Fix computed headers not updating, improve logout

lib/stores/auth_store.dart

@@ -14,13 +14,13 @@ abstract class _AuthBase with Store {
   /// Secure storage to store tokens.
   final _storage = const FlutterSecureStorage();
 
-  /// The current token.
-  String? _token;
-  String? get token => _token;
-
   /// Whether the token is valid or not.
   var _tokenIsValid = false;
 
+  /// The current token.
+  @readonly
+  String? _token;
+
   /// The current user's info.
   @readonly
   UserTwitch? _user;
@@ -115,8 +115,24 @@ abstract class _AuthBase with Store {
     // Set the login status to logged out.
     _isLoggedIn = false;
 
-    // If the default token already exists, set it. Otherwise, get a new default token.
-    _token = await _storage.read(key: 'DEFAULT_TOKEN') ?? await Twitch.getDefaultToken();
+    // If the default token already exists, set it.
+    _token = await _storage.read(key: 'DEFAULT_TOKEN');
+
+    // If the default token does not already exist, get the new default token and store it.
+    // Else, validate the existing token.
+    if (_token == null) {
+      await Twitch.getDefaultToken();
+      await _storage.write(key: 'DEFAULT_TOKEN', value: _token);
+    } else {
+      // Validate the stored token.
+      _tokenIsValid = await Twitch.validateToken(token: _token!);
+
+      // If the stored token is invalid, get a new default token and store it.
+      if (!_tokenIsValid) {
+        await Twitch.getDefaultToken();
+        await _storage.write(key: 'DEFAULT_TOKEN', value: _token);
+      }
+    }
 
     debugPrint('Successfully logged out');
   }

lib/stores/auth_store.g.dart

@@ -17,6 +17,23 @@ mixin _$AuthStore on _AuthBase, Store {
               name: '_AuthBase.headersTwitch'))
       .value;
 
+  final _$_tokenAtom = Atom(name: '_AuthBase._token');
+
+  String? get token {
+    _$_tokenAtom.reportRead();
+    return super._token;
+  }
+
+  @override
+  String? get _token => token;
+
+  @override
+  set _token(String? value) {
+    _$_tokenAtom.reportWrite(value, super._token, () {
+      super._token = value;
+    });
+  }
+
   final _$_userAtom = Atom(name: '_AuthBase._user');
 
   UserTwitch? get user {