Flutter：必须向 Text 小部件提供非空字符串

Question

我有一个应用程序，想查看广告创建者的用户名，但是我在运行它时遇到了一点问题，我收到了这个错误：

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)

虽然我检查了数据库并且文档中已经存在数据，如果我将值更改为文档中的任何其他值，它可以正常工作。

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: (){},
                        ),

这里是 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,
});

这是我通过 AdView 传递 AdModel 的方式：

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,)));
    },

这里是在 Provider 中创建和使用 AdModel 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

你可以打印widget.adModel.username来看看它是否真的是null，你也应该这样做null check像这样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，您将变量的默认值设为 null

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

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

Answer 3

这是因为 widget.adModel.username 为空。你可以通过这样做来解决这个错误

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空，则使用第二个字符串