问动态更改docId时不更新流

EN

class Controller extends GetxController {

  // onInit
  @override
  void onInit() {
    finalNewsModel.bindStream(streamDemo());
    super.onInit();
  }

 
  // list of document ids.
  List docIdList = [
    'USA',
    'New York',
    'Canada',
  ];

  //
  RxString mainDocId = 'USA'.obs;

  // method to change document id based on docId index.
  changeDocId(int index) {
    mainDocId(docIdList[index]);
  }

  //
  Rxn<List<NewsModel>> finalNewsModel = Rxn<List<NewsModel>>();

  //
  List<NewsModel> get newsModelList => finalNewsModel.value;

  //
  Stream<List<NewsModel>> streamDemo() {
    return FirebaseFirestore.instance
        .collection('news')
        .doc('$mainDocId')
        .snapshots()
        .map((ds) {
      var mapData = ds.data();
      List mapList = mapData['list'];
      List<NewsModel> modelList = [];
      mapList.forEach((element) {
        modelList.add(NewsModel.fromMap(element));
      });
      return modelList;
    });
  }
}

class HomeBody extends StatefulWidget {
  @override
  _HomeBodyState createState() => _HomeBodyState();
}

class _HomeBodyState extends State<HomeBody> {
//
  final Controller _controller = Get.put<Controller>(Controller());

  @override
  Widget build(BuildContext context) {
    return Container(
      child: Obx(() {
        if (_controller.newsModelList == null) {
          return Center(
              child: Text(
            'Please try later!',
          ));
        } else if (_controller.newsModelList.isEmpty) {
          return Text('Empty List');
        } else {
          return ListView.builder(
            itemCount: _controller.newsModelList.length,
            itemBuilder: (context, index) {
              final NewsModel _newsModel = _controller.newsModelList[index];
              return MyContainer(
                title: _newsModel.title,
                titleImage: _newsModel.titleImage,
                index: index,
              );
            },
          );
        }
      }),
    );
  }
}

bottomNavigationBar: Container(
        color: Colors.grey[300],
        height: 60.0,
        child: Padding(
          padding: EdgeInsets.all(8.0),
          child: GetBuilder<Controller>(
            builder: (context) => ListView.builder(
              shrinkWrap: true,
              scrollDirection: Axis.horizontal,
              itemCount: _controller.docIdList.length,
              itemBuilder: (context, index) {
                return FavCategoryTags(
                  tagName: _controller.docIdList[index],
                  onpress: () =>_controller.changeDocId(index),
                );
              },
            ),
          ),
        ),
      ),

class NewsModel {
  String title, titleImage, brief, source;
  List aList;

  NewsModel({this.title, this.titleImage, this.brief, this.aList, this.source});

  factory NewsModel.fromMap(dynamic fieldData) {
    return NewsModel(
      title: fieldData['title'],
      titleImage: fieldData['titleImage'],
      brief: fieldData['brief'],
      aList: fieldData['mediaDescList'],
      source: fieldData['source'],
    );
  }
}