Show filter icon in action bar when filtering

app/src/main/java/eu/faircode/email/FragmentMessages.java

@@ -99,6 +99,7 @@ import androidx.appcompat.widget.PopupMenu;
 import androidx.constraintlayout.widget.Group;
 import androidx.core.content.ContextCompat;
 import androidx.core.graphics.ColorUtils;
+import androidx.core.view.MenuItemCompat;
 import androidx.fragment.app.FragmentActivity;
 import androidx.fragment.app.FragmentManager;
 import androidx.fragment.app.FragmentResultListener;
@@ -3893,6 +3894,17 @@ public class FragmentMessages extends FragmentBase implements SharedPreferences.
                 (viewType == AdapterMessage.ViewType.UNIFIED ||
                         (viewType == AdapterMessage.ViewType.FOLDER && !outbox));
 
+        String filter_language = prefs.getString("filter_language", null);
+        boolean filter_active = (filter_seen || filter_unflagged || filter_unknown || !TextUtils.isEmpty(filter_language));
+        MenuItem menuFilter = menu.findItem(R.id.menu_filter);
+        menuFilter.setShowAsAction(folder && filter_active
+                ? MenuItem.SHOW_AS_ACTION_ALWAYS
+                : MenuItem.SHOW_AS_ACTION_NEVER);
+        MenuItemCompat.setIconTintList(menuFilter,
+                folder && filter_active ?
+                        ColorStateList.valueOf(Helper.resolveColor(getContext(), R.attr.colorAccent)) : null);
+        menuFilter.setIcon(folder && filter_active ? R.drawable.twotone_filter_alt_24 : R.drawable.twotone_filter_list_24);
+
         MenuItem menuSearch = menu.findItem(R.id.menu_search);
         menuSearch.setVisible(folder);
 
@@ -4252,6 +4264,10 @@ public class FragmentMessages extends FragmentBase implements SharedPreferences.
                             prefs.edit().putString("filter_language", locale.getLanguage()).apply();
                         }
 
+                        FragmentActivity activity = getActivity();
+                        if (activity != null)
+                            activity.invalidateOptionsMenu();
+
                         loadMessages(true);
 
                         return true;