使用listview恢复片段时出错

Question

我有两个片段LatestFragment和LatestDetailsFragment。LatestFragment显示一个列表视图，而LatestDetailsFragment显示从LatestFragment中单击的列表项的详细信息。现在我想保持LatestFragment的状态，以便从LatestDetailsFragment列表再次显示在onItemClick之前。但是当我尝试恢复时，我得到一个空指针异常。我怎么能实现呢？我实现了一个无限的listview，一次加载20个项目。我想恢复到当前的listview位置。 这是我的LatestFragment代码

public class LatestFragment extends Fragment implements OnItemClickListener,
    OnScrollListener {
ProgressDialog progressDialog;
ArrayList<CarItem> adverts;
TextView loadingmore;

// products JSONArray
JSONArray products = null;

// url to get all products list
private static final String url_latest ="http:...";

// JSON Node names
private static final String TAG_SUCCESS = "success";
private static final String TAG_COUNT = "count";
private static final String TAG_PRODUCTS = "products";
private static final String TAG_HEADING = "tag";
private static final String TAG_DESCRIPTION = "description";
private static final String TAG_PRICE = "price";
private static final String TAG_ID = "item_id";
private static final String TAG_LOCATION = "location";
private static final String TAG_PHOTO = "photo";
private static final String TAG_MODEL = "model";
private static final String TAG_MAKE = "make";
private static final String TAG_REGISTRATION = "registration";
private static final String TAG_CATEGORY = "category";
private static final String TAG_TRANSMISSION = "transmission";
private static final String TAG_FUEL = "fuel";
private static final String TAG_ENGINE = "engine_size";

// Bundle key names
private static final String KEY_DESCRIPTION = "advert_description";
private static final String KEY_ID = "advert_id";
private static final String KEY_RATE = "advert_rate";
private static final String KEY_LOCATION = "advert_location";
private static final String KEY_FUEL = "advert_fuel";
private static final String KEY_ENGINE = "advert_engine";
private static final String KEY_MODEL = "advert_model";
private static final String KEY_MAKE = "advert_make";
private static final String KEY_TRANSMISSION = "advert_transmission";
private static final String KEY_REGISTRATION = "advert_registration";
private static final String KEY_CATEGORY = "advert_category";

ListView latestlist;
CarItem advert;

boolean loadingMore;
int currentPage;
int itemCount;

public LatestFragment() {
    advert = new CarItem();
    loadingMore = false;
    currentPage = 0;
    itemCount = 0;
}

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
        Bundle savedInstanceState) {
    setRetainInstance(true);

    View rootView = inflater.inflate(R.layout.fragment_latest, container,
            false);

    latestlist = (ListView) rootView.findViewById(R.id.listView_latest);
    loadingmore = (TextView) rootView
            .findViewById(R.id.latest_textView_more);

    new GetLatest().execute();

    latestlist.setOnItemClickListener(this);
    latestlist.setOnScrollListener(this);
    return rootView;
}

@Override
public void onItemClick(AdapterView<?> parent, View view, int position,
        long id) {
    Fragment detailsFragment = new LatestDetailsFragment();
    CarsListAdapter adapter = (CarsListAdapter) latestlist.getAdapter();
    advert = (CarItem) parent.getItemAtPosition(position);
    int advert_id = advert.getId();
    String advert_description = advert.getDescription();
    String advert_location = advert.getLocation();
    String advert_price = advert.getPrice();
    String advert_engine = advert.getEngineSize();
    String advert_model = advert.getModel();
    String advert_make = advert.getMake();
    String advert_transmission = advert.getTransmission();
    String advert_fuel = advert.getFuel();
    String advert_registration = advert.getRegistration();
    String advert_category = advert.getCategeory();

    Bundle bundle = new Bundle();
    bundle.putString(KEY_DESCRIPTION, advert_description);
    bundle.putInt(KEY_ID, advert_id);
    bundle.putString(KEY_RATE, advert_price);
    bundle.putString(KEY_LOCATION, advert_location);
    bundle.putString(KEY_ENGINE, advert_engine);
    bundle.putString(KEY_MODEL, advert_model);
    bundle.putString(KEY_MAKE, advert_make);
    bundle.putString(KEY_TRANSMISSION, advert_transmission);
    bundle.putString(KEY_FUEL, advert_fuel);
    bundle.putString(KEY_REGISTRATION, advert_registration);
    bundle.putString(KEY_CATEGORY, advert_category);

    detailsFragment.setArguments(bundle);

