Added a small helper property to deal with controls that fail to update the measurements in response to complex data binding situations

Source/Properties.cs

@@ -0,0 +1,121 @@
+#region Apache License 2.0
+/*
+Nuclex Foundation libraries for .NET
+Copyright (C) 2002-2025 Markus Ewald / Nuclex Development Labs
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+#endregion // Apache License 2.0
+
+using System;
+
+using Avalonia;
+using Avalonia.Controls;
+
+namespace Nuclex.Avalonia {
+
+  /// <summary>Additional properties that can be attached to Avalonia objects</summary>
+  public static class Properties {
+
+    #region class InvalidateMeasureOnChangeObserver
+
+    /// <summary>
+    ///   Invalidates the calculated measurement of the control that is reporting
+    ///   a change to the property value
+    /// </summary>
+    private class InvalidateMeasureOnChangeObserver :
+      IObserver<AvaloniaPropertyChangedEventArgs<object>> {
+
+      /// <summary>The one and only instance you need</summary>
+      public static readonly InvalidateMeasureOnChangeObserver Instance = new();
+
+      /// <summary>Called after all observers have been notified successfully</summary>
+      public void OnCompleted() {}
+
+      /// <summary>Reports when the observed control encountered an error</summary>
+      /// <param name="error">Error the observed control has encountered</param>
+      public void OnError(Exception error) {}
+
+      /// <summary>Reports the updated value of the property to the observer</summary>
+      /// <param name="value">New value the property has assumed</param>
+      public void OnNext(AvaloniaPropertyChangedEventArgs<object> value) {
+        if(value.Sender is Control senderAsControl) {
+          senderAsControl.InvalidateMeasure();
+        }
+      }
+
+    }
+
+    #endregion // class InvalidateMeasureOnChangeObserver
+
+    /// <summary>
+    ///   Invalidates the object's measured size of a control and triggers a new layout pass
+    /// </summary>
+    /// <remarks>
+    ///   <para>
+    ///     If, for some reason, an Avalonia control whose size depends on a data-bound value
+    ///     (for example, a &lt;Border /&gt; that holds a &lt;TextBlock /gt; that uses data
+    ///     binding) does not update its own size, that is an Avalonia bug.
+    ///   </para>
+    ///   <para>
+    ///     By attaching this property to the misbehaving control and data-binding its value
+    ///     to the trigger that should cause a reevaluation of its dimensions, you can work
+    ///     around the issue:
+    ///   </para>
+    ///   <example>
+    ///     <code>
+    ///       &lt;Border
+    ///         CornerRadius="4"
+    ///         Padding="4,0"
+    ///         HorizontalAlignment="Center"
+    ///         nuclex:Properties.InvalidateMeasureOnChange="{Binding Status}"
+    ///       &gt;
+    ///         &lt;TextBlock
+    ///           HorizontalAlignment="Center"
+    ///           Text="{Binding Status}"
+    ///         /&gt;
+    ///       &lt;/Border&gt;
+    ///     </code>
+    ///   </example>
+    /// </remarks>
+    public static readonly AttachedProperty<object> InvalidateMeasureOnChangeProperty = (
+      AvaloniaProperty.RegisterAttached<Control, object>(
+        "InvalidateMeasureOnChange",
+        typeof(Properties),
+        defaultValue: null!,
+        inherits: false
+      )
+    );
+
+    /// <summary>Initializes all static members of the class</summary>
+    static Properties() {
+      InvalidateMeasureOnChangeProperty.Changed.Subscribe(InvalidateMeasureOnChangeObserver.Instance);
+    }
+
+    /// <summary>Reads the value of the 'InvalidateMeasureOnChange' property</summary>
+    /// <param name="control">Control from which the property will be read</param>
+    /// <returns>The current value of the property attached to the control</returns>
+    public static object GetInvalidateMeasureOnChange(Control control) {
+      return control.GetValue(InvalidateMeasureOnChangeProperty);
+    }
+
+    /// <summary>Updates the value of the 'InvalidateMeasureOnChange' property</summary>
+    /// <param name="control">Control on which the property will be updated</param>
+    /// <param name="value">New value to assign to the attached property</param>
+    public static void SetInvalidateMeasureOnChange(Control control, object value) {
+      control.SetValue(InvalidateMeasureOnChangeProperty, value);
+    }
+
+  }
+
+} // namespace Nuclex.Avalonia