Dart 如何在多个视图中同时使用PrimaryScrollController

我正在使用

PrimaryScrollController

在一个定制的有状态小部件中加载项目，该小部件基本上是一个

ListView

，带有代码，可以在用户滚动时从服务器加载项目。我需要

PrimaryScrollController

，这样在iOS上点击应用程序栏的顶部就会返回顶部

我在多个页面中使用相同的小部件，这些页面由

BottomNavigationBar

控制。我还试图保持状态，这样当用户滚动，然后切换到底部导航栏中的另一个选项卡，然后返回到第一个选项卡时，滚动位置和从服务器加载的所有项目仍然存在

为了保持页面的状态，我使用了

索引堆栈

。但是，由于两个页面同时使用

primarycrollcontroller

，我在控制台中收到了一长串相同的错误消息：

flatter:引发了另一个异常：ScrollController连接到多个滚动视图。

我理解这是在告诉我不应该在多个视图中使用相同的

ScrollController

。简单的解决方案是为每个视图创建一个新实例。但是，我必须使用相同的

ScrollController

，因为我需要

PrimaryScrollController

才能在iOS上点击应用程序栏

如果我只是忽略错误，实际的代码按照我希望的方式工作，每个页面都保留自己的状态。 事实上，它没有按照我想要的方式工作。当听众在sperate页面上时，他们不工作

我创建了一个示例应用程序来显示问题：

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

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,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

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

List<Widget> _pages = [
  BodyWidget(key: PageStorageKey(1)),
  BodyWidget(key: PageStorageKey(2)),
];

class _MyHomePageState extends State<MyHomePage> {
  int _currentPage = 0;
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: IndexedStack(
        index: _currentPage,
        children: _pages,
      ),
      bottomNavigationBar: BottomNavigationBar(
          currentIndex: _currentPage,
          onTap: (int index) {
            setState(() {
              _currentPage = index;
            });
          },
          items: [
            BottomNavigationBarItem(
                icon: Icon(Icons.star_border), title: Text('page 1')),
            BottomNavigationBarItem(
                icon: Icon(Icons.crop_square), title: Text('page 2'))
          ]),
    );
  }
}

class BodyWidget extends StatefulWidget {
  BodyWidget({Key key}) : super(key: key);

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

class _BodyWidgetState extends State<BodyWidget> {
  ScrollController _scrollController;
  List<int> numbers = [];

  void dataGenerator() {
    // simulate loading items from server
    var rng = new Random();
    for (var i = 0; i < 100; i++) {
      if (this.mounted) {
        setState(() {
          numbers.add(rng.nextInt(100));
        });
      }
    }
    print('loaded more items');
  }

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

    // This delay is required to get the build context
    Future.delayed(
      Duration.zero,
      () {
        _scrollController = PrimaryScrollController.of(context);
        _scrollController.addListener(() {
          if (_scrollController.position.pixels >=
              (_scrollController.position.maxScrollExtent - 50)) {
            dataGenerator();
          }
        });
        dataGenerator();
      },
    );
  }

  @override
  Widget build(BuildContext context) {
    return ListView.builder(
      itemCount: numbers.length,
      controller: _scrollController,
      itemBuilder: (BuildContext context, int index) {
        return Container(
          child: Card(
            child: Column(
              children: <Widget>[
                SizedBox(
                  height: 15,
                ),
                Row(
                  mainAxisAlignment: MainAxisAlignment.spaceEvenly,
                  children: <Widget>[
                    Text('Random number: ' + numbers[index].toString()),
                    Text('Index: ' + index.toString())
                  ],
                ),
                SizedBox(
                  height: 15,
                ),
              ],
            ),
          ),
        );
      },
    );
  }
}

