reset direct dns when region change

lib/features/config_option/data/config_option_repository.dart

@@ -71,6 +71,7 @@ abstract class ConfigOptions {
   static final directDnsAddress = PreferencesNotifier.create<String, String>(
     "direct-dns-address",
     "1.1.1.1",
+    defaultValueFunction: (ref) => ref.read(region) == Region.cn ? "223.5.5.5" : "1.1.1.1",
     validator: (value) => value.isNotBlank,
   );
 

lib/features/config_option/overview/config_options_page.dart

@@ -146,6 +146,7 @@ class ConfigOptionsPage extends HookConsumerWidget {
                     choices: Region.values,
                     title: t.settings.general.region,
                     presentChoice: (value) => value.present(t),
+                    onChanged: (val) => ref.watch(ConfigOptions.directDnsAddress.notifier).reset(),
                   ),
                   SwitchListTile(
                     title: Text(experimental(t.config.blockAds)),

lib/features/config_option/widget/preference_tile.dart

@@ -61,6 +61,7 @@ class ChoicePreferenceWidget<T> extends StatelessWidget {
     required this.title,
     required this.presentChoice,
     this.validateInput,
+    this.onChanged,
   });
 
   final T selected;
@@ -70,7 +71,7 @@ class ChoicePreferenceWidget<T> extends StatelessWidget {
   final String title;
   final String Function(T value) presentChoice;
   final bool Function(String value)? validateInput;
-
+  final ValueChanged<T>? onChanged;
   @override
   Widget build(BuildContext context) {
     return ListTile(
@@ -88,7 +89,9 @@ class ChoicePreferenceWidget<T> extends StatelessWidget {
         if (selection == null) {
           return;
         }
-        await preferences.update(selection);
+        final out = await preferences.update(selection);
+        onChanged?.call(selection);
+        return out;
       },
     );
   }