DART

singleton in dart

class FileSystemManager {
  static final FileSystemManager _instance = FileSystemManager._internal();
 
  // using a factory is important
  // because it promises to return _an_ object of this type
  // but it doesn't promise to make a new one.
  factory FileSystemManager() {
    return _instance();
  }
  
  // This named constructor is the "real" constructor
  // It'll be called exactly once, by the static property assignment above
  // it's also private, so it can only be called in this class
  FileSystemManager._internal() {
    // initialization logic 
  }
  
  // rest of class as normal, for example:
  void openFile() {}
  void writeFile() {}
}

singleton classes in dart example

class Singleton {
  static Singleton _instance;

  Singleton._internal() {
    _instance = this;
  }

  factory Singleton() => _instance ?? Singleton._internal();
}

dart singleton

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