Flutter / Firebase_Auth：生成函数返回null

问题描述：

当我尝试运行该应用程序时，它会引发错误：一个构建函数返回null 并崩溃。这是已镶嵌的小部件。它怎么可能返回null？以及我该如何解决？

When i try to run the app it throws an error: A build function returned null and crashes. Here is the insterested widget. How is it possible that it returns null ? and how can i fix it ?

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();
      }
    });

  }
}

答

如果您的渲染输出取决于异步加载的内容（例如身份验证状态），应将其存储在对象的状态中：

If your render output depends on something that is loaded asynchronously (such as the authentication state), you should store that in the state of the object:

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 ：

I typically prefer writing the build like this:

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

Flutter / Firebase_Auth：生成函数返回null

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