Замыкание

main.dart

/// https://gist.github.com/dened/3385fd713c1fffcd23057db44b9545c9
/// https://dartpad.dev/3385fd713c1fffcd23057db44b9545c9

typedef Inc = int Function();

Inc intrementer() {
  var i = 0;

  return () {
    i++;
    return i;
  };
}

void main() {
  final increment = intrementer();

  print('hello ${increment()}');
  print('hello ${increment()}');
  print('hello ${increment()}');
  print('hello ${increment()}');
}