导入“包装：颤振/材料.省道”；
导入“dart:math”；
void main（）=>runApp（MyApp（））；
类MyApp扩展了无状态小部件{
//此小部件是应用程序的根。
@凌驾
小部件构建（构建上下文）{
返回材料PP(
标题：“颤振演示”，
主题：主题数据(
主样本：颜色。蓝色，
),
主页：MyHomePage（标题：“颤振演示主页”），
);
}
}
类MyHomePage扩展StatefulWidget{
MyHomePage（{Key，this.title}）：超级（Key:Key）；
最后的字符串标题；
@凌驾
_MyHomePageState createState（）=>\u MyHomePageState（）；
}
列表页面=[
BodyWidget（键：PageStorageKey（1）），
BodyWidget（键：PageStorageKey（2）），
];
类_MyHomePageState扩展状态{
int _currentPage=0；
@凌驾
小部件构建（构建上下文）{
返回脚手架(
appBar:appBar(
标题：文本（widget.title），
),
正文：IndexedStack(
索引：_当前页面，
儿童：_页，
),
底部导航栏：底部导航栏(
currentIndex:_currentPage，
onTap：（int索引）{
设置状态（）{
_currentPage=索引；
});
},
项目：[
底部导航气压计(
图标：图标（图标。星形边框），标题：文本（“第1页”），
底部导航气压计(
图标：图标（Icons.crop_square），标题：文本（第2页）
]),
);
}
}
类BodyWidget扩展了StatefulWidget{
BodyWidget（{Key}）：超级（Key:Key）；
@凌驾
_BodyWidgetState createState（）=>\u BodyWidgetState（）；
}
类_BodyWidgetState扩展了状态{
ScrollController\u ScrollController；
列表编号=[]；
void数据生成器（）{
//模拟从服务器加载项目
var rng=新随机数（）；
对于（变量i=0；i<100；i++）{
如果（本安装）{
设置状态（）{
编号。添加（rng.nextInt（100））；
});
}
}
打印（“加载了更多项目”）；
}
@凌驾
void initState（）{
super.initState（）；
//获取生成上下文需要此延迟
推迟(
持续时间0.0，
() {
_scrollController=PrimaryScrollController.of（上下文）；
_scrollController.addListener（（）{
如果（_scrollController.position.pixels>=
（_scrollController.position.maxScrollExtent-50））{
数据生成器（）；
}
});
数据生成器（）；
},
);
}
@凌驾
小部件构建（构建上下文）{
返回ListView.builder(
itemCount:number.length，
控制器：\ u滚动控制器，
itemBuilder:（构建上下文，int索引）{
返回容器(
孩子：卡片(
子：列(
儿童：[
大小盒子(
身高：15，
),
划船(
mainAxisAlignment:mainAxisAlignment.space，
儿童：[
Text（'随机数：'+数字[index].toString（）），
Text（'Index:'+Index.toString（））
],
),
大小盒子(
身高：15，
),
],
),
),
);
},
);
}
}

