Switching between tabs initstate() called multiple times

Question

Switching between tabs initstate() called multiple times.

i have 4 tabs in my tab barA,B,C and D.

case (1) if i switch in tab like from tab A to B it's working fine.

case (2) but if i'm go to tab A to C then initstate() of tab 'B' called two times

results of case (1)

flutter: A

flutter: B

results of case (2)

flutter: A

flutter: B

flutter: C

flutter: B

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

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

  final String title;

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

class _MyHomePageState extends State<MyHomePage> with SingleTickerProviderStateMixin{

  TabController _controller;

  void initState() {
    super.initState();
    _controller = TabController(length: 4, vsync: this);
    _controller.addListener(_handleSelected);
  }

  bool alarm = false;

// Function for handle tap event of tab
  void _handleSelected() async {
  }


  Widget build(BuildContext context) {
    return DefaultTabController(
      length: 4,
      child: Scaffold(
        appBar: AppBar(
          bottom: TabBar(
            controller: _controller,
            tabs: [
              Tab(text: "A"),
              Tab(text: "B"),
              Tab(text: "C"),
              Tab(text: "D"),
            ],
          ),
          actions: [
            Switch(
              value: alarm,
              onChanged: (value) {
              },
              activeTrackColor: Color(0xffff6b6b),
              activeColor: Color(0xffff0000),
            ),
          ],
        ),
        body: TabBarView(
          controller: _controller,
          children: [
            A(),
            B(),
            C(),
            D(),
          ],
        ),
      ),
    );
  }
}

Answer 1

To keep a stateful widget alive (not rebuild or re-render), you can use AutomaticKeepAliveClientMixin. By this way, you can easily decide which widget need to rebuild by changing ``wantKeepAlive'' parameter.

Here is a example for Class A:

class A extends StatefulWidget {
  @override
  _AState createState() => _AState();
}

class _AState extends State<A> with AutomaticKeepAliveClientMixin{
  bool _isLoading;
  @override
  void initState() {
    super.initState();
    Future.delayed(Duration(seconds: 3)).then((_){
      setState(() {
        _isLoading = false;
      });
    });
  }

  @override
  Widget build(BuildContext context) {
    return Center(
      child: _isLoading == false ?
        Text("A")
        : CircularProgressIndicator(),
    );
  }

  @override
  bool get wantKeepAlive => true;
}

Answer 2

You can use IndexedStack widget to solve this kind of problem.

In _MyHomePageState use one variable to manage index of selected page;

class _MyHomePageState extends State<MyHomePage> with SingleTickerProviderStateMixin{

int _selectedPage;

/////
Your code 
/////

}

In the body of your scaffold implement IndexedStack

body: IndexedStack(
          index:_selectedPage,
          children: [
            A(),
            B(),
            C(),
            D(),
          ],
        ),

Now in _handleSelected () method handle take the latest page index from controller and using setState update the tab bar

void _handleSelected () async {
 int index = _controller.page ;// get index from controller (I am not sure about exact parameter name for selected index) ;
setState((){
_selectedPage = index;
});
}