public NavigatorViewModel(IObservable <int> currentPage, IObservable <int> pageSize)
{
Size = new ReactiveProperty <int>(pageSize);
var obs = currentPage.DistinctUntilChanged().CombineLatest(Size, (a, b) =>
new { page = a, size = b });
Output = NextCommand.WithLatestFrom(obs, (a, b) => new PageRequest(b.page + 1, b.size)).Merge
(PreviousCommand.WithLatestFrom(obs, (a, b) =>
new PageRequest(b.page - 1, b.size))).StartWith(new PageRequest(1, 25)).ToReactiveProperty();
}