ReactiveUI RoutedViewHost for Xamarin-Android

RoutedViewHost.cs

using System;
using System.Reactive.Linq;
using Android.App;
using ReactiveUI;

namespace ReactiveUI
{
    class RoutedViewHost : ReactiveObject
    {
        RoutingState _router;
        public RoutingState Router
        {
            get { return _router; }
            set { this.RaiseAndSetIfChanged(ref _router, value); }
        }

        IObservable<string> _viewContractObservable;
        public IObservable<string> ViewContractObservable
        {
            get { return _viewContractObservable; }
            set { this.RaiseAndSetIfChanged(ref _viewContractObservable, value); }
        }

        Fragment _defaultContent;
        public Fragment DefaultContent
        {
            get { return _defaultContent; }
            set { this.RaiseAndSetIfChanged(ref _defaultContent, value); }
        }

        public IViewLocator ViewLocator { get; set; }

        public RoutedViewHost(FragmentManager fragmentManager, int containerId)
        {
            Fragment viewLastAdded = null;

            ViewContractObservable = Observable.Return(default(string));

            var vmAndContract = this.WhenAnyObservable(x => x.Router.CurrentViewModel)
                .CombineLatest(this.WhenAnyObservable(x => x.ViewContractObservable),
                (vm, contract) => new { ViewModel = vm, Contract = contract, });

            vmAndContract.Subscribe(x =>
            {
                if (viewLastAdded != null)
                    fragmentManager.BeginTransaction().Remove(viewLastAdded).Commit();

                if (x.ViewModel == null)
                {
                    if (DefaultContent != null)
                    {
                        fragmentManager.BeginTransaction().Add(containerId, DefaultContent).Commit();
                    }
                    return;
                }

                var viewLocator = ViewLocator ?? ReactiveUI.ViewLocator.Current;
                var view = viewLocator.ResolveView(x.ViewModel, x.Contract) ?? viewLocator.ResolveView(x.ViewModel);
                view.ViewModel = x.ViewModel;

                var added = view as Fragment;
                if (added != null)
                {
                    viewLastAdded = added;
                }
                else
                {
                    throw new Exception(String.Format("'{0}' must be a Fragment", view.GetType().FullName));
                }

                fragmentManager.BeginTransaction().Add(containerId, viewLastAdded).Commit();
            }, RxApp.DefaultExceptionHandler.OnNext);
        }
    }
}