Using Dart's factory constructor feature to implement the singleton pattern

singleton.dart

class MyClass {
  static final MyClass _singleton = new MyClass._internal();

  factory MyClass() {
    return _singleton;
  }

  MyClass._internal() {
    ... // initialization logic here
  }

  ... // rest of the class
}

// consuming code
MyClass myObj = new MyClass(); // get back the singleton
... 
// another piece of consuming code
MyClass myObj = new MyClass(); // still getting back the singleton

@ascmp Trying to solve exactly the same problem with AppConfig as a singleton using params from relevant json files.
Have you found any elegant solutions?..

I actually came up with something like this:

import 'dart:async';

import 'package:rxdart/rxdart.dart';
import 'package:example/app_config.dart';

class ApplicationConfigurationService {
  static final ApplicationConfigurationService _instance =
      ApplicationConfigurationService._singelton();

  static final _behaviorSubject = BehaviorSubject<AppConfig>();
  final StreamSink<AppConfig> configStreamSink = _behaviorSubject.sink;
  final Stream<AppConfig> configStream = _behaviorSubject.stream;

  factory ApplicationConfigurationService() => _instance;

  ApplicationConfigurationService._singelton();
}

I am writing a command-line program and in the main.dart I'am writing on the StreamSink of my AppConfigService after parsing my JSON file. Due the service is using a BehaviorSubject the last value will be cached and replayed to new subscribers / listeners.

Later when I need the config values I do the following:

final applicationConfigurationService = ApplicationConfigurationService();
...
applicationConfigurationService.configStream.first.then((config) {
      _appConfig = config;
});

Or if you want you can even react to config changes when you need that feature.

I think, one could use the static instance variable also.
MyClass myObj1 = MyClass._singleton; // get back the singleton

This feels like an antipattern, singleton should be implemented with a private constructor and a static singleton class.
Having a constructor that returns the singleton on behind hides this detail from the developer.

The pattern proposed by @theburningmonk actually has a problem.

I just spent twenty minutes chasing down a bug which this pattern obscured.

The problem was that my _internal method was throwing an exception.
In the exception handler I was trying to reference the same factory instance. This caused an npe as _singleton was null (due to the _internal exception) so I got an npe rather than the original exception.

The following pattern avoids this issue:

class MyClass {
  static  MyClass _singleton;

  factory MyClass() => _singleton ??= MyClass._internal();

  MyClass._internal() {
    ... // initialization logic here
  }

  ... // rest of the class
}

The only disadvantage is that _singleton can no longer be final.

Singleton design pattern is intended to create one and only one (single) instance of a class.

If you wanted a single instance with a parameter, how would you do that?

Do you see any issue in this code?

class MyClass  {
  MyClass._(this.parameter);
  final String parameter;

  static MyClass? _instance;

  static void initialize(String parameter) {
    _instance = MyClass._(parameter );
  }

  static MyClass get instance => _instance!; // throw an "initialize first" error

  // access parameter 
}

Singleton design pattern is intended to create one and only one (single) instance of a class.

If you wanted a single instance with a parameter, how would you do that?

Do you see any issue in this code?
This is not a singleton implementation.

See this for reference: Dart Null-Safty Singleton with parameters

Can MyClass extend an abstract class with non-null constructor? If so how?