Prevent duplicate region views and add DefaultView to WPF, Uno, and Avalonia

src/Maui/Prism.Maui/Navigation/Regions/RegionViewRegistry.cs

@@ -14,6 +14,8 @@ namespace Prism.Navigation.Regions;
 public class RegionViewRegistry : IRegionViewRegistry
 {
     private readonly ListDictionary<string, Func<IContainerProvider, object>> _registeredContent = new();
+    private readonly Dictionary<string, HashSet<Type>> _registeredViewTypes = new();
+    private readonly Dictionary<string, HashSet<string>> _registeredTargetNames = new();
     private readonly WeakDelegatesManager _contentRegisteredListeners = new();
 
     /// <summary>
@@ -49,6 +51,11 @@ public IEnumerable<object> GetContents(string regionName, IContainerProvider con
     /// <param name="viewType">Content type to be registered for the <paramref name="regionName"/>.</param>
     public void RegisterViewWithRegion(string regionName, Type viewType)
     {
+        if (!TryAddRegisteredViewType(regionName, viewType))
+        {
+            return;
+        }
+
         RegisterViewWithRegion(regionName, c =>
         {
             var registry = c.Resolve<IRegionNavigationRegistry>();
@@ -67,6 +74,11 @@ public void RegisterViewWithRegion(string regionName, Type viewType)
     /// <param name="targetName">Content type to be registered for the <paramref name="regionName"/>.</param>
     public void RegisterViewWithRegion(string regionName, string targetName)
     {
+        if (!TryAddRegisteredTargetName(regionName, targetName))
+        {
+            return;
+        }
+
         RegisterViewWithRegion(regionName, c =>
         {
             var registry = c.Resolve<IRegionNavigationRegistry>();
@@ -81,10 +93,50 @@ public void RegisterViewWithRegion(string regionName, string targetName)
     /// <param name="getContentDelegate">Delegate used to retrieve the content associated with the <paramref name="regionName"/>.</param>
     public void RegisterViewWithRegion(string regionName, Func<IContainerProvider, object> getContentDelegate)
     {
+        if (IsContentDelegateRegistered(regionName, getContentDelegate))
+        {
+            return;
+        }
+
         _registeredContent.Add(regionName, getContentDelegate);
         OnContentRegistered(new ViewRegisteredEventArgs(regionName, getContentDelegate));
     }
 
+    private bool TryAddRegisteredViewType(string regionName, Type viewType)
+    {
+        if (!_registeredViewTypes.TryGetValue(regionName, out HashSet<Type> viewTypes))
+        {
+            viewTypes = new HashSet<Type>();
+            _registeredViewTypes.Add(regionName, viewTypes);
+        }
+
+        return viewTypes.Add(viewType);
+    }
+
+    private bool TryAddRegisteredTargetName(string regionName, string targetName)
+    {
+        if (!_registeredTargetNames.TryGetValue(regionName, out HashSet<string> targetNames))
+        {
+            targetNames = new HashSet<string>();
+            _registeredTargetNames.Add(regionName, targetNames);
+        }
+
+        return targetNames.Add(targetName);
+    }
+
+    private bool IsContentDelegateRegistered(string regionName, Func<IContainerProvider, object> getContentDelegate)
+    {
+        foreach (var existingDelegate in _registeredContent[regionName])
+        {
+            if (existingDelegate == getContentDelegate)
+            {
+                return true;
+            }
+        }
+
+        return false;
+    }
+
     private void OnContentRegistered(ViewRegisteredEventArgs e)
     {
         try

src/Wpf/Prism.Wpf/Navigation/Regions/RegionManager.cs

@@ -111,6 +111,48 @@ private static void OnSetRegionNameCallback(DependencyObject element, Dependency
             if (!IsInDesignMode(element))
             {
                 CreateRegion(element);
+                RegisterDefaultViewWithRegion(element);
+            }
+        }
+
+        private static void OnDefaultViewChanged(DependencyObject element, DependencyPropertyChangedEventArgs args)
+        {
+            if (!IsInDesignMode(element) && args.NewValue != null)
+            {
+                RegisterDefaultViewWithRegion(element, args.NewValue);
+            }
+        }
+
+        private static void RegisterDefaultViewWithRegion(DependencyObject element)
+        {
+            var defaultView = GetDefaultView(element);
+            if (defaultView != null)
+            {
+                RegisterDefaultViewWithRegion(element, defaultView);
+            }
+        }
+
+        private static void RegisterDefaultViewWithRegion(DependencyObject element, object defaultView)
+        {
+            var regionName = GetRegionName(element);
+            if (string.IsNullOrEmpty(regionName))
+            {
+                return;
+            }
+
+            var regionViewRegistry = ContainerLocator.Container.Resolve<IRegionViewRegistry>();
+
+            if (defaultView is string targetName)
+            {
+                regionViewRegistry.RegisterViewWithRegion(regionName, targetName);
+            }
+            else if (defaultView is Type viewType)
+            {
+                regionViewRegistry.RegisterViewWithRegion(regionName, viewType);
+            }
+            else
+            {
+                regionViewRegistry.RegisterViewWithRegion(regionName, _ => defaultView);
             }
         }
 
