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

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?