Question

看过类似的问题，看不到任何常见的错误。一旦工厂似乎可以毫无问题地创建对象。但是，调用任何方法都会生成NoSuchMethodError。调试了好几天，没了主意。可以使用具有一般布局的数据模型的类似代码，而不会出现任何问题。

这是数据模型的代码

class Performer {
  String avatar, header, name, username;
  int id, subscribePrice;
  bool isRealPerformer,
      isPerformer,
      hasStories,
      hasStream,
      isPaywallRestriction;

  Performer(
      {this.avatar,
      this.header,
      this.name,
      this.username,
      this.id,
      this.subscribePrice,
      this.isRealPerformer,
      this.isPerformer,
      this.hasStories,
      this.hasStream,
      this.isPaywallRestriction});

  factory Performer.fromJson(Map<String, dynamic> performer) {
    return Performer(
        avatar: performer["avatar"],
        header: performer["header"],
        name: performer["name"],
        username: performer["username"],
        id: performer["id"],
        subscribePrice: performer["subscribePrice"],
        isRealPerformer: performer["isRealPerformer"],
        isPerformer: performer["isPerformer"],
        hasStories: performer["hasStories"],
        hasStream: performer["hasStream"],
        isPaywallRestriction: performer["isPaywallRestriction"]);
  }
}

这是填充模型的代码

Future<List<Performer>> getSubscriptions() async {
    List<Performer> performers = [];

    String url = "some API url";

    String res = await _callServer(url);

    if (res.isNotEmpty) {
      List<dynamic> payload = json.decode(res);

      payload.forEach((element) {
        performers.add(new Performer.fromJson(element));
      });
      return performers;

    } else return performers;
  }

Future<Performer> getPerformer(int performerID) async {
    List<Performer> subs = await getSubscriptions();

    Performer performer;

    int prefIndex;

    for (int x = 0; x < subs.length; x++) {
      if (subs[x].id == performerID){
        performer = subs[x];
        break;
      }
    }

    if (performer.avatar != null) {
      print("found ${performer.username}");
      return performer;
    } else return null;
  }

这是基于模型生成UI元素的代码

class ProfilePic extends StatefulWidget {
  final int id;
  ProfilePic({Key key, @required this.id}) : super();

  @override
  State<StatefulWidget> createState() => _profilePicState();
}

class _profilePicState extends State<ProfilePic> {
  Performer performer;

  @override
  void initState() {
    // TODO: implement initState
    super.initState();

    Backend().getPerformer(widget.id).then((value) {
      performer = value;
      setState(() {});
    });

  }

  @override
  Widget build(BuildContext context) {
    print("profile for: ${widget.id}");
    return Container(
      child: performer == null ? Container() : CircleAvatar(
        radius: 30.0,
        backgroundImage:
        NetworkImage(performer.avatar),
        backgroundColor: Colors.transparent,
      ),
    );
  }

Answer 1

在您的getSubscriptions中，尝试

payload.forEach((element) {
        performers.add(new Performer.fromJson(Map.from(element)));
});

Answer 2

代码没什么问题，是将帖子ID而非用户ID返回给函数，因此返回空错误。

NoSuchMethodError：吸气剂“头像”在null上被调用

2 个答案: