我有一个android应用程序示例，根据设置（sharedpreference）中设置的位置（邮政编码）和温度单位，应用程序显示7天天气。
似乎当应用程序第一次获取温度并检查sharedpreference中设置的温度单位是什么时，它会显示为空，ismetric设置为true。可以对utility.ismetric进行改进，以表示没有从sharedpreference获取数据，但我的问题是，为什么在forecastframent的oncreateview的utility.ismetric中首次调用sharedpreference时，sharedpreference为空？
实用工具.ismetric访问共享引用
SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(context);
ForecastFragment调用实用程序.isMetric
boolean isMetric = Utility.isMetric(getActivity());
我有显示这种行为的logcat，如果你想看的话请告诉我。
由于字符数有最大限制，完整的代码可以在https://github.com/gosaliajigar/CSC519/tree/master/CSC519-HW6
前舱碎片
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);
}
}
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);
}
}
我将logcat放在两个文件（utility&forecastframent）中来演示这个问题。这是在安装应用程序时，加载ForecastFragment onCreateView时的第一次日志。
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时的日志。
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，在文件和完整的应用程序中显示了待办事项，但在使用应用程序和理解代码时，我在应用程序中发现了这个错误，并试图理解其发生的原因。这是家庭作业或任何提交材料的一部分。
最佳答案
谢谢丹尼斯和凯兹，我的android应用程序正在settingactivity中的addPreferencesFromResource(R.xml.pref_general)帮助下从xml文件加载默认的共享首选项，这是一个单独的活动，当有人单击settings按钮时它就会启动，因此在按下settings按钮之前，共享首选项不会被填充。
您关于填充默认首选项的问题让我研究了它是如何填充的，应该使用下面的extends PreferenceActivity在forecastfragmentonCreate中填充它…
PreferenceManager.setDefaultValues(context, R.xml.pref_general, false);
这解决了我的问题。我会投票给你的答案！谢谢