Clean up

app/src/main/java/com/kabouzeid/gramophone/adapter/CategoryAdapter.java

@@ -39,8 +39,10 @@ public class CategoryAdapter extends RecyclerView.Adapter<CategoryAdapter.ViewHo
         holder.title.setText(holder.title.getResources().getString(category.id.key));
 
         holder.itemView.setOnClickListener(v -> {
+            if (!(category.visible && isLastCheckedCategory(category))) {
                 category.visible = !category.visible;
                 holder.checkBox.setChecked(category.visible);
+            }
         });
 
         holder.dragView.setOnTouchListener((view, event) -> {
@@ -79,6 +81,15 @@ public class CategoryAdapter extends RecyclerView.Adapter<CategoryAdapter.ViewHo
         return categories;
     }
 
+    private boolean isLastCheckedCategory(Category category) {
+        if (category.visible) {
+            for (Category c : categories) {
+                if (c != category && c.visible) return false;
+            }
+        }
+        return true;
+    }
+
     static class ViewHolder extends RecyclerView.ViewHolder {
         public CheckBox checkBox;
         public TextView title;

app/src/main/java/com/kabouzeid/gramophone/preferences/LibraryPreferenceDialog.java

@@ -41,39 +41,28 @@ public class LibraryPreferenceDialog extends DialogFragment {
         categoriesView.setAdapter(adapter);
         touchHelper.attachToRecyclerView(categoriesView);
 
-        MaterialDialog dialog = new MaterialDialog.Builder(getContext())
+        return new MaterialDialog.Builder(getContext())
                 .title(R.string.library_categories)
                 .customView(view, false)
                 .positiveText(android.R.string.ok)
                 .negativeText(android.R.string.cancel)
                 .neutralText(R.string.reset_action)
                 .autoDismiss(false)
-                .onNeutral((dialog1, action) -> {
+                .onNeutral((dialog, action) -> {
                     adapter.setCategories(PreferenceUtil.getInstance(getContext()).getDefaultLibraryCategories());
                 })
-                .onNegative((dialog12, action) -> dismiss())
+                .onNegative((dialog, action) -> dismiss())
-                .onPositive((dialog13, action) -> {
+                .onPositive((dialog, action) -> {
-                    if (!updateCategories(adapter.getCategories())) {
+                    updateCategories(adapter.getCategories());
-                        new MaterialDialog.Builder(getContext())
-                                .title(R.string.edit_categories)
-                                .content(R.string.at_least_one_category_must_be_enabled)
-                                .positiveText(android.R.string.ok)
-                                .show();
-                    } else {
                     dismiss();
-                    }
                 })
                 .build();
-
-        return dialog;
     }
 
-    private boolean updateCategories(ArrayList<Category> categories) {
+    private void updateCategories(ArrayList<Category> categories) {
-        if (getSelected(categories) == 0) return false;
+        if (getSelected(categories) == 0) return;
 
         PreferenceUtil.getInstance(getContext()).setLibraryCategories(categories);
-
-        return true;
     }
 
     private int getSelected(ArrayList<Category> categories) {

app/src/main/java/com/kabouzeid/gramophone/util/PreferenceUtil.java

@@ -7,6 +7,7 @@ import android.preference.PreferenceManager;
 import android.support.annotation.NonNull;
 import android.support.annotation.StyleRes;
 
+import com.google.gson.Gson;
 import com.kabouzeid.gramophone.R;
 import com.kabouzeid.gramophone.helper.SortOrder;
 import com.kabouzeid.gramophone.model.Category;
@@ -14,13 +15,11 @@ import com.kabouzeid.gramophone.ui.fragments.mainactivity.folders.FoldersFragmen
 import com.kabouzeid.gramophone.ui.fragments.player.NowPlayingScreen;
 
 import java.io.File;
-import java.util.Collections;
 import java.util.ArrayList;
+import java.util.Collections;
 import java.util.HashSet;
 import java.util.Set;
 
-import com.google.gson.Gson;
-
 public final class PreferenceUtil {
     public static final String GENERAL_THEME = "general_theme";
     public static final String DEFAULT_START_PAGE = "default_start_page";
@@ -449,7 +448,7 @@ public final class PreferenceUtil {
 
         SharedPreferences.Editor editor = mPreferences.edit();
         editor.putStringSet(LIBRARY_CATEGORIES, data);
-        editor.commit();
+        editor.apply();
     }
 
     public ArrayList<Category> getLibraryCategories() {