Fixing issue with validation template not updating when current error changes (#3182)

Fixes #3176

Author: Keboo

MaterialDesignThemes.UITests/Samples/Validation/ValidationUpdates.xaml

@@ -0,0 +1,19 @@
+<UserControl x:Class="MaterialDesignThemes.UITests.Samples.Validation.ValidationUpdates"
+             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
+             xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
+             mc:Ignorable="d" 
+             d:DesignHeight="450" d:DesignWidth="800">
+  <Grid>
+    <Grid.RowDefinitions>
+      <RowDefinition />
+      <RowDefinition />
+    </Grid.RowDefinitions>
+
+    <TextBox Text="{Binding Text, UpdateSourceTrigger=PropertyChanged, NotifyOnValidationError=True}"
+             VerticalAlignment="Center"/>
+
+    <Button Command="{Binding CauseErrorsCommand}" Content="Validate" Grid.Row="1" VerticalAlignment="Center" />
+  </Grid>
+</UserControl>

MaterialDesignThemes.UITests/Samples/Validation/ValidationUpdates.xaml.cs

@@ -0,0 +1,53 @@
+using System.Collections;
+using System.ComponentModel;
+using System.ComponentModel.DataAnnotations;
+using CommunityToolkit.Mvvm.ComponentModel;
+using CommunityToolkit.Mvvm.Input;
+
+namespace MaterialDesignThemes.UITests.Samples.Validation;
+
+/// <summary>
+/// Interaction logic for ValidationUpdates.xaml
+/// </summary>
+public partial class ValidationUpdates 
+{
+    public ValidationUpdates()
+    {
+        DataContext = new ValidationUpdatesViewModel();
+        InitializeComponent();
+    }
+}
+
+public partial class ValidationUpdatesViewModel : ObservableObject, INotifyDataErrorInfo
+{
+    [ObservableProperty]
+    private string? _text;
+
+    private string? Error { get; set; }
+
+    public bool HasErrors => Error != null;
+
+    public event EventHandler<DataErrorsChangedEventArgs>? ErrorsChanged;
+
+    public IEnumerable GetErrors(string? propertyName)
+    {
+        if (propertyName == nameof(Text))
+        {
+            if (Error != null)
+            {
+                return new[] { Error };
+            }
+        }
+        return Enumerable.Empty<string>();
+    }
+
+    [RelayCommand]
+    private async Task CauseErrors()
+    {
+        Error = "Some error";
+        ErrorsChanged?.Invoke(this, new DataErrorsChangedEventArgs(nameof(Text)));
+        await Task.Delay(100);
+        Error += " + more";
+        ErrorsChanged?.Invoke(this, new DataErrorsChangedEventArgs(nameof(Text)));
+    }
+}

MaterialDesignThemes.UITests/WPF/TextBoxes/TextBoxTests.cs

@@ -1,6 +1,7 @@
 using System.ComponentModel;
 using System.Globalization;
 using System.Windows.Media;
+using MaterialDesignThemes.UITests.Samples.Validation;
 
 namespace MaterialDesignThemes.UITests.WPF.TextBoxes;
 
@@ -590,6 +591,26 @@ public async Task TextBox_MultiLineAndFixedHeight_RespectsVerticalContentAlignme
 
         recorder.Success();
     }
+
+    [Fact]
+    [Description("Issue 3176")]
+    public async Task ValidationErrorTemplate_WithChangingErrors_UpdatesValidation()
+    {
+        await using var recorder = new TestRecorder(App);
+
+        IVisualElement userControl = await LoadUserControl<ValidationUpdates>();
+        var textBox = await userControl.GetElement<TextBox>();
+        var button = await userControl.GetElement<Button>();
+        await button.LeftClick();
+
+        await Wait.For(async() =>
+        {
+            var errorViewer = await textBox.GetElement("DefaultErrorViewer");
+            var textBlock = await errorViewer.GetElement<TextBlock>();
+
+            Assert.Equal("Some error + more", await textBlock.GetText());
+        });
+    }
 }
 
 public class NotEmptyValidationRule : ValidationRule

MaterialDesignThemes.Wpf/Themes/MaterialDesignTheme.ValidationErrorTemplate.xaml

@@ -32,7 +32,7 @@
                    HorizontalAlignment="{Binding ElementName=Placeholder, Path=AdornedElement.(wpf:ValidationAssist.HorizontalAlignment)}"
                    FontSize="{Binding ElementName=Placeholder, Path=AdornedElement.(wpf:ValidationAssist.FontSize)}"
                    Foreground="{DynamicResource MaterialDesignValidationErrorBrush}"
-                   Text="{Binding CurrentItem.ErrorContent, Mode=OneTime}"
+                   Text="{Binding /ErrorContent, Mode=OneTime}"
                    TextWrapping="Wrap"
                    UseLayoutRounding="false" />
       </Border>
@@ -46,7 +46,7 @@
                      HorizontalAlignment="{Binding ElementName=Placeholder, Path=AdornedElement.(wpf:ValidationAssist.HorizontalAlignment)}"
                      FontSize="{Binding ElementName=Placeholder, Path=AdornedElement.(wpf:ValidationAssist.FontSize)}"
                      Foreground="{DynamicResource MaterialDesignValidationErrorBrush}"
-                     Text="{Binding CurrentItem.ErrorContent, Mode=OneTime}"
+                     Text="{Binding /ErrorContent, Mode=OneTime}"
                      TextWrapping="Wrap"
                      UseLayoutRounding="false" />
         </Border>