java - Android Fragment 中的 Volley 请求队列(getApplicationContext 可能会产生 NullPointerException)

Question

我只想使用 fragment 内的recyclerview从本地主机加载数据，但没有显示任何内容，并且它说getApplicationContext可能会产生NullPointerException。

错误出现在

Volley.newRequestQueue(getActivity().getApplicationContext()).add(stringRequest);

我在主要 Activity 上尝试了代码，它工作正常。

public class UpdateFragment extends Fragment {

private static final String URL = "http://192.168.1.32/CAGELCOII_APP/api.php";

RecyclerView recyclerView;
ItemAdapter adapter;

List<Item> itemList;

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


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

    itemList = new ArrayList<>();
    recyclerView = (RecyclerView) view.findViewById(R.id.recyclerView);
    recyclerView.setHasFixedSize(true);
    recyclerView.setLayoutManager(new LinearLayoutManager(getActivity()));

    loadItem();

    adapter = new ItemAdapter(getActivity(), itemList);
    recyclerView.setAdapter(adapter);

    return view;
}

private void loadItem(){

    StringRequest stringRequest = new StringRequest(Request.Method.GET, URL,
            new Response.Listener<String>() {
                @Override
                public void onResponse(String response) {

                    try {
                        JSONArray products = new JSONArray(response);

                        for(int i =0; i < products.length(); i++){
                            JSONObject productObject = products.getJSONObject(i);

                            int id = productObject.getInt("id");
                            String description = productObject.getString("description");
                            String agency = productObject.getString("agency");
                            String date = productObject.getString("date");
                            String time = productObject.getString("time");
                            String image = productObject.getString("image");

                            Item item = new Item(id, description, agency, date, time, image);
                            itemList.add(item);
                        }

                        adapter = new ItemAdapter(getActivity(), itemList);
                        recyclerView.setAdapter(adapter);


                    } catch (JSONException e) {
                        e.printStackTrace();
                    }
                }
            },
            new Response.ErrorListener() {
                @Override
                public void onErrorResponse(VolleyError error) {
                    Toast.makeText(getActivity(), error.getMessage(), Toast.LENGTH_SHORT).show();
                }
            });

    Volley.newRequestQueue(getActivity().getApplicationContext()).add(stringRequest);

}

}

Answer 1

调用 getActivity() 时，您必须确保 fragment isAdded() 已添加到 Activity，否则您将收到 NullPointerException因为 getActivity() 返回托管该 fragment 的 Activity 。

如果您始终想使用应用程序上下文，由于其非死亡生命周期，您可以使用此静态函数在整个应用程序中检索它:

App.java

public class App extends Application {
    private static Context sAppContext;

    public void onCreate() {
        super.onCreate();
        sAppContext = this;
    }

    public static Context getContext() {
        return sAppContext;
    }
}

AndroidManifest.xml

...
    <application
        android:name=".App"
        ...>
    </application>

您可以在代码中使用它:

Volley.newRequestQueue(App.getContext()).add(stringRequest);