@@ -167,6 +209,52 @@ public static void SetRegionManager(DependencyObject target, IRegionManager valu
             target.SetValue(RegionManagerProperty, value);
         }
 
+        /// <summary>
+        /// Identifies the DefaultView attached property.
+        /// </summary>
+        /// <remarks>
+        /// Sets the default view to be displayed in a region when it is created.
+        /// This can be a view name, a type, or an instance of the view.
+        /// </remarks>
+#if !AVALONIA
+        public static readonly DependencyProperty DefaultViewProperty =
+            DependencyProperty.RegisterAttached("DefaultView", typeof(object), typeof(RegionManager),
+                new PropertyMetadata(defaultValue: null, propertyChangedCallback: OnDefaultViewChanged));
+#else
+        public static readonly AvaloniaProperty DefaultViewProperty =
+            AvaloniaProperty.RegisterAttached<AvaloniaObject, object>("DefaultView", typeof(RegionManager));
+#endif
+
+        /// <summary>
+        /// Sets the <see cref="DefaultViewProperty"/> attached property for the specified region target.
+        /// </summary>
+        /// <param name="regionTarget">The object that will host the default view.</param>
+        /// <param name="viewNameTypeOrInstance">
+        /// The default view to display in the region. This can be a view name, a type, or an instance of the view.
+        /// </param>
+        public static void SetDefaultView(DependencyObject regionTarget, object viewNameTypeOrInstance)
+        {
+            if (regionTarget == null)
+                throw new ArgumentNullException(nameof(regionTarget));
+
+            regionTarget.SetValue(DefaultViewProperty, viewNameTypeOrInstance);
+        }
+
+        /// <summary>
+        /// Gets the value of the <see cref="DefaultViewProperty"/> attached property for the specified region target.
+        /// </summary>
+        /// <param name="regionTarget">The object that hosts the default view.</param>
+        /// <returns>
+        /// The default view associated with the region. This can be a view name, a type, or an instance of the view.
+        /// </returns>
+        public static object GetDefaultView(DependencyObject regionTarget)
+        {
+            if (regionTarget == null)
+                throw new ArgumentNullException(nameof(regionTarget));
+
+            return regionTarget.GetValue(DefaultViewProperty);
+        }
+
         /// <summary>
         /// Identifies the RegionContext attached property.
         /// </summary>
@@ -272,6 +360,7 @@ static RegionManager()
             // TODO: Could this go into the default constructor?
             RegionNameProperty.Changed.Subscribe(args => OnSetRegionNameCallback(args?.Sender, args));
             RegionContextProperty.Changed.Subscribe(args => OnRegionContextChanged(args?.Sender, args));
+            DefaultViewProperty.Changed.Subscribe(args => OnDefaultViewChanged(args?.Sender, args));
         }
 #endif
 

src/Wpf/Prism.Wpf/Navigation/Regions/RegionViewRegistry.cs

@@ -13,6 +13,8 @@ public class RegionViewRegistry : IRegionViewRegistry
     {
         private readonly IContainerProvider _container;
         private readonly ListDictionary<string, Func<IContainerProvider, object>> _registeredContent = new ListDictionary<string, Func<IContainerProvider, object>>();
+        private readonly Dictionary<string, HashSet<Type>> _registeredViewTypes = new Dictionary<string, HashSet<Type>>();
+        private readonly Dictionary<string, HashSet<string>> _registeredTargetNames = new Dictionary<string, HashSet<string>>();
         private readonly WeakDelegatesManager _contentRegisteredListeners = new WeakDelegatesManager();
 
