Flutter making singleton class and getting Non-nullable instance field error

Question

Below is a simple class which i want to wrap dio class is singleton, as i enabled analyzer i get this error:

Non-nullable instance field '_instance' must be initialized. (Documentation) Try adding an initializer expression, or add a field initializer in this constructor, or mark it 'late'.

my code:

const baseApiUrl = 'http://localhost';

class BaseDio {
  BaseDio._();

  static BaseDio _instance;

  static BaseDio getInstance() {
    return _instance;
  }

  final cacheManager = DioCacheManager(CacheConfig(baseUrl: baseApiUrl));
  final BaseOptions options = BaseOptions(
    connectTimeout: 20000,
    receiveTimeout: 30000,
  );

  Dio getDio() {
    final dio = Dio(options)
      ..interceptors.addAll(
        [

          PrettyDioLogger(
            requestHeader: true,
            requestBody: true,
            responseBody: true,
            responseHeader: false,
            compact: false,
          ),

          cacheManager.interceptor as Interceptor,
          LogInterceptor(responseBody: false),
        ],
      );

    return dio;
  }
}

how can i resolve this problem for analyzer ?

Yuri H · Accepted Answer · 2021-07-15 12:45:52Z

1

You can use a factory method to create a singleton in Dart:

class BaseDio {
  static final BaseDio _singleton = BaseDio._internal();
  final int value = 42;

  factory BaseDio() => _singleton;

  BaseDio._internal() {
    // private constructor that creates the singleton instance
  }
}

final baseDioSingleton = BaseDio();
print(baseDioSingleton.value);

edited Jul 15, 2021 at 12:45

answered Jul 14, 2021 at 16:15

Yuri H

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2 Comments

DolDurma Over a year ago

Thanks, how can i access to _singleton from outside of this class now? for example BaseDio.getInstance().getDio()

Yuri H Over a year ago

The factory method call BaseDio() returns the _singleton value. The last string in the example code illustrates that

Lucas Josino · Accepted Answer · 2021-07-14 16:13:55Z

0

Non-nullable instance field '_instance' must be initialized.

Add a value to _instance or make it nullable:

BaseDio _instance = someValue;

or

static BaseDio? _instance;

answered Jul 14, 2021 at 16:13

Lucas Josino

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