class Transient<T> {
  T _value;

  Transient(T value) {
    this._value = value;
  }

  void use(Function(T value) block) {
    assert(block != null);
    if (_value != null) {
      block(_value);
      _value = null;
    }
  }
}

class Event {
  Object _flag = Object();

  void consume(Function block) {
    assert(block != null);
    if (_flag != null) {
      block();
      _flag = null;
    }
  }
}