android - Flutter/Dart Static variables lost / keep getting reinitialized

Question

I am trying out things with Flutter/Dart right now. But my static variables keep getting reinitialised when accessed from another class.

I have a class, in its separate dart source file, holding the server status, declared as such:

class ServerStatus{
  static int newestBinary;
  static bool serverUp;
}

I initialised them @ main() by

ServerStatus.newestBinary = 20;
ServerStatus.serverUp = true;

. Afterwards, when I try to access them at another page in my application, the variables 'newestBinary' and 'serverUp' both became null, as if they are reinitalised. (If I declare them like static int newestBinary = 10;, then reassign ServerStatus.newestBinary = 20; at main(), it would still show up as 10 at another page in my application.

My application did not quit or stop between the two operations. Under what circumstances would static variables be reinitalised?

If I have to hold global and commonly used information for the application, what would be the best way to do it other than using static variables?

Thanks in advance.

Answer 1

I toyed around for an hour and realise what appears to be the reason. Apparently when I do:

import 'package:flutter_test_app/main.dart';

It is different from

import 'main.dart';

Even if both source files belong to the same package.

So in the end my test code looks like:

main.dart:

import 'package:flutter/material.dart';
import 'pageA.dart';
import 'pageB.dart';
import 'pageH.dart';

void main() {
  runApp(new MyApp());
}

class MyApp extends StatelessWidget {

  static bool testFlag = false;
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {

    testFlag = true;
    ThemeData mainTheme = new ThemeData(
      primarySwatch: Colors.cyan,
    );
    print("testFlag @ MyApp: " + testFlag.toString());
    MaterialApp mainApp = new MaterialApp(
      title: 'Instabazaar',
      theme: mainTheme,
      home: new HomePage(title: 'Instabazaar'),
    );

    return mainApp;
  }
}

class HomePage extends StatefulWidget {

  final String title;
  HomePage({Key key, this.title}) : super(key: key);

  @override
  _HomePageState createState() {

    return new _HomePageState();
  }
}

class _HomePageState extends State<HomePage> {
  int _currentPageID = 0; // 0=home, 1=pageA, 2=pageB



  @override
  Widget build(BuildContext context) {

    print("testFlag @ HomePage: " + MyApp.testFlag.toString());


    AppBar appBar = new AppBar(
        title: new Text("TestApp"),
        centerTitle: true,
    );

    BottomNavigationBar bottomNavigationBar = new BottomNavigationBar(
        type: BottomNavigationBarType.shifting,
        items: <BottomNavigationBarItem>[
          new BottomNavigationBarItem(icon: new Icon(Icons.home), title: new Text('Home'), backgroundColor: Theme.of(context).accentColor),
          new BottomNavigationBarItem(icon: new Icon(Icons.explore), title: new Text('PageA'), backgroundColor: Colors.purple),
          new BottomNavigationBarItem(icon: new Icon(Icons.star), title: new Text('PageB'), backgroundColor: Colors.redAccent),
        ],
        onTap: (i) => setState( () => _currentPageID = i ),
        currentIndex: _currentPageID
    );


    Scaffold mainScaffold = new Scaffold(
      appBar: appBar,
      body: _getNewSubPage(),
      bottomNavigationBar: bottomNavigationBar,
    );
    return mainScaffold;
  }


  //MARK: navigation


  Widget _getNewSubPage(){
    switch (_currentPageID)
    {
      case 1:
        return new pageA();
      case 2:
        return new pageB();
      default:
        return new pageH();
    }
  }


}

pageA.dart / pageB.dart:

import 'package:flutter/material.dart';
import 'package:flutter_test_app/main.dart';

class pageA extends StatefulWidget{
  pageAState createState() => new pageAState();
}


class pageAState extends State<pageA> {

  @override
  Widget build(BuildContext context) {
    print("testFlag @ pageA: " + MyApp.testFlag.toString());
    return new Container();
  }
}

pageH.dart:

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

class pageH extends StatefulWidget{
  pageHState createState() => new pageHState();
}


class pageHState extends State<pageH> {
  @override
  Widget build(BuildContext context) {
    print("testFlag @ pageH: " + MyApp.testFlag.toString());
    return new Container();
  }
}

The only difference is the import statement. However, for pageA/pageB, the print statement would give "false". As for pageH, the print statement would give "true". I have switched around the import statements and it checks out. I am not familiar with how dart actually interprets the code, so I am not sure if it is a dart thing, a setup thing or a flutter thing. I will continue investigating but for now my problem is solved.

Thanks for everyone's help.