ArrayList.size返回0但arryList不为空

Question

我正在尝试通过循环从ArrayList获取数据，但它显示列表为空并且arrayList.size()为0，即使我的adapter.getItemCount()也返回0但是我的arrayList不是空的它在至少2或3个数据，它完全放在RecyclerView中，但arrayList.size()仍为0

    arrayList = Backgroundtask.getList();

    adapter = new RecycleAdapter(arrayList,getContext());

    ItemRecyclerView.setHasFixedSize(true);
    layoutManager = new LinearLayoutManager(getActivity());
    ItemRecyclerView.setLayoutManager(layoutManager);
    ItemRecyclerView.setAdapter(adapter);

    for(int i = 0; i < arrayList.size(); i++){

        int quantity = arrayList.get(i).getQuantity();

        Total += quantity ;
    }
    Log.d("ArrayList Size    " , String.valueOf(arrayList.size()));
    Log.d("getItemCount Size " , String.valueOf(adapter.getItemCount()));

这是我的BackgroundTask：

public ArrayList<DataProvider> getList(){

    StringRequest stringRequest = new StringRequest(Request.Method.POST, jsonURL,

            new Response.Listener<String>() {
                @Override
                public void onResponse(String response) {

                    try {
                        JSONArray jsonArray = new JSONArray(response);

                        for(int i = 0; i < jsonArray.length(); i++){
                            JSONObject jsonObject = jsonArray.getJSONObject(i);
                            DataProvider dataProvider = new DataProvider(jsonObject.getString("Name"),
                                    jsonObject.getDouble("quantity"));
                            arrayList.add(dataProvider);
                        }
                        UserFragment.adapter.notifyDataSetChanged();

                    } catch (JSONException e) {
                        e.printStackTrace();
                    }

                }
            },new Response.ErrorListener(){
        @Override
        public void onErrorResponse(VolleyError volleyError){


        }
    }) {
        @Override
        protected Map<String, String> getParams() throws AuthFailureError {

            Map<String,String> param = new HashMap<String,String>();
            param.put("id",MainActivity.USER_ID.toString());
            return param;
        }
    };

    Singletone.getSingletone(context).addToRequest(stringRequest);
    return arrayList;
}

Answer 1

所以这是一个愚蠢的错误。您的请求和响应是异步的。那是你的getList()方法返回空数组，因为尚未收到响应。但由于您在回复中写了adapter.notifyDatasetChanged()，因此您可以在RecyclerView中看到这些项目。

编辑：

除了实现之外，还有几个内存泄漏。所以我在这里更新代码。

<强> BlankFragment.java

public class BlankFragment extends Fragment implements BackgroundTask.UpdateListInterface{

    private Context mContext;
    private List<DataProvider> arrayList;
    private RecyclerView ItemRecyclerView;
    private RecycleAdapter adapter;

    public BlankFragment() {
        // Required empty public constructor
    }

    @Override
    public void onAttach(Context context) {
        super.onAttach(context);
        this.mContext = context;
    }

    @Override
    public void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        arrayList = new ArrayList<>();
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        return inflater.inflate(R.layout.fragment_blank, container, false);
    }

    @Override
    public void onViewCreated(View view, @Nullable Bundle savedInstanceState) {
        super.onViewCreated(view, savedInstanceState);
        ItemRecyclerView = view.findViewById(R.id.your_rc_view);
    }

    @Override
    public void onActivityCreated(@Nullable Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);
        adapter = new RecycleAdapter(arrayList,getContext());
        ItemRecyclerView.setHasFixedSize(true);
        ItemRecyclerView.setLayoutManager(new LinearLayoutManager(getActivity()));
        ItemRecyclerView.setAdapter(adapter);
        BackgroundTask.updateList(this);
    }

    @Override
    public void updateList(List<DataProvider> dataProviders) {
        arrayList.addAll(dataProviders);
        for(int i = 0; i < arrayList.size(); i++){
            int quantity = arrayList.get(i).getQuantity();
            Total += quantity ;
        }
        adapter.notifyDatasetChanged();
    }

}

<强> BackgroundTask.java

public class BackgroundTask {
    public static void updateList(final UpdateListInterface updateListInterface){
        final List<DataProvider> arrayList = new ArrayList<>();
        StringRequest stringRequest = new StringRequest(Request.Method.POST, jsonURL,
                new Response.Listener<String>() {
                    @Override
                    public void onResponse(String response) {

                        try {
                            JSONArray jsonArray = new JSONArray(response);

                            for(int i = 0; i < jsonArray.length(); i++){
                                JSONObject jsonObject = jsonArray.getJSONObject(i);
                                DataProvider dataProvider = new DataProvider(jsonObject.getString("Name"),
                                        jsonObject.getDouble("quantity"));
                                arrayList.add(dataProvider);
                            }

                            //Here is how you must communicate with your calling class.
                            updateListInterface.updateList(arrayList);
                        } catch (JSONException e) {
                            e.printStackTrace();
                        }

                    }
                },new Response.ErrorListener(){
            @Override
            public void onErrorResponse(VolleyError volleyError){


            }
        }) {
            @Override
            protected Map<String, String> getParams() throws AuthFailureError {

                Map<String,String> param = new HashMap<String,String>();
                param.put("id",MainActivity.USER_ID.toString());
                return param;
            }
        };

        Singletone.getSingletone(context).addToRequest(stringRequest);
    }

    public interface UpdateListInterface{
        void updateList(List<DataProvider> dataProviders);
    }
}