firebase - Flutter FutureBuilder会引发错误，但在仿真中它会按预期工作

Question

我是Flutter的初学者，目前在Flutter中正确实现FutureBuilder时遇到问题。
我正在尝试建立一个用户页面，我的用户信息存储在Firebase中，每次访问该用户页面时，它都会检索当前用户数据并在页面上显示数据。这是我为实现编写的代码:

class UserPage extends StatefulWidget{
  @override
  UserPageState createState() => UserPageState();
}

class UserPageState extends State<UserPage>{
  String userName;
  String userEmail;
  String collegeName;

  Future _infoInit() async {
    userName = await HelperFunctions.getUserNamePreference();
    userEmail = await HelperFunctions.getUserEmailPreference();
    collegeName = await HelperFunctions.getUserCollegePreference();
  }

  Widget userScaffold(BuildContext context, AsyncSnapshot snapshot){
    return Scaffold(
        appBar: AppBar(
          elevation: 0,
          title: Text(
              userName
          ),
          backgroundColor: Colors.lightBlue,
        ),
        body:Center(
            child: Text("This is User Page")
        )
    );
  }

  @override
  Widget build(BuildContext context) {
    return FutureBuilder(
        future: _infoInit(),
        builder: (context,AsyncSnapshot snapshot) => userScaffold(context, snapshot)
    );
  }
}

目前，我编写的唯一部分是在应用程序栏上显示当前登录的用户，并且当我运行代码时，它似乎已成功运行。但是，当我查看android studio控制台时，我可以看到它实际上正面临着错误，我认为这与在FutureBuilder小部件中执行的异步功能有关。

Error Message:
Performing hot reload...
Syncing files to device iPhone 11...
Reloaded 7 of 650 libraries in 397ms.

════════ Exception caught by widgets library ═══════════════════════════════════════════════════════
The following assertion was thrown building FutureBuilder<dynamic>(dirty, state: _FutureBuilderState<dynamic>#772f1):
A non-null String must be provided to a Text widget.
'package:flutter/src/widgets/text.dart':
Failed assertion: line 298 pos 10: 'data != null'

The relevant error-causing widget was: 
  FutureBuilder<dynamic> file:///Users/nossu3751/Downloads/flutter_project/moim_app/lib/user/profile.dart:39:12
When the exception was thrown, this was the stack: 
#2      new Text (package:flutter/src/widgets/text.dart:298:10)
#3      UserPageState.userScaffold (package:moimapp/user/profile.dart:26:18)
#4      UserPageState.build.<anonymous closure> (package:moimapp/user/profile.dart:41:54)
#5      _FutureBuilderState.build (package:flutter/src/widgets/async.dart:732:55)
#6      StatefulElement.build (package:flutter/src/widgets/framework.dart:4619:28)

更确切地说，它表示我要在FutureBuilder中使用的userName为null，即使我相信我已经通过运行_infoInit()方法分配了该值，并且实际上确实在模拟器上正确显示了该值。

如果有人可以让我知道我在这里做错了什么，以及我可以做些什么来阻止此消息再次出现，我将不胜感激。提前非常感谢您!

Answer 1

您需要在构建器中使用ConnectionState。查看以下代码模板:(当前，您的构建器返回userScaffold，而无需等待将来完成)

return FutureBuilder(
        future: yourFuture(),
        builder: (context, snapshot) {
          if (snapshot.connectionState == ConnectionState.done) {
            // future complete
            // if error or data is false return error widget
            if (snapshot.hasError) {
              return _buildErrorWidget('SOMETHING WENT WRONG, TAP TO RELOAD');
            }

            // return data widget
            return _buildDataWidget();

            // return loading widget while connection state is active
          } else
            return _buildLoadingWidget();
        },
      );