Question

运行应用程序时，HomeNewsPage第一次变成红页，控制台显示一条错误消息，大约1秒钟，HomeNewsPage恢复正常，我从HomeProvider那里获得了数据。我一直在寻找为什么找不到原因的原因，请帮我看看代码，我的英语不是很好，非常感谢，

还有

HomeProvider provider = Provider.of（上下文，听：false ）；

设置listen = true _HomeNewsPageBuildState＃ build（）方法将执行两次

设置listen = true _HomeNewsPageBuildState＃ build（）方法将执行一次

我不知道为什么

这是页面

class HomeNewsPage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    HomeProvider provider = Provider.of<HomeProvider>(context, listen: false);
    return _HomeNewsPageBuild(provider);
  }
}

class _HomeNewsPageBuild extends StatefulWidget {
  final HomeProvider provider;
  _HomeNewsPageBuild(this.provider);
  @override
  _HomeNewsPageBuildState createState() => _HomeNewsPageBuildState();
}


class _HomeNewsPageBuildState extends State<_HomeNewsPageBuild> {
  HomeProvider _provide;

  @override
  void initState() {
    print('HomeNewsPage initState');
    _provide ??= widget.provider;
    getHomeNewsList(_provide);
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    print("HomeNewsPage--build");
    return Scaffold(
      backgroundColor: UniteColors.bg_color,
      body: _initView(),
    );
  }

  Consumer<HomeProvider> _initView() {
    return Consumer<HomeProvider>(
      builder: (_, HomeProvider value, __) {
        return _provide.newsList.length > 0 ? _buildListView() : UiHelper.initLoading(false);
      },
    );
  }

 Widget _buildListView() {
    return ListView(
      children: <Widget>[
        _newsBigItem(),
        Divider(height: 1.0, color: UniteColors.divider_color),
        _newsSmallItem(),
      ],
    );
  }
}

这是家庭提供者

class HomeProvider extends BaseProvider {
  final HomeRepo _homeRepo = HomeRepo();

  List<DataList> _newsList;

  List<DataList> get newsList => _newsList;

  set newsList(List<DataList> value) {
    _newsList = value;
    notifyListeners();
  }

  ///GetNewsListData
  Observable getHomeNewsList() => _homeRepo
      .getHomeNewsList()
      .doOnData((result) {
        var data = json.decode(result.toString());
        HomeEntity homeEntity = EntityFactory.generateOBJ(data);
        this.newsList = homeEntity.dataList;
      })
      .doOnError((e) {
        print(e.toString());
      })
      .doOnListen(() =>{})
      .doOnDone(() =>{});
}

这是错误消息

 I/flutter: HomeNewsPage initState
 I/flutter: doOnListen
 I/flutter: HomeNewsPage--build
 I/flutter: ══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════
 I/flutter: The following NoSuchMethodError was thrown building Consumer<HomeProvider> 
 (dirty, dependencies:
 I/flutter: [InheritedProvider<HomeProvider>]):
 I/flutter: The getter 'length' was called on null.
 I/flutter: Receiver: null
 I/flutter: Tried calling: length
 I/flutter: When the exception was thrown, this was the stack:

 I/flutter: #0      Object.noSuchMethod (dart:core-patch/object_patch.dart:50:5)
 I/flutter: #1      _HomeNewsPageBuildState._initView.<anonymous closure> (package:pear_video/app/pages/home/home_news_page.dart:48:34)
 I/flutter: #2      Consumer.build (package:provider/src/consumer.dart:47:12)
 I/flutter: #3      StatelessElement.build (package:flutter/src/widgets/framework.dart:3885:28)
 I/flutter: #4      ComponentElement.performRebuild 
      (package:flutter/src/widgets/framework.dart:3835:15)
 #5..............
 #6................
......

Answer 1

最初，newsList的值为null，即当您收到错误消息时，因为null没有名为length的属性，因此发生了The getter 'length' was called on null.错误。 / p>

在调用getHomeNewsList（可能正在调用一个api）之后，这也需要一些时间，之后它会为List分配一个_newsList值。

现在，在newList返回List的情况下重新渲染您的UI，_newsList的属性为length。

一种不会出现初始错误的方法是用空的List初始化List<DataList> _newsList = [];。

info1=x;info2=y;.....;info1=.;info2=.;.....

遇到rrousselGit /提供商的问题

这是页面

这是家庭提供者

这是错误消息

1 个答案: