未处理的异常 ScrollController 未附加到任何滚动视图

Question

我总是收到这个错误，不明白为什么会这样。该应用程序启动非常好，但是当我尝试登录时它卡住并抛出此错误。谁能告诉我我做错了什么。

E/flutter ( 3070): [ERROR:flutter/lib/ui/ui_dart_state.cc(157)] Unhandled Exception: 'package:flutter/src/widgets/scroll_controller.dart': Failed assertion: line 110 pos 12: '_positions.isNotEmpty': ScrollController not attached to any scroll views.
[38;5;244mE/flutter ( 3070): #0      _AssertionError._doThrowNew  (dart:core-patch/errors_patch.dart:42:39)[39;49m
[38;5;244mE/flutter ( 3070): #1      _AssertionError._throwNew  (dart:core-patch/errors_patch.dart:38:5)[39;49m
[38;5;244mE/flutter ( 3070): #2      ScrollController.position[39;49m
[38;5;244mE/flutter ( 3070): #3      PageController.animateToPage[39;49m
[38;5;248mE/flutter ( 3070): #4      _PreviewMobilePortraitState.initState.<anonymous closure>[39;49m
[38;5;244mE/flutter ( 3070): #5      _rootRunUnary  (dart:async/zone.dart:1134:38)[39;49m
[38;5;244mE/flutter ( 3070): #6      _CustomZone.runUnary  (dart:async/zone.dart:1031:19)[39;49m
[38;5;244mE/flutter ( 3070): #7      _CustomZone.runUnaryGuarded  (dart:async/zone.dart:933:7)[39;49m
[38;5;244mE/flutter ( 3070): #8      _CustomZone.bindUnaryCallbackGuarded.<anonymous closure>  (dart:async/zone.dart:970:26)[39;49m
[38;5;244mE/flutter ( 3070): #9      _rootRunUnary  (dart:async/zone.dart:1138:13)[39;49m
[38;5;244mE/flutter ( 3070): #10     _CustomZone.runUnary  (dart:async/zone.dart:1031:19)[39;49m
[38;5;244mE/flutter ( 3070): #11     _CustomZone.bindUnaryCallback.<anonymous closure>  (dart:async/zone.dart:954:26)[39;49m
[38;5;244mE/flutter ( 3070): #12     _Timer._runTimers  (dart:isolate-patch/timer_impl.dart:384:19)[39;49m
[38;5;244mE/flutter ( 3070): #13     _Timer._handleMessage  (dart:isolate-patch/timer_impl.dart:418:5)[39;49m
[38;5;244mE/flutter ( 3070): #14     _RawReceivePortImpl._handleMessage  (dart:isolate-patch/isolate_patch.dart:174:12)[39;49m

我在 PreviewMobilePortraitState 中的代码如下所示：

@override
  void initState() {
    super.initState();
    //cycle through pages
    Timer.periodic(Duration(seconds: 4), (Timer timer) {
      if (_currentPage < 3) {
        _currentPage++;
      } else {
        _currentPage = 0;
      }

      _pageController.animateToPage(
        _currentPage,
        duration: Duration(milliseconds: 400),
        curve: Curves.easeInCubic,
      );
    });
  }

Answer 1

当您调用_pageController.animateToPage 时，PageView 尚未准备好
您可以使用addPostFrameCallback 并检查_pageController.hasClients

代码sn-p

@override
void initState() {
    super.initState();
    WidgetsBinding.instance.addPostFrameCallback((_) {
      Timer.periodic(Duration(seconds: 4), (Timer timer) {
        if (_currentPage < 3) {
          _currentPage++;
        } else {
          _currentPage = 0;
        }

        if(_pageController.hasClients) {
          _pageController.animateToPage(
            _currentPage,
            duration: Duration(milliseconds: 400),
            curve: Curves.easeInCubic,
          );
        }

