Singleton classes in dart from dartpad

gistfile1.txt

class MySingleton {
  int resource = 5;
  
  // Singleton pattern
  static final MySingleton _singleton = new MySingleton._internal();

  factory MySingleton({int newResource}) {
    // Called everytime an object is created.
    // If an instance of the class already exists, then returns the existing reference,
    // else creates a new reference and returns it.
    print ("singlton factory constructor");
    if (newResource != null) _singleton.resource = newResource;
    return _singleton;
  }

  // Name doesn't have to be _internal, just a function private to the class
  MySingleton._internal() {
    // This is called only the first time
    print ("private internal function");
  }
  
  // Getter
  get getResource => resource;
  
  // Setter
  set setResource(int newResource) {
    resource = newResource;
  }
}

main() {
  print ("First call");
  MySingleton a = new MySingleton(newResource: 4);
  print (a.getResource);
  print ("");
  
  print ("Second call");
  // New just gets the earlier reference
  MySingleton b = new MySingleton();
  print (a.getResource);
  print (b.getResource);
  print ("Setting a new value using the setter");
  b.setResource = 9;
  print (a.getResource);
  print (b.getResource);
  print ("");
  
  print ("Third call");
  MySingleton c = new MySingleton();
  print (a.getResource);
  print (b.getResource);
  print (c.getResource);
  print ("");
  
  print ("Fourth call, with constructor initialization");
  MySingleton d = new MySingleton(newResource: 10);
  print (a.getResource);
  print (b.getResource);
  print (c.getResource);
  print (d.getResource);
}

/// Output:

First call
singlton factory constructor
private internal function
4

Second call
singlton factory constructor
4
4
Setting a new value using the setter
9
9

Third call
singlton factory constructor
9
9
9

Fourth call, with constructor initialization
singlton factory constructor
10
10
10
10