    FragmentManager fragmentManager = getActivity()
            .getSupportFragmentManager();
    fragmentManager.beginTransaction()
            .replace(R.id.content_frame, detailsFragment)
            .addToBackStack("latest").commit();

}

protected class GetLatest extends
        AsyncTask<LatestFragment, Void, LatestFragment> {

    @Override
    protected void onPreExecute() {
        super.onPreExecute();
        progressDialog = new ProgressDialog(getActivity());
        progressDialog.show();
    }

    @Override
    protected LatestFragment doInBackground(LatestFragment... params) {
        adverts = getLatest();
        return null;
    }

    @Override
    protected void onPostExecute(LatestFragment params) {
        super.onPostExecute(params);

        if (adverts.size() != 0) {
            latestlist.setAdapter(new CarsListAdapter(getActivity(),
                    adverts));
            currentPage = currentPage + 1;
        }
        progressDialog.dismiss();
    }
}

public ArrayList<CarItem> getLatest() {
    InputStream is = null;
    String result = "";
    ArrayList<CarItem> results = new ArrayList<CarItem>();

    ArrayList<NameValuePair> nameValuePairs = new ArrayList<NameValuePair>();
    nameValuePairs.add(new BasicNameValuePair("currentPage", String
            .valueOf(currentPage)));
    // http post
    try {
        HttpClient httpclient = new DefaultHttpClient();
        HttpPost httppost = new HttpPost(url_latest);
        httppost.setEntity(new UrlEncodedFormEntity(nameValuePairs));
        HttpResponse response = httpclient.execute(httppost);
        HttpEntity entity = response.getEntity();
        is = entity.getContent();

    } catch (Exception e) {
        Log.e("log_tag", "Error in connection " + e.toString());
        // results.setText("Error in connection");
    }
    // convert response to string
    try {
        BufferedReader reader = new BufferedReader(new InputStreamReader(
                is, "iso-8859-1"), 8);
        StringBuilder sb = new StringBuilder();
        String line = null;
        while ((line = reader.readLine()) != null) {
            sb.append(line + "\n");
        }
        is.close();
        result = sb.toString();
    } catch (Exception e) {
        Log.e("log_tag", "Error converting result " + e.toString());
    }

    // Check your log cat for JSON reponse
    Log.d("All Products: ", result);

    try {
        JSONObject json = new JSONObject(result);

        // Checking for SUCCESS TAG
        int success = json.getInt(TAG_SUCCESS);
        if (success == 1) {
            // products found
            // Getting Array of Products
            itemCount = json.getInt(TAG_COUNT);
            products = json.getJSONArray(TAG_PRODUCTS);

            // looping through All Products
            for (int i = 0; i < products.length(); i++) {
                JSONObject c = products.getJSONObject(i);

                // Storing each json item in variable
                int id = c.getInt(TAG_ID);
                String location = c.getString(TAG_LOCATION);
                String photo_url = c.getString(TAG_PHOTO);
                String price = c.getString(TAG_PRICE);
                String description = c.getString(TAG_DESCRIPTION);
                String header = c.getString(TAG_HEADING);
                String registration = c.getString(TAG_REGISTRATION);
                String make = c.getString(TAG_MAKE);
                String model = c.getString(TAG_MODEL);
                String category = c.getString(TAG_CATEGORY);
                String transmission = c.getString(TAG_TRANSMISSION);
                String fuel = c.getString(TAG_FUEL);
                String engine_size = c.getString(TAG_ENGINE);