      });
    });
}

模拟这种情况的工作演示

完整的测试代码

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

class Feature {
  String name;
  Color color;
  String header;
  String subHeader;
  String imgUrl;

  Feature({this.name, this.color, this.header, this.subHeader, this.imgUrl});
}

List<Feature> featuredList = [
  Feature(
      name: "a",
      color: Colors.pink,
      header: "ah",
      subHeader: "a sub",
      imgUrl: "https://picsum.photos/250?image=9"),
  Feature(
      name: "b",
      color: Colors.blue,
      header: "bh",
      subHeader: "b sub",
      imgUrl: "https://picsum.photos/250?image=10"),
  Feature(
      name: "c",
      color: Colors.yellow,
      header: "ch",
      subHeader: "c sub",
      imgUrl: "https://picsum.photos/250?image=11")
];

class WidgetFeatured extends StatefulWidget {
  @override
  _WidgetFeaturedState createState() => _WidgetFeaturedState();
}

class _WidgetFeaturedState extends State<WidgetFeatured> {
  int _currentPage = 0;

  final PageController _pageController = PageController(
    initialPage: 0,
  );

  @override
  void initState() {
    super.initState();
    WidgetsBinding.instance.addPostFrameCallback((_) {
      Timer.periodic(Duration(seconds: 4), (Timer timer) {
        if (_currentPage < 3) {
          _currentPage++;
        } else {
          _currentPage = 0;
        }

        if(_pageController.hasClients) {
          _pageController.animateToPage(
            _currentPage,
            duration: Duration(milliseconds: 400),
            curve: Curves.easeInCubic,
          );
        }

      });
    });
  }

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

  _onPageChanged(int index) {
    setState(() {
      _currentPage = index;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Container(
      height: 200,
      child: PageView.builder(
        physics: BouncingScrollPhysics(),
        scrollDirection: Axis.horizontal,
        controller: _pageController,
        onPageChanged: _onPageChanged,
        itemBuilder: (context, index) => WidgetFeaturedItem(index),
        itemCount: featuredList.length,
      ),
    );
  }
}

class WidgetFeaturedItem extends StatelessWidget {
  final int indexItem;

  WidgetFeaturedItem(this.indexItem);

  @override
  Widget build(BuildContext context) {
    return Container(
      padding: const EdgeInsets.all(20),
      color: featuredList[indexItem].color,
      width: double.infinity,
      height: 180,
      child: Stack(
        children: <Widget>[
          Row(
            children: <Widget>[
              Column(
                mainAxisAlignment: MainAxisAlignment.center,
                children: <Widget>[
                  Text(
                    featuredList[indexItem].header,
                    style: TextStyle(
                      fontSize: 20,
                      fontWeight: FontWeight.bold,
                      color: Colors.white,
                    ),
                  ),
                  Text(
                    featuredList[indexItem].subHeader,
                    style: TextStyle(
                      fontSize: 16,
                      fontWeight: FontWeight.bold,
                      color: Colors.white,
                    ),
                  ),
                  FlatButton(
                    onPressed: () {},
                    padding: const EdgeInsets.symmetric(
                      horizontal: 15,
                      vertical: 3,
                    ),
                    color: Theme.of(context).primaryColor,
                    child: Text(
                      'Order now',
                      style: TextStyle(
                        color: Colors.white,
                        fontSize: 12,
                        fontWeight: FontWeight.w100,
                      ),
                    ),
                  ),
                ],
              ),
              Image.network(
                featuredList[indexItem].imgUrl,
                height: 120,
                width: 120,
                //width: 335,
                fit: BoxFit.cover,
              ),
            ],
          )
        ],
      ),
    );
  }
}

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      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();
}

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;

  void _incrementCounter() {
    setState(() {
      _counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Expanded(child: WidgetFeatured()),
            Text(
              'You have pushed the button this many times:',
            ),
            Text(
              '$_counter',
              style: Theme.of(context).textTheme.headline4,
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ),
    );
  }
}