protected void Unmonitor(DirtifiableObject dirtifiable)
{
var sub = _monitored[dirtifiable];
sub.Dispose();
_monitored.Remove(dirtifiable);
}
protected void Monitor(DirtifiableObject dirtifiable)
{
var sub = dirtifiable.Dirtied.Subscribe(_ => MarkDirty());
_monitored.Add(dirtifiable, sub);
}