Flutter: await a function before Build

Your title does not really match your question. In fact, you basically provided 3 questions:

How to await a function before/in build?: You don't, build can be called multiple times, it is not really in your control, when or how it is called, and you should not make assumptions about it. Instead, you can use FutureBuilder to build 2 different widgets, depending on whether the Future resolved or not.

How to automatically navigate on page entry?: Trigger the method to check and navigate in initState, but you can't prevent the first page from building / showing (since you navigated on that page first).

How to do conditional navigation?: You first await the future, and then navigate.

For your use case: You want to do conditional navigation, so before you navigate to your login page (presumably the first page in your app), provide something like a splash page, where you check if the user is logged in. And after the future resolved, you do the navigation to either profile or login screen. You can wrap this in a function to reuse this conditional navigation on other pages, where you want to land either in the login or the profile page.

You can copy paste run full code below
You can get logined in main() and do Navigate in initialRoute
code snippet

Future<void> main() async {
  WidgetsFlutterBinding.ensureInitialized();

  SharedPreferences prefs = await SharedPreferences.getInstance();

  logined = prefs.getBool(_StorageLogined) ?? false;
  GetUserNum = prefs.getInt(_StorageUserNum) ?? 0;

  runApp(MyApp());
}
...
initialRoute: logined == false ? "/login" : "/profile",
      routes: {
        '/profile': (context) => ShowProfile(
              title: "demo",
            ),
        "/login": (context) => Login(),
      },

full code

import 'package:flutter/material.dart';
import 'package:shared_preferences/shared_preferences.dart';

bool logined;
int GetUserNum;
String _StorageLogined = "Logined";
String _StorageUserNum = "UserNum";

Future<void> main() async {
  WidgetsFlutterBinding.ensureInitialized();

  SharedPreferences prefs = await SharedPreferences.getInstance();

  logined = prefs.getBool(_StorageLogined) ?? false;
  GetUserNum = prefs.getInt(_StorageUserNum) ?? 0;

  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      initialRoute: logined == false ? "/login" : "/profile",
      routes: {
        '/profile': (context) => ShowProfile(
              title: "demo",
            ),
        "/login": (context) => Login(),
      },
    );
  }
}

class Login extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("Login "),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text(
              'Login Page',
            ),
          ],
        ),
      ),
    );
  }
}

class ShowProfile extends StatefulWidget {
  ShowProfile({Key key, this.title}) : super(key: key);

  final String title;

  @override
  _ShowProfileState createState() => _ShowProfileState();
}

class _ShowProfileState extends State<ShowProfile> {
  int _counter = 0;

  void _incrementCounter() {
    setState(() {
      _counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text(
              'You have pushed the button this many times:',
            ),
            Text(
              '$_counter',
              style: Theme.of(context).textTheme.headline4,
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ),
    );
  }
}

You can call the LoginCheck function in the initState lifecycle method. Thus the function will be executed before the build method. see below

• LoginCheck now returns the value of logined and GetUserNum as a list

Future<List<dynamic>> LoginCheck() async{
    SharedPreferences prefs = await SharedPreferences.getInstance();

    bool logined = prefs.getBool(_StorageLogined) ?? false;
    int GetUserNum = prefs.getInt(_StorageUserNum) ?? 0;

//     if(logined == true) {
//       Navigator.of(context).push(MaterialPageRoute(
//          builder: (context) => ShowProfile(UserNum: GetUserNum,)));
  
  return [logined,GetUserNum];
 
}

• You then call LoginCheck in your initState method as below:

    bool logined;
    int GetUserNum;
    initState(){
      super.initState();
  
      LoginedCheck().then((results){  
          setState((){
            logined = results[0];
            GetUserNum = results[1];
          });
  
        if(logined==true)Navigator.of(context).push(
        MaterialPageRoute( builder: (context) => ShowProfile(UserNum: GetUserNum))
        );
      });
  
  
    }  
  
  
  
    Widget build(BuildContext context) {
  
    return Scaffold(
      resizeToAvoidBottomPadding: false,
      body: logined==false?Stack( //if logined is not true we display the login page else we show a loader while we naviagate to the user's page
        children: <Widget>[
          Container(
            height: double.infinity,
            width: double.infinity,
            decoration: BoxDecoration(
                gradient: LinearGradient(
                  begin: Alignment.topCenter,
                  end: Alignment.bottomCenter,
                  colors: [
                    Color(0xFF73AEF5),
                    Color(0xFF61A4F1),
                    Color(0xFF478DE0),
                    Color(0xFF398AE5),
                  ],
                  stops: [0.1,0.4,0.7,0.9],
                )),
            child: Center(
              child: Column(
                mainAxisAlignment: MainAxisAlignment.center,
                children: <Widget>[
                  SizedBox(
                    height: 10,
                  ),
                  Icon(
                    Icons.account_circle,
                    size: 150.0,
                  ),
                  SizedBox(height: 30),
                  Container(
                    width: 300,
                    child: Theme(
                        data: ThemeData(
                          primaryColor: Colors.white,
                          primaryColorDark: Colors.yellowAccent,
                        ),
                        child: TextField(
                          controller: _LtextFieldAccount,
                          decoration: InputDecoration(
                              border: OutlineInputBorder(),
                              hintText: '請輸入帳號',
                              prefixIcon: const Icon(Icons.person)),
                        )),
                  ),
                  SizedBox(height: 20),
                  Container(
                    width: 300,
                    child: Theme(
                        data: ThemeData(
                          primaryColor: Colors.white,
                          primaryColorDark: Colors.yellowAccent,
                        ),
                        child: TextField(
                          controller: _LtextFieldID,
                          decoration: InputDecoration(
                              border: OutlineInputBorder(),
                              hintText: '請輸入密碼',
                              prefixIcon: const Icon(Icons.lock)),
                        )),
                  ),
                  SizedBox(height: 30),
                  RaisedButton(
                    shape: RoundedRectangleBorder(
                        borderRadius: BorderRadius.circular(25.0)),
                    elevation: 4.0,
                    color: Colors.white,
                    child: Container(
                      padding: EdgeInsets.only(
                          left: 10,right: 10,top: 10,bottom: 10),
                      child: Text(
                        '登入',
                        style:
                        TextStyle(fontSize: 25,color: Colors.blueAccent),
                      ),
                    ),
                    onPressed: Login,
                  ),
                  SizedBox(height: 20),
                  RaisedButton(
                      shape: RoundedRectangleBorder(
                          borderRadius: BorderRadius.circular(25.0)),
                      elevation: 4.0,
                      color: Colors.white,
                      child: Container(
                        padding: EdgeInsets.only(
                            left: 10,right: 10,top: 10,bottom: 10),
                        child: Text(
                          '新用戶註冊',
                          style:
                          TextStyle(fontSize: 25,color: Colors.blueAccent),
                        ),
                      ),
                      onPressed: (){
                        Navigator.of(context).push(MaterialPageRoute(
                            builder: (context) => RegisterPage()));
                      })
                ],
              ),
            ),
          ),
        ],
      ):Center(child:CircularProgressIndicator()),
      bottomNavigationBar: BottomNavigationBar(
        onTap: onTabTapped,
        currentIndex: _currentIndex, //thiswillbesetwhenanewtabistapped
        items: [
          new BottomNavigationBarItem(
            icon: new Icon(Icons.school),
            title: new Text('大學'),
          ),
          new BottomNavigationBarItem(
            icon: new Icon(Icons.directions_subway),
            title: new Text('交通'),
          ),
          new BottomNavigationBarItem(
            icon: new Icon(Icons.person),
            title: new Text('個人'),
          )
        ],
      ),
    );
  }