Explain the Singleton pattern concept and its main features.

The pattern Singleton guarantees that a class has only one instance and provides a global access point to it. This is useful for managing centralized states or resources, such as application settings or database managers.

Main features of Singleton:

• Single instance: the class controls the creation of the object and prevents creating more than one.
• Global accessibility: the instance is accessible from anywhere in the program via a static method or property.
• Lazy initialization: the object is created upon first access.

Example implementation of Singleton in Dart (Flutter):

class Singleton {
  Singleton._privateConstructor();

  static final Singleton _instance = Singleton._privateConstructor();

  factory Singleton() {
    return _instance;
  }

  void someMethod() {
    print('Calling Singleton method');
  }
}

void main() {
  var s1 = Singleton();
  var s2 = Singleton();
  print(identical(s1, s2)); // true
}

Here, the constructor is private, and access to the instance is through a factory constructor that always returns the same object.