java - Fragment中第一次调用SharedPreferences为空

Question

我有一个示例 android 应用程序，根据位置(邮政编码)和在设置 (SharedPreference) 中设置的温度单位，该应用程序显示 7 天的天气。

似乎当应用程序第一次获取温度并检查 SharedPreference 中设置的温度单位时，它是空的并且 isMetric 设置为 TRUE。可以改进 Utility.isMetric 以表示没有从 SharedPreference 获取数据，但我的问题是为什么在从 ForecastFragment 的 onCreateView 首次调用 Utility.isMetric 时 SharedPreference 为空？

Utility.isMetric 访问 SharedPreference

SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(context);

ForecastFragment 调用 Utility.isMetric

boolean isMetric = Utility.isMetric(getActivity());

我有显示此行为的 logcat，如果您想查看，请告诉我。

因为没有最大限制。的字符，完整的代码可以在https://github.com/gosaliajigar/CSC519/tree/master/CSC519-HW6访问

预测 fragment

public class ForecastFragment extends Fragment implements LoaderCallbacks<Cursor> {

    private SimpleCursorAdapter mForecastAdapter;

    private static final int FORECAST_LOADER = 0;
    private String mLocation;

    // For the forecast view we're showing only a small subset of the stored data.
    // Specify the columns we need.
    private static final String[] FORECAST_COLUMNS = {
            // In this case the id needs to be fully qualified with a table name, since
            // the content provider joins the location & weather tables in the background
            // (both have an _id column)
            // On the one hand, that's annoying.  On the other, you can search the weather table
            // using the location set by the user, which is only in the Location table.
            // So the convenience is worth it.
            WeatherEntry.TABLE_NAME + "." + WeatherEntry._ID,
            WeatherEntry.COLUMN_DATETEXT,
            WeatherEntry.COLUMN_SHORT_DESC,
            WeatherEntry.COLUMN_MAX_TEMP,
            WeatherEntry.COLUMN_MIN_TEMP,
            LocationEntry.COLUMN_LOCATION_SETTING
    };


    // These indices are tied to FORECAST_COLUMNS.  If FORECAST_COLUMNS changes, these
    // must change.
    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 onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        // Add this line in order for this fragment to handle menu events.
        setHasOptionsMenu(true);
    }

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

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();
        if (id == R.id.action_refresh) {
            updateWeather();
            return true;
        }
        return super.onOptionsItemSelected(item);
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        final String[] columns = {WeatherEntry.COLUMN_DATETEXT,
                WeatherEntry.COLUMN_SHORT_DESC,
                WeatherEntry.COLUMN_MAX_TEMP,
                WeatherEntry.COLUMN_MIN_TEMP
        };

        final int[] viewIDs = {R.id.list_item_date_textview,
                R.id.list_item_forecast_textview,
                R.id.list_item_high_textview,
                R.id.list_item_low_textview
        };

        // The SimpleCursorAdapter will take data from the database through the
        // Loader and use it to populate the ListView it's attached to.
        mForecastAdapter = new SimpleCursorAdapter(
                getActivity(),
                R.layout.list_item_forecast,
                null,
                columns,
                viewIDs,
                0);
        mForecastAdapter.setViewBinder(new SimpleCursorAdapter.ViewBinder() {
            @Override
            public boolean setViewValue(View view, Cursor cursor, int columnIndex) {
                boolean isMetric = Utility.isMetric(getActivity());
                switch (columnIndex) {
                    case COL_WEATHER_MAX_TEMP: {
                        String high = Utility.formatTemperature(
                                cursor.getDouble(cursor.getColumnIndex(WeatherEntry.COLUMN_MAX_TEMP)), isMetric);
                        ((TextView) view).setText(high);
                        return true;
                    }
                    case COL_WEATHER_MIN_TEMP: {
                        // we have to do some formatting and possibly a conversion
                        String low = Utility.formatTemperature(
                                cursor.getDouble(cursor.getColumnIndex(WeatherEntry.COLUMN_MIN_TEMP)), isMetric);
                        ((TextView) view).setText(low);
                        return true;
                    }
                    case COL_WEATHER_DATE: {
                        String dateString = Utility.formatDate(
                                cursor.getString(cursor.getColumnIndex(WeatherEntry.COLUMN_DATETEXT)));
                        ((TextView) view).setText(dateString);
                        return true;
                    }
                }
                return false;
            }
        });

