Get current tab of DefaultTabController

If you wrap your Scaffold inside of a Builder, you'll be able to access your DefaultTabController within the proper context. You can then retrieve the tab index with DefaultTabController.of(context).index.

  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'pari',
      debugShowCheckedModeBanner: false,
      theme: widget._themeData,
      home: DefaultTabController(
        length: 4,
        child: Builder(builder: (BuildContext context) {
          return Scaffold(
            appBar: AppBar(
              title: Text('pari'),
              bottom: TabBar(
                  isScrollable: true,
                  tabs: [Text('0'), Text('1'), Text('2'), Text('3')]),
            ),
            body: _buildBody(),
            floatingActionButton: FloatingActionButton(
              onPressed: () {
                print(
                    'Current Index: ${DefaultTabController.of(context).index}');
              },
            ),
          );
        }),
      ),
    );
  }

You can also get the index of the tab by using the onTap property of the DefaultTabController.

Widget build(BuildContext context) {
  return new MaterialApp(
    title: 'pari',
    debugShowCheckedModeBanner: false,
    theme: widget._themeData,
    home: DefaultTabController(
      length: widget._tabs.length,
      child: Scaffold(
        appBar: AppBar(
          title: Text('pari'),
          bottom: TabBar(
            isScrollable: true,
            onTap: (int index) {
               print('index is $index');
             }
            tabs: widget._tabs,
          ),
        ),
        body: _buildBody(),
        floatingActionButton: FloatingActionButton(
            onPressed: addWagerTap,
        ),
      )
    ),
  );
}