这是引发的错误：

flutter: ══╡ EXCEPTION CAUGHT BY FOUNDATION LIBRARY ╞════════════════════════════════════════════════════════
flutter: The following assertion was thrown while dispatching notifications for ScrollController:
flutter: ScrollController attached to multiple scroll views.
flutter: 'package:flutter/src/widgets/scroll_controller.dart': Failed assertion: line 111 pos 12:
flutter: '_positions.length == 1'
flutter:
flutter: Either the assertion indicates an error in the framework itself, or we should provide substantially
flutter: more information in this error message to help you determine and fix the underlying cause.
flutter: In either case, please report this assertion by filing a bug on GitHub:
flutter:   https://github.com/flutter/flutter/issues/new?template=BUG.md
flutter:
flutter: When the exception was thrown, this was the stack:
flutter: #2      ScrollController.position 
package:flutter/…/widgets/scroll_controller.dart:111
flutter: #3      _BodyWidgetState.initState.<anonymous closure>.<anonymous closure>
package:primaryscrollcontroller_test/main.dart:97
flutter: #4      ChangeNotifier.notifyListeners 
package:flutter/…/foundation/change_notifier.dart:206
flutter: #5      ChangeNotifier.notifyListeners 
package:flutter/…/foundation/change_notifier.dart:206
flutter: #6      ScrollPosition.notifyListeners 
package:flutter/…/widgets/scroll_position.dart:696
flutter: #7      ScrollPosition.setPixels 
package:flutter/…/widgets/scroll_position.dart:218
flutter: #8      ScrollPositionWithSingleContext.setPixels 
package:flutter/…/widgets/scroll_position_with_single_context.dart:84
flutter: #9      ScrollPositionWithSingleContext.applyUserOffset 
package:flutter/…/widgets/scroll_position_with_single_context.dart:127
flutter: #10     ScrollDragController.update 
package:flutter/…/widgets/scroll_activity.dart:372
flutter: #11     ScrollableState._handleDragUpdate 
package:flutter/…/widgets/scrollable.dart:496
flutter: #12     DragGestureRecognizer.handleEvent.<anonymous closure> 
flutter: #13     GestureRecognizer.invokeCallback 
package:flutter/…/gestures/recognizer.dart:166
flutter: #14     DragGestureRecognizer.handleEvent 
package:flutter/…/gestures/monodrag.dart:182
flutter: #15     PointerRouter._dispatch 
package:flutter/…/gestures/pointer_router.dart:73
flutter: #16     PointerRouter.route 
package:flutter/…/gestures/pointer_router.dart:101
flutter: #17     _WidgetsFlutterBinding&BindingBase&GestureBinding.handleEvent 
package:flutter/…/gestures/binding.dart:221
flutter: #18     _WidgetsFlutterBinding&BindingBase&GestureBinding.dispatchEvent 
package:flutter/…/gestures/binding.dart:199
flutter: #19     _WidgetsFlutterBinding&BindingBase&GestureBinding._handlePointerEvent 
package:flutter/…/gestures/binding.dart:156
flutter: #20     _WidgetsFlutterBinding&BindingBase&GestureBinding._flushPointerEventQueue 
package:flutter/…/gestures/binding.dart:102
flutter: #21     _WidgetsFlutterBinding&BindingBase&GestureBinding._handlePointerDataPacket 
package:flutter/…/gestures/binding.dart:86
flutter: #25     _invoke1  (dart:ui/hooks.dart:233:10)
flutter: #26     _dispatchPointerDataPacket  (dart:ui/hooks.dart:154:5)
flutter: (elided 5 frames from class _AssertionError and package dart:async)
flutter:
flutter: The ScrollController sending notification was:
flutter:   ScrollController#be798(2 clients)
flutter: ════════════════════════════════════════════════════════════════════════════════════════════════════
flutter: Another exception was thrown: ScrollController attached to multiple scroll views.
flutter: Another exception was thrown: ScrollController attached to multiple scroll views.
flutter: Another exception was thrown: ScrollController attached to multiple scroll views.
flutter: Another exception was thrown: ScrollController attached to multiple scroll views.

颤振：══╡ 基金会例外╞════════════════════════════════════════════════════════
颤振：为ScrollController发送通知时引发了以下断言：
颤振：连接到多个滚动视图的滚动控制器。
颤振：“包：颤振/src/widgets/scroll\u controller.dart”：失败的断言：第111行位置12:
颤振：'_positions.length==1'
颤振：
颤振：要么断言表明框架本身存在错误，要么我们应该提供
颤振：此错误消息中的更多信息可帮助您确定并修复根本原因。
颤振：无论哪种情况，请在GitHub上提交一个bug来报告这一断言：
颤振：https://github.com/flutter/flutter/issues/new?template=BUG.md
颤振：
颤振：当异常w
ScrollController scrollController;

final scrollPositions = scrollController.positions;

final scrollPositionImInterested = scrollPositions
        .elementAt(scrollController.positions.length - 1);