LoaderManager.LoaderCallbacks<Cursor> 似乎无法正常工作

Question

我想在我的应用中利用加载器作为

public class ForecastFragment extends Fragment implements LoaderManager.LoaderCallbacks<Cursor>{

    private ListView forecasteListView;
    private SimpleCursorAdapter forecasteAdapter;

    private String mLocation;
    private static final int FORE_CAST_LOADER = 404;

    private static final String[] FORE_CAST_COLUMNS = {
       WeatherContract.WeatherEntry.TABLE_NAME + "." + WeatherContract.WeatherEntry._ID,
       WeatherContract.WeatherEntry.COLUMN_DATE,
       WeatherContract.WeatherEntry.COLUMN_SHORT_DESC,
       WeatherContract.WeatherEntry.COLUMN_MAX_TEMP,
       WeatherContract.WeatherEntry.COLUMN_MIN_TEMP,
       WeatherContract.LocationEntry.COLUMN_LOCATION_SETTING

    };

    public static final int COL_WEATHER_ID = 0;
    public static final int COL_WEATHER_DATE = 1;
    public static final int COL_WEATHER_DESC = 2;
    public static final int COL_WEATHER_MAX_TEMP = 3;
    public static final int COL_WEATHER_MIN_TEMP = 4;
    public static final int COL_LOCATION_SETTING = 5;

    public ForecastFragment() {
    }

    @Override
    public void onActivityCreated(Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);
        getLoaderManager().initLoader(FORE_CAST_LOADER, null, this);
    }

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setHasOptionsMenu(true);
    }

    @Override
    public void onCreateOptionsMenu(Menu menu, MenuInflater inflater) {
        inflater.inflate(R.menu.forcast_menu, menu);
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        if (item.getItemId() == R.id.action_refresh){
           updateWeather();
            return true;
        }
        return super.onOptionsItemSelected(item);
    }

    private void updateWeather() {
        FetchWeatherTask forecastWeather = new FetchWeatherTask(getActivity());

        SharedPreferences sharedPreferences = PreferenceManager.getDefaultSharedPreferences(getActivity());
        String cityName = sharedPreferences.getString(getString(R.string.pref_city_key),getString(R.string.pref_city_default));
        forecastWeather.execute(cityName, null, null);
    }

    @Override
    public void onStart() {
        super.onStart();
        updateWeather();
    }

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

        forecasteAdapter = new SimpleCursorAdapter(getActivity(),
                R.layout.list_item_forecast,
                null,
                new String[]{WeatherContract.WeatherEntry.COLUMN_DATE,
                        WeatherContract.WeatherEntry.COLUMN_SHORT_DESC,
                        WeatherContract.WeatherEntry.COLUMN_MAX_TEMP,
                        WeatherContract.WeatherEntry.COLUMN_MIN_TEMP
                },
                new int[]{R.id.list_item_date_textView,
                        R.id.list_item_forecast_desc_textView,
                        R.id.list_item_high_textView,
                        R.id.list_item_low_textView
                }, 0

        );

        Log.e("onCreateView()", "List Adapter is of " + forecasteAdapter.getCount() + " size");

        View rootView = inflater.inflate(R.layout.fragment_main, container, false);    
        forecasteListView = (ListView) rootView.findViewById(R.id.listView_forecast);    
        return rootView ;
    }

    @Override
    public Loader<Cursor> onCreateLoader(int id, Bundle args) {
        String sortOrder = WeatherContract.WeatherEntry.COLUMN_DATE + " ASC";

        mLocation = Utility.getPreferredLocation(getActivity());

        Uri weatherForLocationUri = WeatherContract.WeatherEntry.buildWeatherLocationWithStartDate(mLocation, System.currentTimeMillis());

        Log.e("FORE_CAST_FRAGMENT", "Uri : " + weatherForLocationUri.toString() );
        return new CursorLoader(getActivity(), weatherForLocationUri, FORE_CAST_COLUMNS, null, null, sortOrder);
    }

    @Override
    public void onLoadFinished(Loader<Cursor> loader, Cursor data) {
        Log.e("Loader<Cursor>", "Size of Cursor is " + data.getCount());
        forecasteAdapter.swapCursor(data);
    }

    @Override
    public void onLoaderReset(Loader<Cursor> loader) {
        forecasteAdapter.swapCursor(null);
    }
}

上面的代码运行没有错误/异常，首先当App启动方法时

@Override
    public void onLoadFinished(Loader<Cursor> loader, Cursor data) {
        Log.e("Loader<Cursor>", "Size of Cursor is " + data.getCount());
        forecasteAdapter.swapCursor(data);
    }

打印

E/Loader<Cursor>﹕ Size of Cursor is 0

它不会从数据库中获取数据并填充 ListView。在这种情况下，我们非常欢迎任何帮助。

免责声明 我正在学习 Developing Android Apps - By Udacity 课程中的教程。

Answer 1

您是否尝试过调试您的应用程序并检查批量插入是否正常工作以及是否在表中插入数据。

由于表中没有数据，游标将一直获取 0 行。 尝试在正在执行批量插入查询的适配器类中保留一些日志语句。如果正在创建条目，那么您传递以从表中获取数据的 URI 中可能存在一些问题。

希望这对您有所帮助。 干杯。