         /// <summary>
@@ -57,6 +59,11 @@ public IEnumerable<object> GetContents(string regionName, IContainerProvider con
         /// <param name="viewType">Content type to be registered for the <paramref name="regionName"/>.</param>
         public void RegisterViewWithRegion(string regionName, Type viewType)
         {
+            if (!TryAddRegisteredViewType(regionName, viewType))
+            {
+                return;
+            }
+
             RegisterViewWithRegion(regionName, _ => CreateInstance(viewType));
         }
 
@@ -67,6 +74,11 @@ public void RegisterViewWithRegion(string regionName, Type viewType)
         /// <param name="getContentDelegate">Delegate used to retrieve the content associated with the <paramref name="regionName"/>.</param>
         public void RegisterViewWithRegion(string regionName, Func<IContainerProvider, object> getContentDelegate)
         {
+            if (IsContentDelegateRegistered(regionName, getContentDelegate))
+            {
+                return;
+            }
+
             _registeredContent.Add(regionName, getContentDelegate);
             OnContentRegistered(new ViewRegisteredEventArgs(regionName, getContentDelegate));
         }
@@ -79,8 +91,15 @@ public void RegisterViewWithRegion(string regionName, Func<IContainerProvider, o
         /// <param name="regionName">The name of the region to associate the view with.</param>
         /// <param name="targetName">The type of the view to register with the </param>
         /// <returns>The <see cref="IRegionManager"/>, for adding several views easily</returns>
-        public void RegisterViewWithRegion(string regionName, string targetName) =>
+        public void RegisterViewWithRegion(string regionName, string targetName)
+        {
+            if (!TryAddRegisteredTargetName(regionName, targetName))
+            {
+                return;
+            }
+
             RegisterViewWithRegion(regionName, c => c.Resolve<object>(targetName));
+        }
 
         /// <summary>
         /// Creates an instance of a registered view <see cref="Type"/>.
@@ -94,6 +113,41 @@ protected virtual object CreateInstance(Type type)
             return view;
         }
 
+        private bool TryAddRegisteredViewType(string regionName, Type viewType)
+        {
+            if (!_registeredViewTypes.TryGetValue(regionName, out HashSet<Type> viewTypes))
+            {
+                viewTypes = new HashSet<Type>();
+                _registeredViewTypes.Add(regionName, viewTypes);
+            }
+
+            return viewTypes.Add(viewType);
+        }
+
+        private bool TryAddRegisteredTargetName(string regionName, string targetName)
+        {
+            if (!_registeredTargetNames.TryGetValue(regionName, out HashSet<string> targetNames))
+            {
+                targetNames = new HashSet<string>();
+                _registeredTargetNames.Add(regionName, targetNames);
+            }
+
+            return targetNames.Add(targetName);
+        }
+
+        private bool IsContentDelegateRegistered(string regionName, Func<IContainerProvider, object> getContentDelegate)
+        {
+            foreach (Func<IContainerProvider, object> existingDelegate in _registeredContent[regionName])
+            {
+                if (existingDelegate == getContentDelegate)
+                {
+                    return true;
+                }
+            }
+
+            return false;
+        }
+
         private void OnContentRegistered(ViewRegisteredEventArgs e)
         {
             try

tests/Avalonia/Prism.Avalonia.Tests/Regions/Behaviors/AutoPopulateRegionBehaviorFixture.cs

@@ -11,6 +11,10 @@ namespace Prism.Avalonia.Tests.Regions.Behaviors
 {
     public class AutoPopulateRegionBehaviorFixture
     {
+        private class MockContentObject
+        {
+        }
+
         [Fact]
         public void ShouldGetViewsFromRegistryOnAttach()
         {
@@ -64,6 +68,27 @@ public void NullRegionThrows()
 
         }
 
+        [Fact]
+        public void WhenSameViewTypeRegisteredTwice_RegionContainsSingleView()
+        {
+            var containerMock = new Mock<IContainerExtension>();
+            ContainerLocator.SetContainerExtension(containerMock.Object);
+            containerMock.Setup(c => c.Resolve(typeof(MockContentObject))).Returns(new MockContentObject());
+            var registry = new RegionViewRegistry(containerMock.Object);
+            registry.RegisterViewWithRegion("MyRegion", typeof(MockContentObject));
+            registry.RegisterViewWithRegion("MyRegion", typeof(MockContentObject));
+
+            var region = new Region { Name = "MyRegion" };
+            var behavior = new AutoPopulateRegionBehavior(registry)
+            {
+                Region = region
+            };
+
+            behavior.Attach();
+
+            Assert.Single(region.Views);
+        }
+
         [Fact]
         public void CanAttachBeforeSettingName()
         {