pressed method Null safety

Implementation

@override
void pressed(LimitedState arg) {
  final p = _pressed;
  if (p != null) {
    p(arg as ActiveState);
  }
  // Note the downcast.  LimitedState implementations need to ensure that
  // this method is only called on LimitedOperation instances.  As long as
  // that invariant holds, this bit of covariance increases static checking,
  // because it ensures that LimtedState pressed functions don't call any of
  // the methods declared lower in the hierarchy (like handleCHS or, notably,
  // the methods related to stack lift).  This simplifies reasoning about the
  // state machine, and avoids a bunch of null handleXXX methods in states
  // that don't use them, but it does come at the prices of a little less
  // static type safety.
  //
  // See ArgInputState._buttonDown and ProgramEntry._buttonDown to see how
  // they robustly guarantee that the covariant relationship isn't violated.
}