget> {
  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      body: Column(
        crossAxisAlignment: CrossAxisAlignment.center,
        children: <Widget>[
          _buildTopBannerWidget(),
          _loginButton(),
          _registerButton(),
        ],
      ),
    );
  }

  // 顶部图片
  _buildTopBannerWidget() {
    return Container(
      width: MediaQuery.of(context).size.width,
      height: 300,
      child: Image.asset(
        "images/123.png",
        fit: BoxFit.cover,
      ),
    );
  }

  // 登录Button
  _loginButton() {
    return Padding(
      padding: const EdgeInsets.only(top: 30, left: 40, right: 40),
      child: FlatButton(
        color: Colors.blue,
        highlightColor: Colors.blue,
        textColor: Colors.white,
        shape:RoundedRectangleBorder(borderRadius: BorderRadius.circular(20.0)),
        onPressed: () {
          Navigator.of(context).pushNamed("startLogin");
        },
        child: Container(
          height: 44,
          width: MediaQuery.of(context).size.width,
          alignment: Alignment.center,
          child: Text("登录",
            style: TextStyle(fontSize: 18.0, fontWeight: FontWeight.w300),
          ),
        ),
      ),
    );
  }

  // 注册Button
  _registerButton() {
    return Padding(
      padding: const EdgeInsets.only(top: 30, left: 40, right: 40),
      child: FlatButton(
        color: Color(0xFFcccccc),
        highlightColor: Color(0xFFcccccc),
        textColor: Colors.blue,
        shape:RoundedRectangleBorder(borderRadius: BorderRadius.circular(20.0)),
        onPressed: () {
          Navigator.of(context).pushNamed("register");
        },
        child: Container(
          height: 44,
          width: MediaQuery.of(context).size.width,
          alignment: Alignment.center,
          child: Text("手机号注册",
            style: TextStyle(fontSize: 18.0, fontWeight: FontWeight.w300),
          ),
        ),
      ),
    );
  }
}

三、首页的实现；

class home extends StatefulWidget {
  @override
  State<StatefulWidget> createState() {
    // TODO: implement createState
    return page();
  }
}

class page extends State<StatefulWidget> {

  int _selectedIndex = 0;

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: AppBar(title: Text("首页"),),
      bottomNavigationBar: BottomNavigationBar(
        items: <BottomNavigationBarItem>[
          BottomNavigationBarItem(icon: Icon(Icons.home), title: Text('公园')),
          BottomNavigationBarItem(icon: Icon(Icons.business), title: Text('电台')),
          BottomNavigationBarItem(icon: Icon(Icons.school), title: Text('消息')),
          BottomNavigationBarItem(icon: Icon(Icons.cloud), title: Text('我的')),
        ],
        currentIndex: _selectedIndex,
        selectedItemColor: Colors.blue,
        showUnselectedLabels: true,
        unselectedItemColor: Colors.black54,
        onTap: _onItemTapped,
      ),
    );
  }

  void _onItemTapped(int index) {
    setState(() {
      _selectedIndex = index;
    });
  }

}