sclass OnStableValue<A> extends WaitForStableValue<A> {
  long period;
  A value;
  long valueSince;
  O runnable;
  A lastStableValue;
  
  *() {}
  *(long *period, O *runnable) {}
  *(double seconds, O *runnable) { period = toMS(seconds); }
  
  synchronized void set(A a) {
    if (neq(a, value)) {
      value = a;
      valueSince = sysNow();
      doLater(period, r check); // TODO: slightly dicey
    }
  }
  
  synchronized void check() {
    A a = get();
    if (neq(a, lastStableValue)) {
      lastStableValue = a;
      callF(runnable, a);
    }
  }
  
  synchronized A get() {
    ret valueSince != 0 && sysNow() >= valueSince+period ? value : null;
  }
  
  public bool has() { ret get() != null; }
}