你可以用两种方法来做到这一点。
1.您可以使用API。
1.我不知道这是否是标准的，但你可以在用户每次打开应用时执行silent login，静默日志在没有用户交互的情况下登录到用户帐户，这样你就有了一个新的令牌。

Future<String> refreshToken() async {
    print("Token Refresh");
    final GoogleSignInAccount googleSignInAccount =
        await googleSignIn.signInSilently();
    final GoogleSignInAuthentication googleSignInAuthentication =
        await googleSignInAccount.authentication;

    final AuthCredential credential = GoogleAuthProvider.getCredential(
      accessToken: googleSignInAuthentication.accessToken,
      idToken: googleSignInAuthentication.idToken,
    );
    final AuthResult authResult = await auth.signInWithCredential(credential);

    return googleSignInAuthentication.accessToken; // New refreshed token
  }

手动获取刷新令牌
apiKey -来自firebase的API密钥，idToken -未过期的ID令牌，提供商-“google.com”、“apple.com”等

final url = Uri.parse('https://identitytoolkit.googleapis.com/v1/accounts:signInWithIdp?key=$apiKey');

final response = await http.post(
  url,
  headers: {'Content-type': 'application/json'},
  body: jsonEncode({
    'postBody': 'id_token=$token&providerId=$provider',
    'requestUri': 'http://localhost',
    'returnIdpCredential': true,
    'returnSecureToken': true
  })
);
if (response.statusCode != 200) {
  throw 'Refresh token request failed: ${response.statusCode}';
}

final data = Map<String, dynamic>.of(jsonDecode(response.body));
if (data.containsKey('refreshToken')) {
   // here is your refresh token, store it in a secure way
} else {
  throw 'No refresh token in response';
}