firebase - Flutter/Firebase_Auth:生成函数返回null

Question

当我尝试运行该应用程序时，它将引发错误:A build function returned null并崩溃。这是已镶嵌的小部件。它怎么可能返回null？我该如何解决？

class Wrapper extends StatefulWidget {
  @override
  _WrapperState createState() => _WrapperState();
}

class _WrapperState extends State<Wrapper> {

  @override
  Widget build(BuildContext context) {
    var auth = FirebaseAuth.instance;
    auth.onAuthStateChanged.listen((user) {
      if (user != null) {
        print("user is logged in");
        return HomeScreen();
        
      } else {
        print("user is not logged in");
        return LoginScreen();
      }
    });

  }
}

Answer 1

如果渲染输出依赖于异步加载的内容(例如身份验证状态)，则应将其存储在对象的状态中:

class _WrapperState extends State<Wrapper> {

  public _WrapperState() {
    FirebaseAuth.instance.onAuthStateChanged.listen((user) {
      setState(() {
        this.user = user
      });
  }

  @override
  Widget build(BuildContext context) {
    if (user != null) {
      return HomeScreen();        
    } else {
      return LoginScreen();
    }
  }
}

我通常更喜欢这样编写 build:

@override
Widget build(BuildContext context) {
  return (user != null) ? HomeScreen() : LoginScreen();
}