Question

我有一个应用程序，想要查看广告创建者的用户名，但是运行它时遇到了一些问题，我收到此错误：

A non-null String must be provided to a Text widget. 'package:flutter/src/widgets/text.dart': Failed assertion: line 298 pos 10: 'data != null'

The relevant error-causing widget was:    AdView file:///Users/ahmedhussain/Downloads/khamsat/Client%20Apps/HPX-KSA/hpx_ksa/lib/Screens/custom_ad_tile.dart:23:79 When the exception was thrown, this was the stack: 
#2      new Text (package:flutter/src/widgets/text.dart:298:10)
#3      _AdViewState.build (package:hpxksa/Screens/ad_view.dart:134:34)

尽管我检查了数据库，并且文档中已经有数据，但是如果我将值更改为文档中的任何其他值，它将正常工作。

这是我的代码：

class AdView extends StatefulWidget {

  final AdModel adModel;
  final String submittedStr;

  AdView({this.adModel, this.submittedStr});

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

return StreamProvider<List<AdModel>>.value(
      value: DatabaseService().ads,
        child: Scaffold(
          body: Column(
            children: <Widget>[
              Row(
                      children: <Widget>[
                        InkWell(
                          child: Text(widget.adModel.username  , style: TextStyle(color: kPrimaryColor),),
                          onTap: (){},
                        ),
                        InkWell(
                          child: Text(widget.adModel.location, style: TextStyle(color: kPrimaryColor),),
                          onTap: (){},
                        ),

这里的AdModel类：

class AdModel{
  String category;
  String location;
  String adName;
  String adImage;
  String userId;
  String username;

  AdModel({
    this.category,
    this.location,
    this.adName,
    this.adImage,
    this.userId,
    this.username,
});

这是我如何通过AdView传递AdModel：

class CustomAdTile extends StatefulWidget {
  final AdModel adModel;

  CustomAdTile({this.adModel});

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

class _CustomAdTileState extends State<CustomAdTile> {
  @override
  Widget build(BuildContext context) {
    return Padding(
      padding: const EdgeInsets.symmetric(vertical: 5.0),
      child: InkWell(
        onTap: (){
          print('card pressed');
          Navigator.push(context, new MaterialPageRoute(builder: (context) => AdView(adModel: widget.adModel,)));
    },

在此处创建AdModel并在提供者中使用的AdList：

  class _AdsListState extends State<AdsList> {
  @override
  Widget build(BuildContext context) {

    final ads = Provider.of<List<AdModel>>(context);

    return ListView.builder(
        scrollDirection: Axis.vertical,
        itemCount:(ads == null) ? 0 :  ads.length,
        itemBuilder: (context, index){
          return
            CustomAdTile(adModel: ads[index],);
        });
  }
}

Answer 1

您可以打印widget.adModel.username来查看它是否真的null，并且还应该像这样null check进行widget?.adModel?.username ?? 'value is null'

return StreamProvider<List<AdModel>>.value(
      value: DatabaseService().ads,
        child: Scaffold(
          body: Column(
            children: <Widget>[
              Row(
                      children: <Widget>[
                        InkWell(
                          child: Text(widget?.adModel?.username ?? 'username is null' , style: TextStyle(color: kPrimaryColor),),
                          onTap: (){},
                        ),
                        InkWell(
                          child: Text(widget?.adModel?.location ?? 'location is null', style: TextStyle(color: kPrimaryColor),),
                          onTap: (){},
                        ),

Answer 2

widget.adModel没有为您提供更新的值，您获取的变量的默认值为null

Text(widget.adModel.username?? ""  , style: TextStyle(color: kPrimaryColor),),

Text(widget.adModel.location?? "", style: TextStyle(color: kPrimaryColor),),

Answer 3

这是因为因为widget.adModel.username为null。您可以通过执行此操作来解决该错误

Text('${widget.adModel.username}' ?? 'Username not available' , style: TextStyle(color: kPrimaryColor),

Text('${widget.adModel.location}' ?? 'Location not available', style: TextStyle(color: kPrimaryColor),

??这基本上意味着如果第一个为null，则使用第二个字符串

Flutter：必须将非空字符串提供给“文本”小部件

3 个答案: