颤振中TabView的动态子级

Question

我正在尝试构建具有列表作为子项的选项卡式视图。

类别标签和列表内容都将从数据库中获取。

我正在从呼叫者页面传递标签，并成功地将它们作为列表传递。现在，我正在尝试加载列表，并且构建了一个小部件（myList），该小部件成功返回了Future ListView。

问题有两个：

1. 每次我向左或向右滑动时，列表都会自动重建，而我只希望构建一次
2. 如何使用我制作的使标签的子代实际反映标签的代码，并根据我拥有的类别进行动态加载？

现在我的代码是这样的：

import 'package:flutter/material.dart';
import 'package:flutter_app/ui/menu_category_list.dart';

// Each TabBarView contains a _Page and for each _Page there is a list
// of _CardData objects. Each _CardData object is displayed by a _CardItem.

List<Tab> Tabs(List<String> l){
  List<Tab> list;
  for (String c in l) {
    list.add(new Tab(text: c));
  }
  return list;
}



class TabsDemo extends StatelessWidget {

  const TabsDemo({ Key key , this.categorie}) : super(key: key);

  final List<Tab> categorie;

  @override
  Widget build(BuildContext ctxt) {
    return new MaterialApp(
      title: "Nice app",
      home: new DefaultTabController(
      length: 5,
      child: new Scaffold(
        appBar: new AppBar(
          title: new Text("Title"),
          bottom: new TabBar(
            tabs:
              categories,
              //new Tab(text: "First Tab"),
              //new Tab(text: "Second Tab"),

          ),

        ),
        body: new TabBarView(
            children: [
              new MenuCategoryList(),
              new MenuCategoryList(),
              new MenuCategoryList(),
              new MenuCategoryList(),
              new MenuCategoryList()
            ]
        )
      ),
    )
    );
  }
}

当前结果

提前谢谢

Answer 1

绝对真正的List<E>.generate最佳解决方案。

如果您需要修改数组，就会出现问题。它们包括在修改数组时您没有机会使用相同的控制器。

对于这种情况，您可以使用下一个自定义小部件：

import 'package:flutter/material.dart';

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

  class MyApp extends StatelessWidget {
    @override
    Widget build(BuildContext context) {
      return MaterialApp(
        title: 'Flutter Demo',
        home: MyHomePage(),
      );
    }
  }

  class MyHomePage extends StatefulWidget {
    @override
    _MyHomePageState createState() => _MyHomePageState();
  }

  class _MyHomePageState extends State<MyHomePage> {
    List<String> data = ['Page 0', 'Page 1', 'Page 2'];
    int initPosition = 1;

    @override
    Widget build(BuildContext context) {
      return Scaffold(
        body: SafeArea(
          child: CustomTabView(
            initPosition: initPosition,
            itemCount: data.length,
            tabBuilder: (context, index) => Tab(text: data[index]),
            pageBuilder: (context, index) => Center(child: Text(data[index])),
            onPositionChange: (index){
              print('current position: $index');
              initPosition = index;
            },
            onScroll: (position) => print('$position'),
          ),
        ),
        floatingActionButton: FloatingActionButton(
          onPressed: () {
            setState(() {
              data.add('Page ${data.length}');
            });
          },
          child: Icon(Icons.add),
        ),
      );
    }
  }

  /// Implementation

  class CustomTabView extends StatefulWidget {
    final int itemCount;
    final IndexedWidgetBuilder tabBuilder;
    final IndexedWidgetBuilder pageBuilder;
    final Widget stub;
    final ValueChanged<int> onPositionChange;
    final ValueChanged<double> onScroll;
    final int initPosition;

    CustomTabView({
      @required this.itemCount,
      @required this.tabBuilder,
      @required this.pageBuilder,
      this.stub,
      this.onPositionChange,
      this.onScroll,
      this.initPosition,
    });

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

  class _CustomTabsState extends State<CustomTabView> with TickerProviderStateMixin {
    TabController controller;
    int _currentCount;
    int _currentPosition;

    @override
    void initState() {
      _currentPosition = widget.initPosition ?? 0;
      controller = TabController(
        length: widget.itemCount,
        vsync: this,
        initialIndex: _currentPosition,
      );
      controller.addListener(onPositionChange);
      controller.animation.addListener(onScroll);
      _currentCount = widget.itemCount;
      super.initState();
    }

