在不覆盖旧项目的情况下获取更多项目来列表视图

Question

我想使用Hacker News API对我的应用程序进行分页，我希望在向下滚动并将新项目添加到首页中的旧项目时加载更多数据。

这就是项目页面的代码，我使用一个bloc模式我wend变量<代码> size to the the the event ant the bloc中的事件i在第一个加载中仅获取6个项目，当我向下滚动时，将6个项目添加到我的循环...

    class _TopPageState extends State<TopPage> {
  @override
  void initState() {
    _scrollController.addListener(() {
      if (_scrollController.position.pixels ==
          _scrollController.position.maxScrollExtent) {
        size = size + 6;
        context.read<ItemBloc>().add(GetTopEvent(size));
      }
    });
    super.initState();
  }

  @override
  void dispose() {
    super.dispose();
    _scrollController.dispose();
  }

  ScrollController _scrollController = new ScrollController();
  int size = 6;
  bool isLoading = false;`enter code here`



@override
Widget build(BuildContext context) {


  return Scaffold(
    body: Center(
        child: BlocBuilder<ItemBloc, ItemState>(
          builder: (context, state) {

            if (state.topState == RequestState.LOADING) {
              return const CircularProgressIndicator();
            } else if (state.topState == RequestState.ERROR) {
              print(state.errorMsg);
              return Container();
            }else if (state.topState == RequestState.NONE) {
              context.read<ItemBloc>().add(GetTopEvent(size));
              return Container();
            }
            else if (state.topState == RequestState.LOADED) {
              return NotificationListener<ScrollNotification>(
                onNotification: (notification) {
                  if (notification is ScrollUpdateNotification) {
                    _getMoreData(notification.metrics);
                  }
                  return true;
                },
                child: ListView.builder(
                    controller: _scrollController,
                    padding: const EdgeInsets.symmetric(vertical: 8.0),
                    itemCount: state.reposTop.length,
                    itemBuilder: (context, index) {
                      return GestureDetector(
                          child: Item(state.reposTop[index]),
                          onTap: () {
                            if (state.reposTop[index].url != null) {
                              Navigator.push(
                                context,
                                MaterialPageRoute(
                                    builder: (context) =>
                                        WebViewPage(state.reposTop[index])),
                              );

                            } else if (state.reposTop[index].text != null) {
                              Navigator.push(
                                context,
                                MaterialPageRoute(
                                    builder: (context) =>
                                        ItemDetails(state.reposTop[index])),
                              );
                            }
                          });
                    }),
              );

            } else {
              return Container();
            }
          },
        ),
      ),

  );
}



void _getMoreData(ScrollMetrics metrics) {
    if (_scrollController.position.pixels ==
        _scrollController.position.minScrollExtent) {
      //&& size != 5

      if( size - 6 <=0){
        context.read<ItemBloc>().add(GetTopEvent(size));
      }
      else{
        size = size - 6;
      }
      context.read<ItemBloc>().add(GetTopEvent(size));
    }
  }
}