在 Flutter 中呈现小部件之前，如何等待异步 function 完成执行

Question

On my main.dart file, I want to check if a user is logged so as to direct him to the appropriate screen. I am using SharedPrefence to store user detail from Firebase. How do I tell my function to wait until my SharedPreference async function finishes executing before it can render the appropriate widget.

Below is my code:

  Widget _gotoHomeScreen() {
  AuthService.getuserPrefEmail().then((email) {
  print(email);
  AuthService.email = email;
  if (email == null) {
    return LoginScreen();
  } else {
    AuthService.uid = email;
    return HomeMenuScreen();
  }
});

}

Answer 1

You don't have to wait for the build, you should build something, to show to the user that the app is loading something (or a blank screen), and then rebuild when the funcion ends.

You could have a Widget variable that is set to a default when you create the widget, say with a CircularProgressIndicator , then change it with setState, something like this:

class YourWidgetState extends State<YourWidget> {
  Widget _body = CircularProgressIndicator();  // Default Body

  @override
  void initState(){
    _gotoHomeScreen();
  }

  @override
  Widget build(BuildContext context){
    return _body;
  }

  Widget _gotoHomeScreen() {
    AuthService.getuserPrefEmail().then((email){
      AuthService.email = email;
      if (email == null) {
        setState(() => _body = LoginScreen());
      } else {
        AuthService.uid = email;
        setState(() => _body = HomeMenuScreen());
      }
    });
  }
}

---

Another way would be to use a variable to inform you about the loading situation, like bool finishedLoading , and call setState to change the value when it's done, using the email variable you set to know when the user is logged in, and do a conditional build, like this:

bool loading = true;

@override
Widget build(BuildContext context){
  if(loading) return CircularProgressIndicator();

  if(AuthService.email == null)
    return LoginScreen();
  else
    return HomeMenuScreen();
}

Widget _gotoHomeScreen() {
  AuthService.getuserPrefEmail().then((email){
    AuthService.email = email;
    if (email != null) {
     AuthService.uid = email;
    }
    setState((){ loading = false; });
  });
}

Answer 2

Use a simple FutureBuilder!

https://api.flutter.dev/flutter/widgets/FutureBuilder-class.html

FutureBuilder<Email>(
future: AuthService.getuserPrefEmail(),
 builder: (BuildContext context, AsyncSnapshot<String> snapshot) {
  switch (snapshot.connectionState) {
    case ConnectionState.active:
    case ConnectionState.waiting:
     return CircularProgressIndicator();
    case ConnectionState.done:
     if (snapshot.hasError) {
       return Text('Error: ${snapshot.error}');
      }
     .... here route to your screen or set it how you want
  }
 },
)

Answer 3

Have a async firebase function?

Ans=> Use FutureBuilder or StreamBuilder

---

StreamBuilder

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: StreamBuilder<QuerySnapshot>(
          stream: FirebaseFirestore
                  .instance.
                  .collection('users') // 👈 Your desired collection name here
                  .snapshots(), 
          builder: (BuildContext context, AsyncSnapshot<QuerySnapshot> snapshot) {
            if (snapshot.hasError) {
              return const Text('Something went wrong');
            }
            if (snapshot.connectionState == ConnectionState.waiting) {
              return const Text("Loading");
            }
            return ListView(
                children: snapshot.data!.docs.map((DocumentSnapshot document) {
              Map<String, dynamic> data =
                  document.data()! as Map<String, dynamic>;
              return ListTile(
                title: Text(data['fullName']), // 👈 Your valid data here
              );
            }).toList());
          },
        ),
      ),
    );
  }

---

FutureBuilder

@override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
          child: FutureBuilder<QuerySnapshot>(
        future: FirebaseFirestore
                .instance
                .collection('users') // 👈 Your collection name here
                .get(), 
        builder: (_, snapshot) {
          if (snapshot.hasError) return Text('Error = ${snapshot.error}');
          if (snapshot.connectionState == ConnectionState.waiting) {
            return const Text("Loading");
          }
          return ListView(
              children: snapshot.data!.docs.map((DocumentSnapshot document) {
            Map<String, dynamic> data = document.data()! as Map<String, dynamic>;
            return ListTile(
              title: Text(data['avatar']), // 👈 Your valid data here
            );
          }).toList());
        },
      )),
    );
  }