Pattern for updating TabController length from child widget

EDIT:

Since March/2019 this answer is no longer valid. A pull request have included a check and now the TabController's length must match the Tab count.

So it's definitely worth trying the Salvatore's solution, but if your tabs are really dynamic and may change a lot, in my opinion the only solution now is Collin Jackson's one.

I found my own way of updating the TabController's length.

I initialize it in initState() with a fake length, that works only like a placeholder. When I know the right length that the TabController should have, I initialize it again, but now with the right length. Here is what I mean:

TabController _tabController;

@override
  void initState() {
    super.initState();

    // Initialize the TabController with a fake length
    _tabController = TabController(vsync: this, length: 0);

    // Retrieve the number of tabs that will be displayed in TabBar
    _retrieveNumOfTabs().then((numOfTabs) {
      // Initialize the TabController with the right length
      _tabController = TabController(vsync: this, length: numOfTabs);

     // Add here all the controller's listeners
    });
  }

With DefaultTabController:

  List<String> tabNames = ["a", "b", "C"];

  @override
  Widget build(BuildContext context) {
    return DefaultTabController(
        length: tabNames.length,
        child: Scaffold(
          // add new tab on click
          floatingActionButton: FloatingActionButton(
              onPressed: () => setState(() {
                    tabNames.add("c");
                  })),
          appBar: AppBar(bottom: TabBar(tabs: tabNames.map((x) => Tab(text: x)).toList())),
          body: TabBarView(children: tabNames.map((x) => Container(child: Text(x))).toList()),
        ));
  }

If you are using slivers also add:

TabBar(controller: DefaultTabController.of(context), ...)

With TabController:

  List<String> tabNames = ["a", "b", "C"];
  TabController _controller;

  @override
  void initState() {
    super.initState();
    _controller = TabController(vsync: this, length: tabNames.length);
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      // add new tab on click
      floatingActionButton: FloatingActionButton(
          onPressed: () => setState(() {
                tabNames.add("c");
                // might be good idea to dispose old tabController before
                _controller = TabController(vsync: this, length: tabNames.length);
              })),
      appBar: AppBar(bottom: TabBar(controller: _controller, tabs: tabNames.map((x) => Tab(text: x)).toList())),
      body: TabBarView(controller: _controller, children: tabNames.map((x) => Container(child: Text(x))).toList()),
    );
  }

I use this pattern for changing tabbar length with my items

class MyWidget extends StatefulWidget {
  const MyWidget({Key? key}) : super(key: key);

  @override
  State<MyWidget> createState() => _MyWidgetState();
}

class _MyWidgetState extends State<MyWidget>
    with SingleTickerProviderStateMixin {
  bool inited = false;
  TabController? tabcontroller;
  final List<OrderTabbarWidget> tabbarWidgets = [];
  _initTabbarWidgets() async {
    setState(() {
      inited = false;
    });
    await Future.delayed(const Duration(
        milliseconds: 100)); //some time i give error if this method not exists
    final List<OrderTabbarWidget> _widgest = [];
    int getIndex() => _widgest.length;
    if (condition1)
      _widgest.add(OrderTabbarWidget(NewOrderAddressPage(), getIndex(),
          const Icon(Icons.location_on), OrderTabType.address));
    _widgest.add(OrderTabbarWidget(const NewOrderItemsView(), getIndex(),
        const Icon(Icons.category), OrderTabType.items));
    if (condition2)
      _widgest.add(OrderTabbarWidget(
          const SubmitOrderDiscountPage(),
          getIndex(),
          const Icon(CustomIcons.discount_tag_svgrepo_com),
          OrderTabType.discount));

    if (condition3)
      _widgest.add(OrderTabbarWidget(NewOrderDefaultShipping(), getIndex(),
          const Icon(Icons.local_shipping), OrderTabType.shipping));
    _widgest.add(OrderTabbarWidget(const NewOrderSubmitPage(), getIndex(),
        const Icon(Icons.checklist_rtl_outlined), OrderTabType.submit));
    _widgest.add(OrderTabbarWidget(const NewOrderPaymentPage(), getIndex(),
        const Icon(Icons.payment), OrderTabType.payment));

    tabbarWidgets
      ..clear()
      ..addAll(_widgest);
    tabcontroller?.dispose();
    tabcontroller = null;
    tabcontroller =
        new TabController(length: tabbarWidgets.length, vsync: this);
    setState(() {
      inited = true;
    });
  }

  @override
  void initState() {
    _resetMyTabbar();
    super.initState();
  }

  _resetMyTabbar() {
    /** fetch items or anything need for tabbar **/
    _initTabbarWidgets();
  }

  @override
  Widget build(BuildContext context) {
    if (!inited) return const NormalProgress();
    return Scaffold(
      floatingActionButton: FloatingActionButton(
        onPressed: _resetMyTabbar,
        child: const Icon(Icons.refresh),
      ),
      appBar: AppBar(
        bottom: TabBar(
            controller: tabcontroller,
            tabs: List.generate(
                tabbarWidgets.length, (index) => tabbarWidgets[index].tab)),
      ),
      body: TabBarView(
          controller: tabcontroller,
          children: List.generate(
              tabbarWidgets.length, (index) => tabbarWidgets[index].widget)),
    );
  }
}

OrderTabbarWidget

class OrderTabbarWidget {
  final Widget widget;
  final int index;
  final Widget tab;
  final OrderTabType type;
  bool get isShipping => type == OrderTabType.shipping;
  OrderTabbarWidget(this.widget, this.index, this.tab, this.type);
}