Dart callback

App.dart

main(List<String> args) {

  Button btn = new Button();
  btn.setCallback((a){
    print(a);
  });
  for(int i=0;i<4;++i){
    btn.tap();
  }
}

class Button {

  int _count = 0;

  Function(int) f;

  void tap(){
    ++_count;
    if(f!=null && (_count%2) == 0){
      f(_count);
    }
  }

  int getCount(){
    return _count;
  }

  void setCallback(Function(int) f){
    this.f = f;
  }
}