Add RoutedViewHost control implementation

src/Avalonia.ReactiveUI/RoutedViewHost.cs

@@ -0,0 +1,157 @@
+using System;
+using System.Reactive.Disposables;
+using System.Reactive.Linq;
+using Avalonia.Animation;
+using Avalonia.Controls;
+using Avalonia.Styling;
+using ReactiveUI;
+using Splat;
+
+namespace Avalonia
+{
+    /// <summary>
+    /// This control hosts the View associated with a Router, and will display
+    /// the View and wire up the ViewModel whenever a new ViewModel is navigated to.
+    /// </summary>
+    public class RoutedViewHost : UserControl, IActivatable, IEnableLogger
+    {
+        /// <summary>
+        /// The router dependency property.
+        /// </summary>
+        public static readonly AvaloniaProperty<RoutingState> RouterProperty =
+            AvaloniaProperty.Register<RoutedViewHost, RoutingState>(nameof(Router));
+        
+        /// <summary>
+        /// The default content property.
+        /// </summary> 
+        public static readonly AvaloniaProperty<object> DefaultContentProperty =
+            AvaloniaProperty.Register<RoutedViewHost, object>(nameof(DefaultContent));
+    
+        private readonly IAnimation _fadeOutAnimation = CreateOpacityAnimation(1d, 0d, TimeSpan.FromSeconds(0.25));
+        private readonly IAnimation _fadeInAnimation = CreateOpacityAnimation(0d, 1d, TimeSpan.FromSeconds(0.25));
+    
+        /// <summary>
+        /// Initializes a new instance of the <see cref="RoutedViewHost"/> class.
+        /// </summary>
+        public RoutedViewHost()
+        {
+            this.WhenActivated(disposables =>
+            {
+                this.WhenAnyObservable(x => x.Router.CurrentViewModel)
+                    .DistinctUntilChanged()
+                    .Subscribe(HandleViewModelChange)
+                    .DisposeWith(disposables);
+            });
+        }
+        
+        /// <summary>
+        /// Gets or sets the ReactiveUI view locator used by this router.
+        /// </summary>
+        public IViewLocator ViewLocator { get; set; }
+    
+        /// <summary>
+        /// Gets or sets the <see cref="RoutingState"/> of the view model stack.
+        /// </summary>
+        public RoutingState Router
+        {
+            get => GetValue(RouterProperty);
+            set => SetValue(RouterProperty, value);
+        }
+        
+        /// <summary>
+        /// Gets or sets the content displayed whenever there is no page currently routed.
+        /// </summary>
+        public object DefaultContent
+        {
+            get => GetValue(DefaultContentProperty);
+            set => SetValue(DefaultContentProperty, value);
+        }
+    
+        /// <summary>
+        /// Duplicates the Content property with a private setter.
+        /// </summary>
+        public new object Content
+        {
+            get => base.Content;
+            private set => base.Content = value;
+        }
+    
+        /// <summary>
+        /// Invoked when ReactiveUI router navigates to a view model.
+        /// </summary>
+        /// <param name="viewModel">ViewModel to which the user navigates.</param>
+        /// <exception cref="Exception">
+        /// Thrown when ViewLocator is unable to find the appropriate view.
+        /// </exception>
+        private void HandleViewModelChange(IRoutableViewModel viewModel)
+        {
+            if (viewModel == null)
+            {
+                this.Log().Info("ViewModel is null, falling back to default content.");
+                UpdateContent(DefaultContent);
+                return;
+            }
+    
+            var viewLocator = ViewLocator ?? ReactiveUI.ViewLocator.Current;
+            var view = viewLocator.ResolveView(viewModel);
+            if (view == null) throw new Exception($"Couldn't find view for '{viewModel}'. Is it registered?");
+    
+            this.Log().Info($"Ready to show {view} with autowired {viewModel}.");
+            view.ViewModel = viewModel;
+            UpdateContent(view);
+        }
+    
+        /// <summary>
+        /// Updates the content with transitions.
+        /// </summary>
+        /// <param name="newContent">New content to set.</param>
+        private async void UpdateContent(object newContent)
+        {
+            await _fadeOutAnimation.RunAsync(this, null);
+            Content = newContent;
+            await _fadeInAnimation.RunAsync(this, null);
+        }
+    
+        /// <summary>
+        /// Creates opacity animation for this routed view host.
+        /// </summary>
+        /// <param name="from">Opacity to start from.</param>
+        /// <param name="to">Opacity to finish with.</param>
+        /// <param name="duration">Duration of the animation.</param>
+        /// <returns>Animation object instance.</returns>
+        private static IAnimation CreateOpacityAnimation(double from, double to, TimeSpan duration) 
+        {
+            return new Avalonia.Animation.Animation
+            {
+                Duration = duration,
+                Children =
+                {
+                    new KeyFrame
+                    {
+                        Setters =
+                        {
+                            new Setter
+                            {
+                                Property = OpacityProperty,
+                                Value = from
+                            }
+                        },
+                        Cue = new Cue(0d)
+                    },
+                    new KeyFrame
+                    {
+                        Setters =
+                        {
+                            new Setter
+                            {
+                                Property = OpacityProperty,
+                                Value = to
+                            }
+                        },
+                        Cue = new Cue(1d)
+                    }
+                }
+            };
+        }
+    }
+}