    @override
    void didUpdateWidget(CustomTabView oldWidget) {
      if (_currentCount != widget.itemCount) {
        controller.animation.removeListener(onScroll);
        controller.removeListener(onPositionChange);
        controller.dispose();

        if (widget.initPosition != null) {
          _currentPosition = widget.initPosition;
        }

        if (_currentPosition > widget.itemCount - 1) {
            _currentPosition = widget.itemCount - 1;
            _currentPosition = _currentPosition < 0 ? 0 : 
            _currentPosition;
            if (widget.onPositionChange is ValueChanged<int>) {
               WidgetsBinding.instance.addPostFrameCallback((_){
                if(mounted) {
                  widget.onPositionChange(_currentPosition);
                }
               });
            }
         }

        _currentCount = widget.itemCount;
        setState(() {
          controller = TabController(
            length: widget.itemCount,
            vsync: this,
            initialIndex: _currentPosition,
          );
          controller.addListener(onPositionChange);
          controller.animation.addListener(onScroll);
        });
      } else if (widget.initPosition != null) {
        controller.animateTo(widget.initPosition);
      }

      super.didUpdateWidget(oldWidget);
    }

    @override
    void dispose() {
      controller.animation.removeListener(onScroll);
      controller.removeListener(onPositionChange);
      controller.dispose();
      super.dispose();
    }

    @override
    Widget build(BuildContext context) {
      if (widget.itemCount < 1) return widget.stub ?? Container();

      return Column(
        crossAxisAlignment: CrossAxisAlignment.stretch,
        children: <Widget>[
          Container(
            alignment: Alignment.center,
            child: TabBar(
              isScrollable: true,
              controller: controller,
              labelColor: Theme.of(context).primaryColor,
              unselectedLabelColor: Theme.of(context).hintColor,
              indicator: BoxDecoration(
                border: Border(
                  bottom: BorderSide(
                    color: Theme.of(context).primaryColor,
                    width: 2,
                  ),
                ),
              ),
              tabs: List.generate(
                widget.itemCount,
                    (index) => widget.tabBuilder(context, index),
              ),
            ),
          ),
          Expanded(
            child: TabBarView(
              controller: controller,
              children: List.generate(
                widget.itemCount,
                    (index) => widget.pageBuilder(context, index),
              ),
            ),
          ),
        ],
      );
    }

    onPositionChange() {
      if (!controller.indexIsChanging) {
        _currentPosition = controller.index;
        if (widget.onPositionChange is ValueChanged<int>) {
          widget.onPositionChange(_currentPosition);
        }
      }
    }

    onScroll() {
      if (widget.onScroll is ValueChanged<double>) {
        widget.onScroll(controller.animation.value);
      }
    }
  }

Answer 2

您可以使用它List<E>.generate来实现。

import 'package:flutter/material.dart';

假设您从呼叫者页面传递了一组类别。假设这是您的类别列表。

List<String> categories = ["a", "b", "c", "d", "e", "f", "g", "h"];

然后，您可以执行类似的操作以实现您的期望。

class TabsDemo extends StatefulWidget {
  @override
  _TabsDemoState createState() => _TabsDemoState();
}

class _TabsDemoState extends State<TabsDemo> {
  TabController _controller;

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

    @override
    Widget build(BuildContext ctxt) {
      return new MaterialApp(
        home: DefaultTabController(
            length: categories.length,
            child: new Scaffold(
              appBar: new AppBar(
                title: new Text("Title"),
                bottom: new TabBar(
                  isScrollable: true,
                    tabs: List<Widget>.generate(categories.length, (int index){
                  print(categories[0]);
                  return new Tab(icon: Icon(Icons.directions_car), text: "some random text");

                }),

              ),
            ),

        body: new TabBarView(
             children: List<Widget>.generate(categories.length, (int index){
                print(categories[0]);
                return new Text("again some random text");

             }),
          )
       ))
      );
  }

您还可以将不同组的小部件设置为“选项卡”的视图。您可以创建页面列表并遵循相同的方法。