/ReactiveUI.Routing/RoutingState.cs

using System;
using System.Linq;
using System.Reactive;
using System.Reactive.Concurrency;
using System.Reactive.Linq;
using System.Reactive.Subjects;
using System.Runtime.Serialization;
using System.Windows.Input;
using ReactiveUI.Xaml;

namespace ReactiveUI.Routing
{
    /// <summary>
    /// RoutingState manages the ViewModel Stack and allows ViewModels to
    /// navigate to other ViewModels.
    /// </summary>
    [DataContract]
    public class RoutingState : ReactiveObject, IRoutingState
    {
        [field: IgnoreDataMember]
        bool rxObjectsSetup = false;
        [IgnoreDataMember] ReactiveCollection<IRoutableViewModel> _NavigationStack;

        /// <summary>
        /// Represents the current navigation stack, the last element in the
        /// collection being the currently visible ViewModel.
        /// </summary>
        [DataMember]
        public ReactiveCollection<IRoutableViewModel> NavigationStack {
            get { return _NavigationStack; }
            protected set { _NavigationStack = value; }
        }

        /// <summary>
        /// Navigates back to the previous element in the stack.
        /// </summary>
        [IgnoreDataMember]
        public IReactiveCommand NavigateBack { get; protected set; }

        /// <summary>
        /// Navigates to the a new element in the stack - the Execute parameter
        /// must be a ViewModel that implements IRoutableViewModel.
        /// </summary>
        [IgnoreDataMember]
        public INavigateCommand Navigate { get; protected set; }

        /// <summary>
        /// Navigates to a new element and resets the navigation stack (i.e. the
        /// new ViewModel will now be the only element in the stack) - the
        /// Execute parameter must be a ViewModel that implements
        /// IRoutableViewModel.
        /// </summary>
        [IgnoreDataMember]
        public INavigateCommand NavigateAndReset { get; protected set; }

        public RoutingState()
        {
            _NavigationStack = new ReactiveCollection<IRoutableViewModel>();
            setupRx();
        }

        [OnDeserialized]
#if WP7
        public
#endif
        void setupRx(StreamingContext sc) { setupRx();  }
        void setupRx()
        {
            if (rxObjectsSetup) return;

            NavigateBack = new ReactiveCommand(
                NavigationStack.CollectionCountChanged.StartWith(_NavigationStack.Count).Select(x => x > 0));
            NavigateBack.Subscribe(_ =>
                NavigationStack.RemoveAt(NavigationStack.Count - 1));

            Navigate = new NavigationReactiveCommand();
            Navigate.Subscribe(x => {
                var vm = x as IRoutableViewModel;
                if (vm == null) {
                    throw new Exception("Navigate must be called on an IRoutableViewModel");
                }

                NavigationStack.Add(vm);
            });

            NavigateAndReset = new NavigationReactiveCommand();
            NavigateAndReset.Subscribe(x => {
                NavigationStack.Clear();
                Navigate.Execute(x);
            });

            rxObjectsSetup = true;
        }
    }

    class NavigationReactiveCommand : ReactiveCommand, INavigateCommand { }

    public static class RoutingStateMixins
    {
        /// <summary>
        /// Generates a routing Uri based on the current route state
        /// </summary>
        /// <returns></returns>
        public static string GetUrlForCurrentRoute(this IRoutingState This)
        {
            return "app://" + String.Join("/", This.NavigationStack.Select(x => x.UrlPathSegment));
        }

        /// <summary>
        /// Locate the first ViewModel in the stack that matches a certain Type.
        /// </summary>
        /// <returns>The matching ViewModel or null if none exists.</returns>
        public static T FindViewModelInStack<T>(this IRoutingState This)
            where T : IRoutableViewModel
        {
            return This.NavigationStack.Reverse().OfType<T>().FirstOrDefault();
        }

        /// <summary>
        /// Returns the currently visible ViewModel
        /// </summary>
        public static IRoutableViewModel GetCurrentViewModel(this IRoutingState This)
        {
            return This.NavigationStack.LastOrDefault();
        }

        /// <summary>
        /// Returns an Observable that signals ViewModel changes.
        /// </summary>
        public static IObservable<IRoutableViewModel> ViewModelObservable(this IRoutingState This)
        {
            return This.NavigationStack.CollectionCountChanged
                .Select(_ => This.GetCurrentViewModel())
                .StartWith(This.GetCurrentViewModel());
        }

        public static void Go<T>(this INavigateCommand This, string key = null)
            where T : IRoutableViewModel
        {
            This.Execute(RxApp.GetService<T>(key));
        }
    }
}