5.12 Coding Task to Restore scroll position on rotation (quiz)

Author: Lyla

app/src/main/java/com/example/android/sunshine/app/ForecastFragment.java

@@ -37,6 +37,12 @@
  * Encapsulates fetching the forecast and displaying it as a {@link ListView} layout.
  */
 public class ForecastFragment extends Fragment implements LoaderManager.LoaderCallbacks<Cursor> {
+    private ForecastAdapter mForecastAdapter;
+
+    private ListView mListView;
+    private int mPosition = ListView.INVALID_POSITION;
+
+    private static final String SELECTED_KEY = "selected_position";
 
     private static final int FORECAST_LOADER = 0;
     // For the forecast view we're showing only a small subset of the stored data.
@@ -71,8 +77,6 @@ public class ForecastFragment extends Fragment implements LoaderManager.LoaderCa
     static final int COL_COORD_LAT = 7;
     static final int COL_COORD_LONG = 8;
 
-    private ForecastAdapter mForecastAdapter;
-
     /**
      * A callback interface that all activities containing this fragment must
      * implement. This mechanism allows activities to be notified of item
@@ -116,17 +120,18 @@ public boolean onOptionsItemSelected(MenuItem item) {
     @Override
     public View onCreateView(LayoutInflater inflater, ViewGroup container,
                              Bundle savedInstanceState) {
-        // The CursorAdapter will take data from our cursor and populate the ListView.
+
+        // The ForecastAdapter will take data from a source and
+        // use it to populate the ListView it's attached to.
         mForecastAdapter = new ForecastAdapter(getActivity(), null, 0);
 
         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);
-
+        mListView = (ListView) rootView.findViewById(R.id.listview_forecast);
+        mListView.setAdapter(mForecastAdapter);
         // We'll call our MainActivity
-        listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
+        mListView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
 
             @Override
             public void onItemClick(AdapterView<?> adapterView, View view, int position, long l) {
@@ -140,8 +145,21 @@ public void onItemClick(AdapterView<?> adapterView, View view, int position, lon
                                     locationSetting, cursor.getLong(COL_WEATHER_DATE)
                             ));
                 }
+                mPosition = position;
             }
         });
+
+        // If there's instance state, mine it for useful information.
+        // The end-goal here is that the user never knows that turning their device sideways
+        // does crazy lifecycle related things.  It should feel like some stuff stretched out,
+        // or magically appeared to take advantage of room, but data or place in the app was never
+        // actually *lost*.
+        if (savedInstanceState != null && savedInstanceState.containsKey(SELECTED_KEY)) {
+            // The listview probably hasn't even been populated yet.  Actually perform the
+            // swapout in onLoadFinished.
+            mPosition = savedInstanceState.getInt(SELECTED_KEY);
+        }
+
         return rootView;
     }
 
@@ -163,12 +181,29 @@ private void updateWeather() {
         weatherTask.execute(location);
     }
 
+    @Override
+    public void onSaveInstanceState(Bundle outState) {
+        // When tablets rotate, the currently selected list item needs to be saved.
+        // When no item is selected, mPosition will be set to Listview.INVALID_POSITION,
+        // so check for that before storing.
+        if (mPosition != ListView.INVALID_POSITION) {
+            outState.putInt(SELECTED_KEY, mPosition);
+        }
+        super.onSaveInstanceState(outState);
+    }
+
     @Override
     public Loader<Cursor> onCreateLoader(int i, Bundle bundle) {
-        String locationSetting = Utility.getPreferredLocation(getActivity());
+        // 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, filter the query to return weather only for
+        // dates after or including today.
 
         // Sort order:  Ascending, by date.
         String sortOrder = WeatherContract.WeatherEntry.COLUMN_DATE + " ASC";
+
+        String locationSetting = Utility.getPreferredLocation(getActivity());
         Uri weatherForLocationUri = WeatherContract.WeatherEntry.buildWeatherLocationWithStartDate(
                 locationSetting, System.currentTimeMillis());
 
@@ -181,12 +216,17 @@ public Loader<Cursor> onCreateLoader(int i, Bundle bundle) {
     }
 
     @Override
-    public void onLoadFinished(Loader<Cursor> cursorLoader, Cursor cursor) {
-        mForecastAdapter.swapCursor(cursor);
+    public void onLoadFinished(Loader<Cursor> loader, Cursor data) {
+        mForecastAdapter.swapCursor(data);
+        if (mPosition != ListView.INVALID_POSITION) {
+            // If we don't need to restart the loader, and there's a desired position to restore
+            // to, do so now.
+            mListView.smoothScrollToPosition(mPosition);
+        }
     }
 
     @Override
     public void onLoaderReset(Loader<Cursor> cursorLoader) {
         mForecastAdapter.swapCursor(null);
     }
-}
+}