使用flutter和firebase检查用户名并登录

Question

大家好，我有这段代码：

 new ListTile(
            title: new Text("Log In"),
            leading: new Icon(Icons.account_box),
            onTap: () => _signIn()
                .then((FirebaseUser user) => print(user))
                .catchError((e) => print(e)),
            subtitle: StreamBuilder(
              stream: FirebaseAuth.instance.onAuthStateChanged,
              builder: (BuildContext context,
                  AsyncSnapshot<FirebaseUser> snapshot) {
                if (snapshot.connectionState == ConnectionState.waiting) {
                  username = 'loading';
                  return Text(username);
                } else if (!snapshot.hasData) {
                  username = 'you are not logged in';
                  return Text(username);
                } else {
                  username = snapshot.data.displayName;
                  return Text(username);
                }
              },
            ),
          ),

我想用以下代码替换此代码，但我不知道如何正确实现它，以及可以使用哪个流或生成器可以帮助我

 new ListTile(
            title: new Text("Log In"),
            leading: new Icon(Icons.account_box),
            onTap: () => _signIn()
                .then((FirebaseUser user) => print(user))
                .catchError((e) => print(e)),
            subtitle: 
            FirebaseUser user = FirebaseAuth.getInstance().getCurrentUser();
            var username ='';
       if (user != null) {
          username=user;
          return new Text(username)
         } else {
               username='you are not logged in'
                      }
              },
            ),
          ),

这就是我想要的，谢谢