Allow batch updating resources.

samples/ControlCatalog/MainView.xaml.cs

@@ -35,6 +35,8 @@ namespace ControlCatalog
             var themes = this.Find<ComboBox>("Themes");
             themes.SelectionChanged += (sender, e) =>
             {
+                ((IResourceHost)Application.Current).BeginBatchUpdate();
+
                 switch (themes.SelectedIndex)
                 {
                     case 0:
@@ -50,6 +52,8 @@ namespace ControlCatalog
                         Application.Current.Styles[0] = App.DefaultDark;
                         break;
                 }
+
+                ((IResourceHost)Application.Current).EndBatchUpdate();
             };            
 
             var decorations = this.Find<ComboBox>("Decorations");

src/Avalonia.Controls/Application.cs

@@ -43,6 +43,7 @@ namespace Avalonia
         private Styles _styles;
         private IResourceDictionary _resources;
         private bool _notifyingResourcesChanged;
+        private int _resourceBatchUpdate;
         private Action<IReadOnlyList<IStyle>> _stylesAdded;
         private Action<IReadOnlyList<IStyle>> _stylesRemoved;
 
@@ -199,9 +200,24 @@ namespace Avalonia
                    Styles.TryGetResource(key, out value);
         }
 
+        /// <inheritdoc/>
+        void IResourceHost.BeginBatchUpdate() => ++_resourceBatchUpdate;
+
+        /// <inheritdoc/>
+        void IResourceHost.EndBatchUpdate()
+        {
+            if (_resourceBatchUpdate > 0 && --_resourceBatchUpdate == 0)
+            {
+                ResourcesChanged?.Invoke(this, ResourcesChangedEventArgs.Empty);
+            }
+        }
+
         void IResourceHost.NotifyHostedResourcesChanged(ResourcesChangedEventArgs e)
         {
-            ResourcesChanged?.Invoke(this, e);
+            if (_resourceBatchUpdate == 0)
+            {
+                ResourcesChanged?.Invoke(this, e);
+            }
         }
 
         void IStyleHost.StylesAdded(IReadOnlyList<IStyle> styles)

src/Avalonia.Styling/Controls/IResourceHost.cs

@@ -17,6 +17,20 @@ namespace Avalonia.Controls
         /// </summary>
         event EventHandler<ResourcesChangedEventArgs> ResourcesChanged;
 
+        /// <summary>
+        /// Begin batch updating resources hosted by this element.
+        /// </summary>
+        /// <remarks>
+        /// After calling this method, <see cref="ResourcesChanged"/> will not be raised until
+        /// <see cref="EndBatchUpdate"/> is called.
+        /// </remarks>
+        void BeginBatchUpdate();
+
+        /// <summary>
+        /// End batch updating resources hosted by this element.
+        /// </summary>
+        void EndBatchUpdate();
+
         /// <summary>
         /// Notifies the resource host that one or more of its hosted resources has changed.
         /// </summary>

src/Avalonia.Styling/StyledElement.cs

@@ -67,6 +67,7 @@ namespace Avalonia
         private List<IStyleInstance>? _appliedStyles;
         private ITemplatedControl? _templatedParent;
         private bool _dataContextUpdating;
+        private int _resourceBatchUpdate;
 
         /// <summary>
         /// Initializes static members of the <see cref="StyledElement"/> class.
@@ -371,6 +372,18 @@ namespace Avalonia
         /// <inheritdoc/>
         void ILogical.NotifyResourcesChanged(ResourcesChangedEventArgs e) => NotifyResourcesChanged(e);
 
+        /// <inheritdoc/>
+        void IResourceHost.BeginBatchUpdate() => ++_resourceBatchUpdate;
+
+        /// <inheritdoc/>
+        void IResourceHost.EndBatchUpdate()
+        {
+            if (_resourceBatchUpdate > 0 && --_resourceBatchUpdate == 0)
+            {
+                NotifyResourcesChanged();
+            }
+        }
+
         /// <inheritdoc/>
         void IResourceHost.NotifyHostedResourcesChanged(ResourcesChangedEventArgs e) => NotifyResourcesChanged(e);
 
@@ -799,6 +812,11 @@ namespace Avalonia
             ResourcesChangedEventArgs? e = null,
             bool propagate = true)
         {
+            if (_resourceBatchUpdate > 0)
+            {
+                return;
+            }
+
             if (ResourcesChanged is object)
             {
                 e ??= ResourcesChangedEventArgs.Empty;