feat: Implement access token migration from legacy SharedPreferences to new SharedPreferencesAsync

Author: romaingyh

packages/supabase_flutter/lib/src/local_storage.dart

@@ -78,6 +78,21 @@ class SharedPreferencesLocalStorage extends LocalStorage {
     if (!_useWebLocalStorage) {
       WidgetsFlutterBinding.ensureInitialized();
       _prefs = SharedPreferencesAsync();
+
+      await _maybeMigrateAccessToken();
+    }
+  }
+
+  Future<void> _maybeMigrateAccessToken() async {
+    final legacyPrefs = await SharedPreferences.getInstance();
+
+    if (legacyPrefs.containsKey(persistSessionKey)) {
+      final accessToken = legacyPrefs.getString(persistSessionKey);
+
+      if (accessToken != null) {
+        await legacyPrefs.remove(persistSessionKey);
+        await _prefs.setString(persistSessionKey, accessToken);
+      }
     }
   }
 
@@ -118,34 +133,34 @@ class SharedPreferencesLocalStorage extends LocalStorage {
 /// local storage to store pkce flow code verifier.
 class SharedPreferencesGotrueAsyncStorage extends GotrueAsyncStorage {
   SharedPreferencesGotrueAsyncStorage() {
-    _initialize();
+    initialize();
   }
 
-  final Completer<void> _initializationCompleter = Completer();
+  final Completer<void> initializationCompleter = Completer();
 
-  late final SharedPreferences _prefs;
+  late final SharedPreferences prefs;
 
-  Future<void> _initialize() async {
+  Future<void> initialize() async {
     WidgetsFlutterBinding.ensureInitialized();
-    _prefs = await SharedPreferences.getInstance();
-    _initializationCompleter.complete();
+    prefs = await SharedPreferences.getInstance();
+    initializationCompleter.complete();
   }
 
   @override
   Future<String?> getItem({required String key}) async {
-    await _initializationCompleter.future;
-    return _prefs.getString(key);
+    await initializationCompleter.future;
+    return prefs.getString(key);
   }
 
   @override
   Future<void> removeItem({required String key}) async {
-    await _initializationCompleter.future;
-    await _prefs.remove(key);
+    await initializationCompleter.future;
+    await prefs.remove(key);
   }
 
   @override
   Future<void> setItem({required String key, required String value}) async {
-    await _initializationCompleter.future;
-    await _prefs.setString(key, value);
+    await initializationCompleter.future;
+    await prefs.setString(key, value);
   }
 }