add expansion state restoration example

example/lib/main.dart

@@ -11,6 +11,7 @@ Future<void> main() async {
   runApp(
     MultiProvider(
       providers: [
+        Provider.value(value: prefs),
         ChangeNotifierProvider(create: (_) => SettingsController(prefs)),
         ChangeNotifierProvider(create: (_) => SelectedExampleNotifier(prefs)),
       ],

example/lib/src/examples.dart

@@ -8,6 +8,7 @@ import 'examples/drag_and_drop.dart' show DragAndDropTreeView;
 import 'examples/filterable.dart' show FilterableTreeView;
 import 'examples/lazy_loading.dart' show LazyLoadingTreeView;
 import 'examples/minimal.dart' show MinimalTreeView;
+import 'examples/restorable.dart' show RestorableTreeView;
 import 'settings/controller.dart' show SettingsController;
 import 'shared.dart' show IndentGuideType, LineStyle, enumByName;
 
@@ -37,6 +38,7 @@ enum Example {
   filterable('Filterable', Icon(Icons.manage_search_rounded)),
   lazyLoading('Lazy Loading', Icon(Icons.hourglass_top_rounded)),
   minimal('Minimal', Icon(Icons.segment)),
+  restorable('Restorable', Icon(Icons.restore)),
   ;
 
   const Example(this.title, this.icon);
@@ -62,6 +64,7 @@ class ExamplesView extends StatelessWidget {
           Example.filterable => const FilterableTreeView(),
           Example.lazyLoading => const LazyLoadingTreeView(),
           Example.minimal => const MinimalTreeView(),
+          Example.restorable => const RestorableTreeView(),
         },
       ),
     );

example/lib/src/examples/restorable.dart

@@ -0,0 +1,112 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_fancy_tree_view/flutter_fancy_tree_view.dart';
+import 'package:provider/provider.dart';
+import 'package:shared_preferences/shared_preferences.dart';
+
+const String expandedIdsKey = 'fftv.restorableTreeController.expandedIds';
+
+class RestorableTreeController extends TreeController<Node> {
+  RestorableTreeController({
+    required super.roots,
+  }) : super(childrenProvider: (Node node) => node.children);
+
+  late final Set<String> expandedIds = <String>{};
+
+  void restoreExpandedIds(SharedPreferences prefs) {
+    final List<String>? storedExpandedIds = prefs.getStringList(expandedIdsKey);
+
+    if (storedExpandedIds != null) {
+      expandedIds.addAll(storedExpandedIds);
+    }
+  }
+
+  void persistExpandedIds(SharedPreferences prefs) {
+    prefs.setStringList(expandedIdsKey, expandedIds.toList());
+  }
+
+  @override
+  bool getExpansionState(Node node) => expandedIds.contains(node.id);
+
+  @override
+  void setExpansionState(Node node, bool expanded) {
+    expanded ? expandedIds.add(node.id) : expandedIds.remove(node.id);
+  }
+}
+
+class Node {
+  Node({required this.id, this.children = const <Node>[]});
+
+  final String id;
+  final List<Node> children;
+}
+
+class RestorableTreeView extends StatefulWidget {
+  const RestorableTreeView({super.key});
+
+  @override
+  State<RestorableTreeView> createState() => _RestorableTreeViewState();
+}
+
+class _RestorableTreeViewState extends State<RestorableTreeView> {
+  late final RestorableTreeController treeController;
+  late final SharedPreferences prefs = context.read<SharedPreferences>();
+
+  final Node root = Node(id: '0', children: [
+    Node(id: '1', children: [
+      Node(id: '2', children: [
+        Node(id: '3', children: [
+          Node(id: '4'),
+          Node(id: '5', children: [
+            Node(id: '6'),
+            Node(id: '7', children: [
+              Node(id: '8'),
+            ]),
+          ]),
+        ]),
+        Node(id: '9'),
+      ]),
+      Node(id: '10'),
+    ]),
+    Node(id: '11'),
+  ]);
+
+  @override
+  void initState() {
+    super.initState();
+    treeController = RestorableTreeController(roots: <Node>[root]);
+    treeController.restoreExpandedIds(prefs);
+  }
+
+  @override
+  void dispose() {
+    treeController.persistExpandedIds(prefs);
+    treeController.dispose();
+    super.dispose();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return TreeView<Node>(
+      treeController: treeController,
+      nodeBuilder: (BuildContext context, TreeEntry<Node> entry) {
+        return TreeIndentation(
+          entry: entry,
+          child: Row(
+            children: [
+              FolderButton(
+                key: Key('FolderButton#${entry.node.id}'),
+                isOpen: entry.hasChildren ? entry.isExpanded : null,
+                onPressed: entry.hasChildren
+                    ? () => treeController.toggleExpansion(entry.node)
+                    : null,
+              ),
+              Flexible(
+                child: Text('Node #${entry.node.id}'),
+              ),
+            ],
+          ),
+        );
+      },
+    );
+  }
+}