flutter 异常 : ScrollController attached to multiple scroll views

Question

我的 Flutter 应用程序在离开具有 ScrollController 以控制 NestedScrollView 的页面时抛出异常(ScrollController 附加到多个 ScrollView )而我没有确定我做错了什么。

我用下面的一个简单示例重新创建了异常。我可以从 FirstPage 导航到 SecondPage(并可选择返回)就好了，但是当我从 SecondPage 导航到 ThirdPage 抛出异常。 SecondPage 是包含我的自定义 CollapsingAppBarPage 小部件的页面，我认为它有问题。 CollapsingAppBarPage 在此示例中得到了简化，但在我的真实应用中，它会根据滚动位置更改组件的颜色/大小。在此示例中，当导航开始到 ThirdPage 时调用 _scrollController.offset 时会发生异常。另外，我知道 FirstPage 和 ThirdPage 可以是 Stateless 而不是 StatefulWidgets，但我想让它尽可能像我的应用程序。这是我的完整示例。

import 'package:flutter/material.dart';

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

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

class FirstPage extends StatefulWidget {
  @override
  _FirstPageState createState() => _FirstPageState();
}

class _FirstPageState extends State<FirstPage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("First Page"),
      ),
      body: Center(
        child: RaisedButton(
          child: Text("Navigate Next"),
          onPressed: () async {
            await Navigator.push(context,
                MaterialPageRoute(builder: (context) => SecondPage())
            );
          },
        ),
      ),
    );
  }
}

class SecondPage extends StatefulWidget {
  @override
  _SecondPageState createState() => _SecondPageState();
}

class _SecondPageState extends State<SecondPage> {
  @override
  Widget build(BuildContext context) {
    return Material(
      child: CollapsingAppBarPage(
        titleText: "Second Page",
        bodyCreator: (context) {
          return ListView(
            children: <Widget>[
              Center(
                child: RaisedButton(
                  child: Text("Navigate Next"),
                  onPressed: () async {
                    await Navigator.push(context,
                        MaterialPageRoute(builder: (context) => ThirdPage())
                    );
                  },
                ),
              )
            ],
          );
        },
      ),
    );
  }
}

class ThirdPage extends StatefulWidget {
  @override
  _ThirdPageState createState() => _ThirdPageState();
}

class _ThirdPageState extends State<ThirdPage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(
            title: Text("Third Page")
        ),
        body: Container()
    );
  }
}

typedef CollapsingAppBarBodyCreator = Widget Function(BuildContext context);

class CollapsingAppBarPage extends StatefulWidget {

  final String titleText;
  final CollapsingAppBarBodyCreator bodyCreator;

  CollapsingAppBarPage({
    Key key,
    this.titleText,
    @required this.bodyCreator,
  }) : super(key: key);

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

class _CollapsingAppBarPageState extends State<CollapsingAppBarPage> {

  static const _kExpandedHeight = 200.0;

  ScrollController _scrollController;

  @override
  void initState() {
    super.initState();
    _scrollController = ScrollController()
      ..addListener(() {
        setState(() {
          // force a refresh so the app bar can be updated
        });
      });
  }

  @override
  void dispose() {
    _scrollController.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return NestedScrollView(
      controller: _scrollController,
      headerSliverBuilder: _createSliverAppBar,
      body: widget.bodyCreator(context),
    );
  }

  List<Widget> _createSliverAppBar(BuildContext context, bool innerBoxIsScrolled) {
    // change the icon color as the page scrolls
    var collapsePercent = _getAppBarCollapsePercent();
    int rgb = ((1.0 - collapsePercent) * 255).round();
    var color = Color.fromARGB(255, rgb, rgb, rgb);

    return <Widget>[
      SliverAppBar(
        expandedHeight: _kExpandedHeight,
        pinned: true,
        iconTheme: IconThemeData(color: color),
        title: Text(widget.titleText),
      )
    ];
  }

  double _getAppBarCollapsePercent() {
    if (!_scrollController.hasClients)
      return 0.0;

    return (_scrollController.offset / (_kExpandedHeight - kToolbarHeight)).clamp(0.0, 1.0);
  }
}

这里是异常的截断跟踪:

flutter: ══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════
flutter: The following assertion was thrown building Builder(dirty):
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/src/widgets/scroll_controller.dart:111:12)
flutter: #3      ScrollController.offset (package:flutter/src/widgets/scroll_controller.dart:118:24)
flutter: #4      _CollapsingAppBarPageState._getAppBarCollapsePercent (package:flutter_scroll_test/main.dart:160:31)
flutter: #5      _CollapsingAppBarPageState._createSliverAppBar (package:flutter_scroll_test/main.dart:142:27)
flutter: #6      NestedScrollView._buildSlivers (package:flutter/src/widgets/nested_scroll_view.dart:271:20)
flutter: #7      _NestedScrollViewState.build.<anonymous closure> (package:flutter/src/widgets/nested_scroll_view.dart:347:29)
flutter: #8      Builder.build (package:flutter/src/widgets/basic.dart:5736:41)
flutter: #9      StatelessElement.build (package:flutter/src/widgets/framework.dart:3774:28)
flutter: #10     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3721:15)

Answer 1

如果在您的情况下，同一 View 上有多个 ListView，您也会遇到此错误。要解决它，您只需将以下属性添加到每个 listView 构建器中:

controller: ScrollController(),

之后你将拥有:

ListView.builder(
        controller: ScrollController(),//just add this line
        itemCount: items.length,
        itemBuilder: (context, index) {
          return ListTile(
            title: Text(items[index]),
          );

来源是这个链接:Click here