Easy to manage access tokens and refresh tokens

token_store.dart

class VariableTokenStore<Token> {
  /// validate token is valid
  /// - true = ok
  /// - false = not ok
  /// - neverthrow
  final Future<bool> Function(Token token) checkToken;

  /// fetch a new token
  /// - null = require authenticate
  /// - not null = ok
  /// - neverthrow
  final Future<Token?> Function(Token? oldToken) fetchToken;

  VariableTokenStore(this.checkToken, this.fetchToken);

  Token? token;

  Token? getToken() {
    return this.token;
  }

  void setToken(Token token) {
    this.token = token;
  }

  void clearToken() {
    this.token = null;
  }

  /// get or refresh this store
  /// - not null = ok
  /// - null = require authenticate
  /// - neverthrow
  Future<Token?> check() {
    return Future.sync(() async {
      final token = this.token;
      if (token == null) {
        this.token = await fetchToken(token);
      } else if (!await checkToken(token)) {
        this.token = await fetchToken(token);
      }
      return this.token;
    });
  }
}

token_store_test.dart

void main() {
  test("basic", () async {
    var version = 1;
    final tokenStore = VariableTokenStore<Map<String, dynamic>>(
      (token) => Future.value(token["version"] == version),
      (old) => Future.value(version <= 2 ? {"version": version} : null),
    );

    expect(tokenStore.getToken(), null);
    await tokenStore.check(); // refresh success
    expect(tokenStore.getToken()?["version"], version);

    expect((await tokenStore.check())?["version"], version); // check success

    version++;
    expect((await tokenStore.check())?["version"], version); // refresh success

    version++;
    expect((await tokenStore.check()), null); // should login
  });
}