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

        // Get a reference to the ListView, and attach this adapter to it.
        ListView listView = (ListView) rootView.findViewById(R.id.listview_forecast);
        listView.setAdapter(mForecastAdapter);
        listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {

            @Override
            public void onItemClick(AdapterView<?> adapterView, View view, int position, long l) {
                Cursor cursor = mForecastAdapter.getCursor();
                if (cursor != null && cursor.moveToPosition(position)) {
                    Intent intent = new Intent(getActivity(), DetailActivity.class)
                            .putExtra(DetailActivity.DATE_KEY, cursor.getString(COL_WEATHER_DATE));
                    startActivity(intent);
                }
            }
        });

        return rootView;
    }

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

    private void updateWeather() {
        String location = Utility.getPreferredLocation(getActivity());
        // update weather only if location is not an EMPTY string
        if (location != null
                && location.length() > 0) {
            new FetchWeatherTask(getActivity()).execute(location, SettingsActivity.FORECAST_DAYS);
        }
    }

    @Override
    public void onResume() {
        super.onResume();
        if (mLocation != null && !mLocation.equals(Utility.getPreferredLocation(getActivity()))) {
            getLoaderManager().restartLoader(FORECAST_LOADER, null, this);
        }
    }

    @Override
    public Loader<Cursor> onCreateLoader(int id, Bundle args) {
        // This is called when a new Loader needs to be created.  This
        // fragment only uses one loader, so we don't care about checking the id.

        // To only show current and future dates, get the String representation for today,
        // and filter the query to return weather only for dates after or including today.
        // Only return data after today.
        String startDate = WeatherContract.getDbDateString(new Date());

        // Sort order:  Ascending, by date.
        String sortOrder = WeatherEntry.COLUMN_DATETEXT + " ASC";

        mLocation = Utility.getPreferredLocation(getActivity());

        if (mLocation == null || mLocation.length() == 0) {
            mLocation = "00000";
        }

        Uri weatherForLocationUri = WeatherContract.WeatherEntry.buildWeatherLocationWithStartDate(mLocation, startDate);
        Log.d(getString(R.string.app_name), weatherForLocationUri.toString());

        // Now create and return a CursorLoader that will take care of
        // creating a Cursor for the data being displayed.
        return new CursorLoader(
                getActivity(),
                weatherForLocationUri,
                FORECAST_COLUMNS,
                null,
                null,
                sortOrder
        );
    }

    @Override
    public void onLoadFinished(Loader<Cursor> loader, Cursor data) {
        mForecastAdapter.swapCursor(data);
    }

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

效用

public class Utility {
    public static String getPreferredLocation(Context context) {
        SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(context);
        return prefs.getString(context.getString(R.string.pref_location_key),
                context.getString(R.string.pref_location_default));
    }

    public static boolean isMetric(Context context) {
        SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(context);
        return prefs.getString(context.getString(R.string.pref_units_key),
                context.getString(R.string.pref_units_metric))
                .equals(context.getString(R.string.pref_units_metric));
    }

    static String formatTemperature(double temperature, boolean isMetric) {
        double temp;
        if ( !isMetric ) {
            temp = 9*temperature/5+32;
        } else {
            temp = temperature;
        }
        return String.format("%.0f", temp);
    }

    static String formatDate(String dateString) {
        Date date = WeatherContract.getDateFromDb(dateString);
        return DateFormat.getDateInstance().format(date);
    }
}

我已将 logcat 放入两个文件(Utility 和 ForecastFragment)中以演示该问题。这是在安装应用程序时第一次加载 ForecastFragment onCreateView 时的 logcat。

