Java 在片段中第一次调用SharedReference时为空

我有一个示例android应用程序，根据设置（SharedReference）中设置的位置（邮政编码）和温度单位，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SharedReference中设置的温度单位时，它是空的，isMetric设置为TRUE。Utility.isMetric可以改进为表示没有从SharedReference获取数据，但我的问题是，为什么从ForecastFragment的onCreateView首次调用Utility.isMetric时SharedReference是空的

Utility.isMetric访问SharedReference

SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(context);

ForecastFragment调用实用程序.isMetric

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

我有一个logcat显示了这种行为，如果你想看到，请告诉我

由于字符数有最大限制，可以在以下位置访问完整的代码：

前甲板碎片

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);
    }
}

我已经将logcat放在两个文件（实用工具和ForecastFragment）中来演示这个问题。这是安装应用程序后，ForecastFragment onCreateView首次加载时的日志

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: {}

现在我已经从上面两个位置删除了Log.d语句，但是非常欢迎您放置Log语句

---

注意：这是一个家庭作业应用程序，文件和完整的应用程序中显示了待办事项，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。

您的共享引用为空的原因是您没有成功输入任何内容

我建议以这种方式使用SharedReferences，因为这对我很有用：

import android.content.SharedPreferences;

在你的职能中：

SharedPreferences preferences = getApplicationContext().getSharedPreferences("AppPref", MODE_PRIVATE);
SharedPreferences.Editor editor = preferences.edit();

editor.putString("**Input your data here**", **tag your data/give it a name*);//example editor.putString("userPassword", password);

editor.commit();//this one places your string into the sharedpreferences instance you created

要在创建时访问共享首选项，请执行以下操作：

private String data; //this receives your data from sharedpref

@Override
protected void onCreate(Bundle savedInstanceState){
    super.onCreate(savedInstanceState);
    SharedPreferences conn = getSharedPreferences("AppPref", MODE_PRIVATE);
    data = conn.getString("**your data tag**","");
}

---

希望这有帮助。如果希望存储会话中的值，则需要专门为运行时输入的数据创建SharedReferences实例。我不太擅长技术上的解释，所以如果你有任何问题，请澄清

谢谢Dennis和Kaze，我的android应用程序在

addPreferencesFromResource（R.xml.pref_general）的帮助下正在从xml文件加载默认共享首选项

在SettingActivity中，

扩展首选项activity

这是一个单独的活动，当有人单击“设置”按钮时启动，因此在按下“设置”按钮之前不会填充共享首选项

您关于填充默认首选项的问题使我了解了它是如何填充的，并且应该使用

PreferenceManager在ForecastFragment
onCreate
中填充它。setDefaultValues

如下所示

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

---

这解决了我的问题。我将投票支持你的答案！谢谢

您应该使用PreferenceManager中的以下方法从xml加载默认值（如您在注释中所述，默认值是从xml加载的）

---

亲爱的投票人，如果你能至少说几句话，我会非常感激你为什么投票否决这个问题，这样可以帮助我和其他看到这个问题的人理解为什么投票被否决？它包含所有数据、我尝试过的内容和我正在寻找的内容，因此请用几句话说明为什么在不理解问题的情况下否决投票。调用

SharedReferences.getString（…）

或任何您想在

onCreate（…）中获取的值

您首先将数据放在SharedReferences的何处？尝试在创建时获取SharedReferences，但无效。仅供参考-默认共享首选项完全从pref_general.xml.yea读取！这就是我在评论部分要求他做的事。：）我试图发表评论，但正如你所看到的。。我没有足够的代表。我在回复时没有刷新，所以我看不到哈哈。我只是不希望那些真正需要帮助的人被否决。是的，数据是在不同的活动中加载的，只有在单击设置按钮时才会加载，所以我必须在片段中从xml填充默认的共享首选项。谢谢你的指点！Np，看看你是否可以投票表决这个问题，这样它就不会是否定的哈哈哈。我昨天没有投票，所以给出了不止一个答案：）确保你在主要活动的onCreate（）中这样做

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

static void setDefaultValues(Context context, int resId, boolean readAgain)

Sets the default values from an XML preference file by reading the values defined by each Preference item's android:defaultValue attribute.