无法在颤振中加载共享首选项

Question

So I was trying to make a guest mode and user mode, where if the token is null, it will return the guest mode page and vice versa.

The problem is that my token is always null that makes it ends up in the guest mode instead of user mode.

The token is null because when I try to debug, the program skipped the token = prefs.getString(_sessionTokenKey); I have checked the _sessionTokenKey storage and there is a token.

static void loadToken() async {
    SharedPreferences prefs = await SharedPreferences.getInstance();
    token = prefs.getString(_sessionTokenKey);
}

static Widget _homeBuilder(BuildContext context) {
    loadToken();
    print("this is token from route generator");
    print(token);
    if (token == null) {
        return BottomNavBarGuestMode();
    } else {
        return BottomNavBar();
    }
}

static Route<dynamic> generate(RouteSettings settings) {
    switch (settings.name) {
      case Routes.main:
        return MaterialPageRoute(builder: _homeBuilder, settings: settings);
      default:
        return MaterialPageRoute(builder: (_) => RouteErrorPage());
    }    
}

Answer 1

Your loadToken is async. So token is null because you do not wait for an answer of loadToken .

Solution:

Add an await before your your laodToaken() call.

Answer 2

You are not following the correct approach to use shared preferences. Just make a class with your appname and add a static instance for shared preference like this:

class AppName{
static SharedPreferences prefs;
}

now in you main method initialise the prefs, also make your main function as async as follows:

Future<void> main() async {
AppName.prefs = await SharedPreferences.getInstance();
}

Since you have the preferences initiated now, you can use it easily.So now change your loadToken method as follows:

String loadToken(){
    final prefs = AppName.prefs;
    token = prefs.getString(_sessionTokenKey);
    return token;
}

And change your _homeBuilder code as below:

static Widget _homeBuilder(BuildContext context) {
    token = loadToken();
    print("this is token from route generator");
    print(token);
    if (token == null) {
        return BottomNavBarGuestMode();
    } else {
        return BottomNavBar();
    }
}

You might require to import the AppName class wherever required.

Answer 3

发现需要使用FutureBuilder()，这样load data方法才能同步运行