Flutter Dynamic TabView

main.dart

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

class MyApp extends StatefulWidget {
  @override
  State<StatefulWidget> createState() => MyAppState();
}

class MyAppState extends State<MyApp> {
  bool isExpanded = false;

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: DefaultTabController(
        length: _calcNumberOfTabs(),
        child: Scaffold(
          appBar: AppBar(
            actions: <Widget>[
              GestureDetector(
                  onTap: _toggleTabs, child: Icon(Icons.add_circle_outline))
            ],
            bottom: TabBar(
              tabs: _buildTabs(),
            ),
            title: Text('Flutter Tabs Example'),
          ),
          body: TabBarView(
            children: _buildChildren(),
          ),
        ),
      ),
    );
  }

  _toggleTabs() {
    print("Toggle called");
    setState(() {
      isExpanded = !isExpanded;
    });
  }

  _buildTabs() {
    List<Widget> tabs = List();
    tabs.add(Tab(icon: Icon(Icons.directions_car)));
    tabs.add(Tab(icon: Icon(Icons.directions_transit)));

    if (isExpanded) {
      tabs.add(Tab(icon: Icon(Icons.directions_bike)));
    }

    return tabs;
  }

  _buildChildren() {
    List<Widget> children = List();

    children.add(FirstScreen());
    children.add(SecondScreen());

    if (isExpanded) {
      children.add(OptionalScreen());
    }

    return children;
  }

  _calcNumberOfTabs() {
    return (isExpanded ? 3 : 2);
  }
}