Dart singleton explained. Try it out in https://dartpad.dev/

singleton.dart

import 'dart:math';

class Randy {
  String say;
  int nr;
  
  // Note: Randy constructs with a RANDOM number
  Randy(say) {
    this.say = say;

    int random = Random().nextInt(999);
    print('Construct Randy with number: $random');
    this.nr = random;
  }
    
  String getSome() {
    return '$say: $nr!';
  }
}

// Single is a singleton class
class Single {
  // It has a property that will be an instance of a normal class
  Randy randy;
  
  // This construct will make sure only 1 instance can ever exist
  Single._singleton();
  static final Single _instance = Single._singleton();

  // The singleton factory method allows to pass initialisation to the singleton
  factory Single(String say) {
    // Here initialise any instance properties on creation of singleton
    _instance.randy = Randy(say);
    return _instance;
  }
}

void main() {
  // The first call will work as expected,
  // printing `First call: <random number>`
  Single single = Single('First init');
  print(single.randy.getSome());
  
  // However when initialising again, even with a new variable (single2)
  // the singleton properties are overwritten and BOTH variables will
  // produce the same output
  Single single2 = Single('Second init');
  
  // The second init overwrites the first
  print(single.randy.getSome());
  print(single2.randy.getSome());
}

The Single singleton class in this example uses a factory constructor. See other examples in this post: https://stackoverflow.com/a/55348216/1783174