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()) {
      print('Service account key already initialized.');
      return _instance;
    }
    print('Initializing service account key.');
    _instance.serviceAccountJsonKey = jsonKey;
    return _instance;
  }

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

  Future<String> getAccessToken() async {
    print('Attempting to get a new access token...');

    try {
      // Parse service account credentials from JSON
      print('Parsing service account credentials...');
      final serviceAccountCredentials = ServiceAccountCredentials.fromJson(
          json.decode(serviceAccountJsonKey));

      // Log service account email (or other non-sensitive information)
      print('Service account email: ${serviceAccountCredentials.email}');

      // Create an authenticated client via the service account
      print('Creating authenticated client via service account...');
      final client = await clientViaServiceAccount(
        serviceAccountCredentials,
        ['https://www.googleapis.com/auth/firebase.messaging'],
      );

      // Log successful client creation
      print('Authenticated client created successfully.');

      // Update the access token and expiry date
      _accessToken = client.credentials.accessToken;
      _expiryDate = client.credentials.accessToken.expiry;

      // Log the obtained token and expiry time
      print('Access token obtained: ${_accessToken!.data}');
      print('Token expiry date: $_expiryDate');

      // Close the client to prevent resource leaks
      print('Closing authenticated client...');
      client.close();

      // Return the newly fetched access token
      return _accessToken!.data;
    } catch (e) {
      // Log error if token fetch fails
      print('Failed to obtain a new access token: $e');
      throw Exception('Failed to obtain a new access token: $e');
    }
  }
}