fix: openfoodfacts#899 - avoid to reload the product in onboarding

Impacted file:
* `preferences_page.dart`: split in two the process - load product and then display it with preferences

packages/smooth_app/lib/pages/onboarding/preferences_page.dart

@@ -18,10 +18,10 @@ class PreferencesPage extends StatefulWidget {
   State<PreferencesPage> createState() => _PreferencesPageState();
 }
 
+// Just here to load the product and pass it to the next Widget
 class _PreferencesPageState extends State<PreferencesPage> {
   late Future<void> _initFuture;
   late Product _product;
-  bool _isProductExpanded = false;
 
   @override
   void didChangeDependencies() {
@@ -39,12 +39,7 @@ class _PreferencesPageState extends State<PreferencesPage> {
   }
 
   @override
-  Widget build(BuildContext context) {
-    final ProductPreferences productPreferences =
-        context.watch<ProductPreferences>();
-    final UserPreferences userPreferences = context.watch<UserPreferences>();
-    final AppLocalizations appLocalizations = AppLocalizations.of(context)!;
-    return FutureBuilder<void>(
+  Widget build(BuildContext context) => FutureBuilder<void>(
         future: _initFuture,
         builder: (BuildContext context, AsyncSnapshot<void> snapshot) {
           if (snapshot.hasError) {
@@ -53,74 +48,97 @@ class _PreferencesPageState extends State<PreferencesPage> {
           if (snapshot.connectionState != ConnectionState.done) {
             return const CircularProgressIndicator();
           }
-          final List<Widget> pageData = <Widget>[
-            Padding(
-              padding: const EdgeInsets.only(
-                right: LARGE_SPACE,
-                left: LARGE_SPACE,
-                bottom: LARGE_SPACE,
-              ),
-              child: Text(
-                appLocalizations.productDataUtility,
-                style: Theme.of(context).textTheme.headline2!.apply(
-                      color: Colors.black,
-                    ),
-              ),
-            ),
-            Container(
-              height: _isProductExpanded ? null : 150,
-              padding: const EdgeInsets.only(
-                bottom: LARGE_SPACE,
-                right: LARGE_SPACE,
-                left: LARGE_SPACE,
-              ),
-              child: GestureDetector(
-                onTap: () => _expandProductCard(),
-                child: SummaryCard(
-                  _product,
-                  productPreferences,
-                  isFullVersion: _isProductExpanded,
-                ),
+          return _Helper(_product);
+        },
+      );
+}
+
+// In order to avoid to reload the product when refreshing the preferences.
+class _Helper extends StatefulWidget {
+  const _Helper(this.product);
+
+  final Product product;
+
+  @override
+  State<_Helper> createState() => _HelperState();
+}
+
+class _HelperState extends State<_Helper> {
+  bool _isProductExpanded = false;
+
+  @override
+  Widget build(BuildContext context) {
+    final ProductPreferences productPreferences =
+        context.watch<ProductPreferences>();
+    final UserPreferences userPreferences = context.watch<UserPreferences>();
+    final AppLocalizations appLocalizations = AppLocalizations.of(context)!;
+    final List<Widget> pageData = <Widget>[
+      Padding(
+        padding: const EdgeInsets.only(
+          right: LARGE_SPACE,
+          left: LARGE_SPACE,
+          bottom: LARGE_SPACE,
+        ),
+        child: Text(
+          appLocalizations.productDataUtility,
+          style: Theme.of(context).textTheme.headline2!.apply(
+                color: Colors.black,
               ),
+        ),
+      ),
+      Container(
+        height: _isProductExpanded ? null : 150,
+        padding: const EdgeInsets.only(
+          bottom: LARGE_SPACE,
+          right: LARGE_SPACE,
+          left: LARGE_SPACE,
+        ),
+        child: GestureDetector(
+          onTap: () => _expandProductCard(),
+          child: SummaryCard(
+            widget.product,
+            productPreferences,
+            isFullVersion: _isProductExpanded,
+          ),
+        ),
+      ),
+    ];
+    pageData.addAll(
+      UserPreferencesFood(
+        productPreferences: productPreferences,
+        setState: setState,
+        context: context,
+        userPreferences: userPreferences,
+        appLocalizations: appLocalizations,
+        themeData: Theme.of(context),
+      ).getContent(),
+    );
+    return Scaffold(
+      body: Stack(
+        children: <Widget>[
+          ListView(
+            // bottom padding is very large because [NextButton] is stacked on top of the page.
+            padding: const EdgeInsets.only(
+              top: LARGE_SPACE,
+              bottom: VERY_LARGE_SPACE * 5,
             ),
-          ];
-          pageData.addAll(UserPreferencesFood(
-            productPreferences: productPreferences,
-            setState: setState,
-            context: context,
-            userPreferences: userPreferences,
-            appLocalizations: appLocalizations,
-            themeData: Theme.of(context),
-          ).getContent());
-          return Scaffold(
-            body: Stack(
-              children: <Widget>[
-                ListView(
-                  // bottom padding is very large because [NextButton] is stacked on top of the page.
-                  padding: const EdgeInsets.only(
-                    top: LARGE_SPACE,
-                    bottom: VERY_LARGE_SPACE * 5,
-                  ),
-                  shrinkWrap: true,
-                  children: pageData,
-                ),
-                const Positioned(
-                  child: Align(
-                    alignment: Alignment.bottomCenter,
-                    child: NextButton(OnboardingPage.PREFERENCES_PAGE),
-                  ),
-                ),
-              ],
+            shrinkWrap: true,
+            children: pageData,
+          ),
+          const Positioned(
+            child: Align(
+              alignment: Alignment.bottomCenter,
+              child: NextButton(OnboardingPage.PREFERENCES_PAGE),
             ),
-          );
-        });
+          ),
+        ],
+      ),
+    );
   }
 
   void _expandProductCard() {
     if (!_isProductExpanded) {
-      setState(() {
-        _isProductExpanded = true;
-      });
+      setState(() => _isProductExpanded = true);
     }
   }
 }