Flutter read shared preferences in main then decide which startup page?

问题

I want to judge which page to start up in main (actually is login page and the home page). So I have to read isLogin in preferences. How to do that in main?

I tied these codes:

Future<Null> checkIsLogin() async {
  String _token = "";
  // If token exist already, then HomePage
  SharedPreferences prefs = await SharedPreferences.getInstance();
  _token = prefs.getString("token");
  print('get token from prefs: ' +  _token);
  if (_token != "" && _token != null) {
    // already login
    print("alreay login.");
    isLogin = true;
  }
}

void main() {
  App.init();
  // if we have token then go to HomePage directly otherwise go to LoginPage.
  Widget _defaultHome = new LoginPage();
  checkIsLogin();
  if (isLogin) {
    _defaultHome = new HomePage();
  }

  runApp(new MaterialApp(
      debugShowCheckedModeBanner: false,
      theme: globalThemeData,
      home: _defaultHome
  ));
}

above code, isLogin is an global variable. There was an error:

Performing full restart...                                       
Restarted app in 2,810ms.
[VERBOSE-2:dart_error.cc(16)] Unhandled exception:
Invalid argument(s)
#0      _StringBase.+ (dart:core/runtime/libstring_patch.dart:245:57)
#1      checkIsLogin (file:///Volumes/xs/awesome/uranus/clients/flutter/flutter_asgard/lib/main.dart:17:34)
<asynchronous suspension>
#2      main (file:///Volumes/xs/awesome/uranus/clients/flutter/flutter_asgard/lib/main.dart:29:3)
#3      _startIsolate.<anonymous closure> (dart:isolate/runtime/libisolate_patch.dart:279:19)
#4      _RawReceivePortImpl._handleMessage (dart:isolate/runtime/libisolate_patch.dart:165:12)

Seems there are issue to call async in main, how to get it to work?

回答1:

Load the Homepage and if the user is not logged in, then replace it with your LoginPage()

@override
  void initState() {
    super.initState();
    checkIsLogin();
 }   


Future<Null> checkIsLogin() async {
    String _token = "";
    SharedPreferences prefs = await SharedPreferences.getInstance();
    _token = prefs.getString("token");
    if (_token != "" && _token != null) {
      print("alreay login.");
      //your home page is loaded
    }
    else
    {
      //replace it with the login page
      Navigator.pushReplacement(
        context,
        MaterialPageRoute(builder: (context) => new LoginPage()),
      );
    }
  }

回答2:

You need await checkIsLogin.

This is my code:

Future<Null> main() async {
  SystemChrome.setPreferredOrientations([
    DeviceOrientation.landscapeRight,
    DeviceOrientation.landscapeLeft,
  ]);
  Screen.keepOn(true);
  SharedService.sharedPreferences = await SharedPreferences.getInstance();
  account = SharedService.sharedPreferences.getString("Account");
  password = SharedService.sharedPreferences.getString("Password");
  runApp(new MyApp());
}

回答3:

This is what i did,

void main() {
  WidgetsFlutterBinding.ensureInitialized();
  SharedPreferences.getInstance().then((instance) {
    StorageService().sharedPreferencesInstance = instance; // Storage service is a service to manage all shared preferences stuff. I keep the instance there and access it whenever i wanted.
    runApp(MyApp());
  });
}

Then in the Material App Build

@override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'App Title',
      home: _checkUserLoggedIn()
          ? HomeScreen()
          : LoginPage(),
    );
  }

_checkUserLoggedIn Function

bool _checkUserLoggedIn() {
    return _storageService.getFromShared('isLoggedIn'); //  Just a get method from shared preferences
}

回答4:

Create a SplashPage that you can pass as a home route in your MaterialApp()

Inside SplashPage, for example initState() you can check for login and than push new route to a Navigator.

SplashPage can just be centered logo, with optional animation.

来源：https://stackoverflow.com/questions/50458526/flutter-read-shared-preferences-in-main-then-decide-which-startup-page

标签

async-await

dart

Flutter