Flutter 异常：附加到多个滚动视图的 ScrollController

Question

我的 Flutter 应用程序在离开具有 ScrollController 控制 NestedScrollView 的页面时抛出异常（附加到多个滚动视图的 ScrollController），我不确定我是什么做错了。

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 时，会抛出异常。 SecondPage 是包含我的自定义 CollapsingAppBarPage 小部件的页面，我认为它有问题。 CollapsingAppBarPage 在此示例中得到简化，但在我的真实应用程序中，它会根据滚动位置更改 colors/sizes 组件。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

您需要在 _getAppBarCollapsePercent() 中添加另一个条件为

 double _getAppBarCollapsePercent() {
    if (!_scrollController.hasClients || _scrollController.positions.length > 1)
      return 0.0;

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

controller 如果控制多个 scrollable，则不能给出 offset。

Answer 2

不要直接使用_scrollController.offset，因为推送路由显然附加了一个滚动位置，当你再次弹出时接近滚动时要检索

所以我们需要把它转成一个状态

这是修改后的完整代码。

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;
  //Offset state <-------------------------------------
  double offset = 0.0 ;



  @override
  void initState() {
    super.initState();
    //print("init state is called");

    _scrollController = ScrollController() //keepScrollOffset: false removed
      ..addListener(() {
        setState(() {
          //<-----------------------------
          offset = _scrollController.offset;
          // 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
    print("_createSliverAppBar is called");
    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 ){
      print("positions is ${_scrollController.positions.length}");
      return 0.0;

    }


    //print("offset is${_scrollController.offset} and positions is ${_scrollController.positions.length}");
    return (offset / (_kExpandedHeight - kToolbarHeight)).clamp(0.0, 1.0);
  }
}

Answer 3

问题是 _scrollController.position 或 _scrollController.offset 无论您在哪里调用 _scrollController.position 都将其替换为 _scrollController.positions.last，无论您在哪里调用 _scrollController.offset将其替换为 _scrollController.positions.last.pixels 你不会得到任何错误。

实际上出现问题是因为它在 _scrollController 的 positions 列表中添加了新元素，我检查了 scroll_controller.dart _scrollController.position returns scrollcontroller.positions.single 和 _scrollController.offset returns _scrollController.position.pixels 这就是我们面临这个问题的原因。

但是，如果您将使用 _scrollController.positions.last，它将 return 最后保存的位置元素，因此您的应用程序将运行顺利并且不会抛出任何错误。

Answer 4

如果在您的情况下，同一视图上有多个 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link：Click here

Flutter 异常：附加到多个滚动视图的 ScrollController

Flutter Exception: ScrollController attached to multiple scroll views

flutter

flutter-sliver