                // creating new CarItem object
                CarItem car = new CarItem();
                car.setHeader(header);
                car.setDescription(description);
                car.setId(id);
                car.setLocation(location);
                car.setPrice(price);
                car.setPhoto(photo_url);
                car.setRegistration(registration);
                car.setMake(make);
                car.setModel(model);
                car.setCategory(category);
                car.setTransmission(transmission);
                car.setEngineSize(engine_size);
                car.setFuel(fuel);

                results.add(car);
            }
        }
    } catch (JSONException e) {
        Log.e("log_tag", "Error parsing data " + e.toString());
    }

    return results;
}

@Override
public void onResume() {
    super.onResume();
    // latestlist.setAdapter(new CarsListAdapter(getActivity(), adverts));
}

@Override
public void onScrollStateChanged(AbsListView view, int scrollState) {
    // TODO Auto-generated method stub

}

@Override
public void onScroll(AbsListView view, int firstVisibleItem,
        int visibleItemCount, int totalItemCount) {
    // what is the bottom iten that is visible
    int lastInScreen = firstVisibleItem + visibleItemCount;
    if ((lastInScreen == totalItemCount) && !(loadingMore)
            && ((currentPage * 20) < itemCount)) {
        loadingmore.setVisibility(View.VISIBLE);
        loadingMore = true;
        new LoadMore().execute();
    }
}

private class LoadMore extends
        AsyncTask<LatestFragment, Void, LatestFragment> {
    CarsListAdapter adapter = (CarsListAdapter) latestlist.getAdapter();
    ArrayList<CarItem> newadverts = new ArrayList<CarItem>();

    @Override
    protected LatestFragment doInBackground(LatestFragment... params) {
        newadverts = getLatest();
        return null;
    }

    protected void onPostExecute(LatestFragment param) {
        super.onPostExecute(param);
        if (newadverts.size() != 0) {
            loadingmore.setVisibility(View.GONE);
            adverts.addAll(newadverts);
            adapter.notifyDataSetChanged();
        }
        loadingMore = false;
    }
}

这是我的Logcat内容：

08-24 10:06:59.158: E/AndroidRuntime(1176): FATAL EXCEPTION: main
08-24 10:06:59.158: E/AndroidRuntime(1176): java.lang.NullPointerException
08-24 10:06:59.158: E/AndroidRuntime(1176):     at com.koncious.safirisha.LatestFragment$LoadMore.onPostExecute(LatestFragment.java:321)
08-24 10:06:59.158: E/AndroidRuntime(1176):     at com.koncious.safirisha.LatestFragment$LoadMore.onPostExecute(LatestFragment.java:1)
08-24 10:06:59.158: E/AndroidRuntime(1176):     at android.os.AsyncTask.finish(AsyncTask.java:631)
08-24 10:06:59.158: E/AndroidRuntime(1176):     at android.os.AsyncTask.access$600(AsyncTask.java:177)
08-24 10:06:59.158: E/AndroidRuntime(1176):     at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:644)
08-24 10:06:59.158: E/AndroidRuntime(1176):     at android.os.Handler.dispatchMessage(Handler.java:99)
08-24 10:06:59.158: E/AndroidRuntime(1176):     at android.os.Looper.loop(Looper.java:137)
08-24 10:06:59.158: E/AndroidRuntime(1176):     at android.app.ActivityThread.main(ActivityThread.java:5041)
08-24 10:06:59.158: E/AndroidRuntime(1176):     at java.lang.reflect.Method.invokeNative(Native Method)
08-24 10:06:59.158: E/AndroidRuntime(1176):     at java.lang.reflect.Method.invoke(Method.java:511)
08-24 10:06:59.158: E/AndroidRuntime(1176):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
08-24 10:06:59.158: E/AndroidRuntime(1176):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
08-24 10:06:59.158: E/AndroidRuntime(1176):     at dalvik.system.NativeStart.main(Native Method)

任何人都请帮忙。

Answer 1

要保存状态，您应该覆盖onSaveInstanceState：

@Override
protected void onSaveInstanceState(Bundle outState) {
    super.onSaveInstanceState(outState);
}

要恢复状态，您应该覆盖onRestoreInstanceState：

@Override
protected void onRestoreInstanceState(Bundle savedInstanceState) {
    super.onRestoreInstanceState(savedInstanceState);
}

点击此链接了解更多信息：

How to use onSavedInstanceState example please