public void adding_an_item_updates_the_current_delta_type_if_it_has_changed()
{
var model = new DoubleCollectionModel();
var observer = model.CreateObserver();
model.Add(1d);
Assert.Equal(DeltaType.None, model.CurrentDeltaType);
model.Add(2d);
Assert.Equal(DeltaType.Increase, model.CurrentDeltaType);
model.Add(1d);
Assert.Equal(DeltaType.Decrease, model.CurrentDeltaType);
model.Add(1d);
Assert.Equal(DeltaType.None, model.CurrentDeltaType);
Assert.Equal(3, observer.GetChangeCount(x => x.CurrentDeltaType));
}