import 'dart:convert';
import 'package:googleapis_auth/auth_io.dart';

class AccessTokenManager {
  static final AccessTokenManager _instance = AccessTokenManager._internal();
  late final String serviceAccountJsonKey;
  AccessToken? _accessToken;
  DateTime? _expiryDate;

  AccessTokenManager._internal();

  factory AccessTokenManager(String jsonKey) {
    if (_instance._isServiceAccountKeyInitialized()) {
      // Prevent re-initialization
      return _instance;
    }
    _instance.serviceAccountJsonKey = jsonKey;
    return _instance;
  }

  bool _isServiceAccountKeyInitialized() {
    try {
      serviceAccountJsonKey; // Access to check if initialized
      return true;
    } catch (e) {
      return false;
    }
  }

  Future<String> getAccessToken() async {
    if (_accessToken != null && DateTime.now().isBefore(_expiryDate!)) {
      return _accessToken!.data;
    }
    try {
      final serviceAccountCredentials = ServiceAccountCredentials.fromJson(
          json.decode(serviceAccountJsonKey));
      final client = await clientViaServiceAccount(
        serviceAccountCredentials,
        ['https://www.googleapis.com/auth/firebase.messaging'],
      );

      _accessToken = client.credentials.accessToken;
      _expiryDate = client.credentials.accessToken.expiry;
      client.close();
      return _accessToken!.data;
    } catch (e) {
      throw Exception('Failed to obtain access token');
    }
  }
}