Como construir um TabView usando Flutter GetX

Question

Estou tentando construir um aplicativo usando Flutter com GetX para gerenciamento de estado, e em uma das minhas páginas eu preciso construir um widget TabView no meio da página, já encontrei um monte de coisas explicando como construir um TabView como widget, como este post e este artigo , mas todos eles estendem um controlador de estado com um SingleTickerProviderStateMixin .

Pelo que entendi lendo a documentação, não devo usar StatefulWidgets e States assim, mas não consigo descobrir como elaborar uma solução usando a arquitetura GetX.

Já experimentei na minha página coisas como:

class CourseDetailPage extends StatelessWidget {
  final TabController _tabController = Get.put(TabController(vsync: null, length: 2));
}

e

class CourseDetailPage extends StatelessWidget {
  final TabController _tabController = TabController(vsync: null, length: 2);
}  

Mas o argumento VSYNC para o TabController não pode ser nulo, e não descobri como não consigo obter um TickerProvider para preenchê-lo.

Answer 1

O seguinte seria uma solução para usar um controlador GetX para controlar um TabView?

GetTicker

Há uma versão Get de SingleTickerProviderMixin que implementa a interface TickerProvider (usando a mesma classe Ticker do Flutter SDK).

Tem um nome cativante: SingleGetTickerProviderMixin

The below example is basically from the Flutter docs for TabController.

From the linked example's StatefulWidget I transplated its contents into a GetxController (MyTabController) and added mixin of SingleGetTickerProviderMixin:

class MyTabController extends GetxController with SingleGetTickerProviderMixin {
  final List<Tab> myTabs = <Tab>[
    Tab(text: 'LEFT'),
    Tab(text: 'RIGHT'),
  ];

  TabController controller;

  @override
  void onInit() {
    super.onInit();
    controller = TabController(vsync: this, length: myTabs.length);
  }

  @override
  void onClose() {
    controller.dispose();
    super.onClose();
  }
}

To use MyTabController inside a Stateless widget:

class MyTabbedWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    final MyTabController _tabx = Get.put(MyTabController());
    // ↑ init tab controller
    
    return Scaffold(
      appBar: AppBar(
        bottom: TabBar(
          controller: _tabx.controller,
          tabs: _tabx.myTabs,
        ),
      ),
      body: TabBarView(
        controller: _tabx.controller,
        children: _tabx.myTabs.map((Tab tab) {
          final String label = tab.text.toLowerCase();
          return Center(
            child: Text(
              'This is the $label tab',
              style: const TextStyle(fontSize: 36),
            ),
          );
        }).toList(),
      ),
    );
  }
}

Here's the rest of the app example to just copy & paste into Android Studio / VisualStudio Code to run:

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

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,
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: HomePage(),
    );
  }
}

class HomePage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('GetX Tab Example'),
      ),
      body: Column(
        children: [
          Expanded(
            flex: 1,
            child: Container(
              alignment: Alignment.center,
              child: Text('Some random stuff'),
            ),
          ),
          Expanded(
            flex: 4,
            child: MyTabbedWidget(),
          )
        ],
      ),
    );
  }
}

class MyTabController extends GetxController with SingleGetTickerProviderMixin {
  final List<Tab> myTabs = <Tab>[
    Tab(text: 'LEFT'),
    Tab(text: 'RIGHT'),
  ];

  TabController controller;

  @override
  void onInit() {
    super.onInit();
    controller = TabController(vsync: this, length: myTabs.length);
  }

  @override
  void onClose() {
    controller.dispose();
    super.onClose();
  }
}

class MyTabbedWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    final MyTabController _tabx = Get.put(MyTabController());
    // ↑ init tab controller
    
    return Scaffold(
      appBar: AppBar(
        bottom: TabBar(
          controller: _tabx.controller,
          tabs: _tabx.myTabs,
        ),
      ),
      body: TabBarView(
        controller: _tabx.controller,
        children: _tabx.myTabs.map((Tab tab) {
          final String label = tab.text.toLowerCase();
          return Center(
            child: Text(
              'This is the $label tab',
              style: const TextStyle(fontSize: 36),
            ),
          );
        }).toList(),
      ),
    );
  }
}

Answer 2

As you said in all articles they extend State with SingleTickerProviderStateMixin because you won't be able to initialize your TabController outside of a State as TabController manage a state (doc).

A workaround would be to not use a variable for your controller and wrap your TabBar and TabView widget tree inside a DefaultTabController.

Here is an example from the official doc:

class MyDemo extends StatelessWidget {
  final List<Tab> myTabs = <Tab>[
    Tab(text: 'LEFT'),
    Tab(text: 'RIGHT'),
  ];

  @override
  Widget build(BuildContext context) {
    return DefaultTabController(
      length: myTabs.length,
      child: Scaffold(
        appBar: AppBar(
          bottom: TabBar(
            tabs: myTabs,
          ),
        ),
        body: TabBarView(
          children: myTabs.map((Tab tab) {
            final String label = tab.text.toLowerCase();
            return Center(
              child: Text(
                'This is the $label tab',
                style: const TextStyle(fontSize: 36),
              ),
            );
          }).toList(),
        ),
      ),
    );
  }
}

By doing so you won't need to have your TabView inside a State but you won't be using GetX either.

Answer 3

I came across the exact issue and it shows that the GetX framework is not ready for many possible scenarios and you should use it cautiously.

The solution I came up with is a wrapper widget. In the proposed example I used HookWidget but you are free to use Stateful widget as well.

class TabWidget extends HookWidget {
  final List<Widget> children;
  final RxInt currentTab;
  final int initialIndex;

  TabWidget({
    @required this.children,
    @required this.currentTab,
    @required this.initialIndex,
  });

  @override
  Widget build(BuildContext context) {
    final controller = useTabController(
      initialLength: children.length,
      initialIndex: initialIndex,
    );
    currentTab.listen((page) {
      controller.animateTo(page);
    });
    return TabBarView(controller: controller, children: children);
  }
}

As you can see there is a Rx variable which controls the state of the tabView. So you have to pass a RxInt from the outside and whenever the value of it changes, the tabView updates respectively.

class TestView extends GetView<TestController> {
  @override
  Widget build(BuildContext context) {
    final screen = 0.obs;
    return SafeArea(
      child: Scaffold(
        appBar: AppBar(
          actions: [
            IconButton(
              icon: Icon(Icons.forward),
              onPressed: () => screen.value = screen.value + 1,
            )
          ],
        ),
        body: TabWidget(
          initialIndex: 1,
          currentTab: screen,
          children: [
             child1,
             child2,
             child3,
             ...,
          ],
        ),
      ),
    );
  }

Right now we take care of the controller by the help of HookWidget. If you are using Stateful widget you have to dispose it correctly.