07-05 17:35:57.562 21604-21604/com.example.android.weather.app D/JIGAR: {}
07-05 17:35:57.563 21604-21604/com.example.android.weather.app D/JIGAR: isMetric true

这是在按下刷新按钮后加载 ForecastFragment onCreateView 时的 logcat。

07-05 17:36:44.433 21604-21604/com.example.android.weather.app D/JIGAR: {}
07-05 17:36:44.435 21604-21604/com.example.android.weather.app D/JIGAR: {}
07-05 17:36:44.435 21604-21604/com.example.android.weather.app D/JIGAR: {}
07-05 17:36:44.436 21604-21604/com.example.android.weather.app D/JIGAR: {}
07-05 17:36:44.445 21604-21604/com.example.android.weather.app D/JIGAR: {}
07-05 17:36:44.449 21604-21604/com.example.android.weather.app D/JIGAR: {}
07-05 17:36:44.450 21604-21604/com.example.android.weather.app D/JIGAR: {}
07-05 17:36:44.452 21604-21604/com.example.android.weather.app D/JIGAR: {}
07-05 17:36:44.458 21604-21604/com.example.android.weather.app D/JIGAR: {}
07-05 17:36:44.458 21604-21604/com.example.android.weather.app D/JIGAR: {}
07-05 17:36:44.458 21604-21604/com.example.android.weather.app D/JIGAR: {}
07-05 17:36:44.458 21604-21604/com.example.android.weather.app D/JIGAR: {}
07-05 17:36:44.459 21604-21604/com.example.android.weather.app D/JIGAR: {}
07-05 17:36:44.460 21604-21604/com.example.android.weather.app D/JIGAR: {}
07-05 17:36:44.460 21604-21604/com.example.android.weather.app D/JIGAR: {}
07-05 17:36:44.460 21604-21604/com.example.android.weather.app D/JIGAR: {}
07-05 17:36:44.461 21604-21604/com.example.android.weather.app D/JIGAR: {}
07-05 17:36:44.461 21604-21604/com.example.android.weather.app D/JIGAR: {}
07-05 17:36:44.461 21604-21604/com.example.android.weather.app D/JIGAR: {}
07-05 17:36:44.461 21604-21604/com.example.android.weather.app D/JIGAR: {}
07-05 17:36:44.462 21604-21604/com.example.android.weather.app D/JIGAR: {}
07-05 17:36:44.462 21604-21604/com.example.android.weather.app D/JIGAR: {}
07-05 17:36:44.462 21604-21604/com.example.android.weather.app D/JIGAR: {}
07-05 17:36:44.463 21604-21604/com.example.android.weather.app D/JIGAR: {}
07-05 17:36:44.463 21604-21604/com.example.android.weather.app D/JIGAR: {}
07-05 17:36:44.464 21604-21604/com.example.android.weather.app D/JIGAR: {}
07-05 17:36:44.464 21604-21604/com.example.android.weather.app D/JIGAR: {}
07-05 17:36:44.464 21604-21604/com.example.android.weather.app D/JIGAR: {}

我现在已经从上述两个位置删除了 Log.d 语句，但非常欢迎您放置日志语句。

注意:这是一个家庭作业应用程序，文件中显示了 TO DO 及其完整的应用程序，但在玩应用程序和理解代码时，我在应用程序中发现了这个错误并试图理解它发生的原因。这不是作业或任何提交的一部分。

Answer 1

感谢 Dennis 和 Kaze，我的 android 应用程序在 addPreferencesFromResource(R.xml.pref_general) 的帮助下从 xml 文件加载默认共享首选项这是一个单独的 Activity ，当有人点击“设置”按钮时它就会启动，因此在按下“设置”按钮之前永远不会填充共享首选项。

你关于填充默认首选项的问题让我研究了它是如何填充的，它应该使用 PreferenceManager.setDefaultValues 在 ForecastFragment onCreate 中填充，如下所示......

PreferenceManager.setDefaultValues(context, R.xml.pref_general, false);

这解决了我的问题。我会给你的答案投票!谢谢