Question

此代码适用于activity但不适用于fragment ..应用程序崩溃时，此操作在片段 ...当我删除getData（）;低于requestQueue应用程序运行但数据未获取..

public class LatestFragment extends Fragment {

    //Creating a List of superheroes
    private List<SuperHero> listSuperHeroes;

    //Creating Views
    private RecyclerView recyclerView;
    private RecyclerView.LayoutManager layoutManager;
    private RecyclerView.Adapter adapter;

    private RequestQueue requestQueue;
    private int requestCount = 1;



    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
           View v = inflater.inflate(R.layout.latest, container, false);
        recyclerView = (RecyclerView) v.findViewById(R.id.recyclerView);
        recyclerView.setHasFixedSize(true);
        layoutManager = new LinearLayoutManager(getActivity());
        recyclerView.setLayoutManager(layoutManager);

        //Initializing our superheroes list
        listSuperHeroes = new ArrayList<>();
        requestQueue = Volley.newRequestQueue(getActivity());

        //Calling method to get data to fetch data
       getData();

        //Adding an scroll change listener to recyclerview
        // RecyclerView.setOnScrollChangeListener(this);

          recyclerView.addOnScrollListener(rVOnScrollListener);

        //initializing our adapter
        adapter = new CardAdapter(listSuperHeroes, getActivity());

        //Adding adapter to recyclerview
        recyclerView.setAdapter(adapter);
        return v;
    }

    //Request to get json from server we are passing an integer here
    //This integer will used to specify the page number for the request ?page = requestcount
    //This method would return a JsonArrayRequest that will be added to the request queue
    private JsonArrayRequest getDataFromServer(int requestCount) {
        //Initializing ProgressBar
        final ProgressBar progressBar = (ProgressBar) getActivity().findViewById(R.id.progressBar1);

        //Displaying Progressbar
        progressBar.setVisibility(View.VISIBLE);
     //  setProgressBarIndeterminateVisibility(true);

        //JsonArrayRequest of volley
        JsonArrayRequest jsonArrayRequest = new JsonArrayRequest(Config.DATA_URL + String.valueOf(requestCount),
                new Response.Listener<JSONArray>() {
                    @Override
                    public void onResponse(JSONArray response) {
                        //Calling method parseData to parse the json response
                        parseData(response);
                        //Hiding the progressbar
                        progressBar.setVisibility(View.GONE);
                    }
                },
                new Response.ErrorListener() {
                    @Override
                    public void onErrorResponse(VolleyError error) {
                        progressBar.setVisibility(View.GONE);
                        //If an error occurs that means end of the list has reached
                        //Toast.makeText(citiesdetail.this, "No More Items Available", Toast.LENGTH_SHORT).show();
                    }
                });

        //Returning the request
        return jsonArrayRequest;
    }

    //This method will get data from the web api
    private void getData() {
        //Adding the method to the queue by calling the method getDataFromServer
        requestQueue.add(getDataFromServer(requestCount));
        //Incrementing the request counter
        requestCount++;
    }

    //This method will parse json data
    private void parseData(JSONArray array) {
        for (int i = 0; i < array.length(); i++) {
            //Creating the superhero object
            SuperHero superHero = new SuperHero();
            JSONObject json = null;
            try {
                //Getting json
                json = array.getJSONObject(i);

                //Adding data to the superhero object
                superHero.setImageUrl(json.getString(Config.TAG_IMAGE_URL));
                superHero.setName(json.getString(Config.TAG_NAME));
                superHero.setPublisher(json.getString(Config.TAG_PUBLISHER));
            } catch (JSONException e) {
                e.printStackTrace();
            }
            //Adding the superhero object to the list
            listSuperHeroes.add(superHero);
        }

        //Notifying the adapter that data has been added or changed
        adapter.notifyDataSetChanged();
    }

    //This method would check that the recyclerview scroll has reached the bottom or not
    private boolean isLastItemDisplaying(RecyclerView recyclerView) {
        if (recyclerView.getAdapter().getItemCount() != 0) {
            int lastVisibleItemPosition = ((LinearLayoutManager) recyclerView.getLayoutManager()).findLastCompletelyVisibleItemPosition();
            if (lastVisibleItemPosition != RecyclerView.NO_POSITION && lastVisibleItemPosition == recyclerView.getAdapter().getItemCount() - 1)
                return true;
        }
        return false;
    }

    //Overriden method to detect scrolling
    private RecyclerView.OnScrollListener rVOnScrollListener = new RecyclerView.OnScrollListener(){
        @Override
        public void onScrollStateChanged(RecyclerView recyclerView,
                                         int newState) {
            super.onScrollStateChanged(recyclerView, newState);
        }

        @Override
        public void onScrolled(RecyclerView recyclerView, int dx, int dy) {
            super.onScrolled(recyclerView, dx, dy);

           if (isLastItemDisplaying(recyclerView)) {
                getData();
            }
        }
    };
}

Answer 1

您正在从onCreateView（）调用getData（）方法，这导致调用parseData（）方法，并且在此方法中您使用了“适配器”对象并且未对其进行初始化。您在onCreateView（）中的getData（）方法之后初始化它。所以这可能导致NullPointerException。

为此，您可以在parseData（）方法中初始化适配器，并仅从那里设置适配器值。

Answer 2

崩溃的原因是getActivity().findViewById().setVisibility()方法中的getDataFromServer()。

为什么从活动视图获取progressBar？可能你的progressBar在片段视图中，所以你的代码应该是这样的：

public class LatestFragment extends Fragment {
    private ProgressBar progressBar;

    ...

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        View v = inflater.inflate(R.layout.latest, container, false);

        ...
        progressBar = (ProgressBar) v.findViewById(R.id.progressBar1);
        ...
    }

    ...

    private JsonArrayRequest getDataFromServer(int requestCount) {
        // You don't need the initialization, it was done in onCreateView() in this code. Remove these lines.
        //final ProgressBar progressBar = (ProgressBar) getActivity().findViewById(R.id.progressBar1);

        //Displaying Progressbar
        progressBar.setVisibility(View.VISIBLE);
        ...
    }
    ...
}

此代码适用于活动，但不适用于片段

2 个答案: