Started adding TextBox error adornments.

src/Avalonia.Controls/TextBox.cs

@@ -35,6 +35,11 @@ namespace Avalonia.Controls
                 o => o.CaretIndex,
                 (o, v) => o.CaretIndex = v);
 
+        public static readonly DirectProperty<TextBox, IEnumerable<Exception>> DataValidationErrorsProperty =
+            AvaloniaProperty.RegisterDirect<TextBox, IEnumerable<Exception>>(
+                nameof(DataValidationErrors),
+                o => o.DataValidationErrors);
+
         public static readonly StyledProperty<bool> IsReadOnlyProperty =
             AvaloniaProperty.Register<TextBox, bool>(nameof(IsReadOnly));
 
@@ -91,6 +96,7 @@ namespace Avalonia.Controls
         private TextPresenter _presenter;
         private UndoRedoHelper<UndoRedoState> _undoRedoHelper;
         private bool _ignoreTextChanges;
+        private IEnumerable<Exception> _dataValidationErrors;
 
         static TextBox()
         {
@@ -147,6 +153,12 @@ namespace Avalonia.Controls
             }
         }
 
+        public IEnumerable<Exception> DataValidationErrors
+        {
+            get { return _dataValidationErrors; }
+            private set { SetAndRaise(DataValidationErrorsProperty, ref _dataValidationErrors, value); }
+        }
+
         public bool IsReadOnly
         {
             get { return GetValue(IsReadOnlyProperty); }
@@ -473,7 +485,18 @@ namespace Avalonia.Controls
         {
             if (property == TextProperty)
             {
-                ((IPseudoClasses)Classes).Set(":error", status.ErrorType != BindingErrorType.None);
+                var classes = (IPseudoClasses)Classes;
+
+                if (status.ErrorType == BindingErrorType.None)
+                {
+                    classes.Remove(":error");
+                    DataValidationErrors = null;
+                }
+                else
+                {
+                    classes.Add(":error");
+                    DataValidationErrors = new[] { status.Error };
+                }
             }
         }
 

src/Avalonia.Themes.Default/Accents/BaseLight.xaml

@@ -18,6 +18,7 @@
     <SolidColorBrush x:Key="ThemeAccentBrush2">#99119EDA</SolidColorBrush>
     <SolidColorBrush x:Key="ThemeAccentBrush3">#66119EDA</SolidColorBrush>
     <SolidColorBrush x:Key="ThemeAccentBrush4">#33119EDA</SolidColorBrush>
+    <SolidColorBrush x:Key="ErrorBrush">Red</SolidColorBrush>
 
     <sys:Double x:Key="ThemeBorderThickness">2</sys:Double>
     <sys:Double x:Key="ThemeDisabledOpacity">0.5</sys:Double>

src/Avalonia.Themes.Default/TextBox.xaml

@@ -36,13 +36,24 @@
                            Opacity="0.5"
                            Text="{TemplateBinding Watermark}"
                            IsVisible="{TemplateBinding Path=Text, Converter={Static StringConverters.NullOrEmpty}}"/>
-                <TextPresenter Name="PART_TextPresenter"
-                               CaretIndex="{TemplateBinding CaretIndex}"
-                               SelectionStart="{TemplateBinding SelectionStart}"
-                               SelectionEnd="{TemplateBinding SelectionEnd}"
-                               Text="{TemplateBinding Text, Mode=TwoWay}"
-                               TextAlignment="{TemplateBinding TextAlignment}"
-                               TextWrapping="{TemplateBinding TextWrapping}"/>
+                <DockPanel LastChildFill="True">
+                  <Panel Name="error" DockPanel.Dock="Right">
+                    <ToolTip.Tip>
+                      <ItemsControl Items="{TemplateBinding DataValidationErrors}" MemberSelector="Message"/>
+                    </ToolTip.Tip>
+                    <!-- TODO: Draw as a path -->
+                    <Ellipse Width="14" Height="14" Stroke="{StyleResource ErrorBrush}" StrokeThickness="1"/>
+                    <Path Data="M7,4l0,5" Stroke="{StyleResource ErrorBrush}" StrokeThickness="2"/>
+                    <Path Data="M7,10l0,2" Stroke="{StyleResource ErrorBrush}" StrokeThickness="2"/>
+                  </Panel>
+                  <TextPresenter Name="PART_TextPresenter"
+                                 CaretIndex="{TemplateBinding CaretIndex}"
+                                 SelectionStart="{TemplateBinding SelectionStart}"
+                                 SelectionEnd="{TemplateBinding SelectionEnd}"
+                                 Text="{TemplateBinding Text, Mode=TwoWay}"
+                                 TextAlignment="{TemplateBinding TextAlignment}"
+                                 TextWrapping="{TemplateBinding TextWrapping}"/>
+                </DockPanel>
               </Panel>
             </ScrollViewer>
             
@@ -58,6 +69,15 @@
     <Setter Property="BorderBrush" Value="{StyleResource ThemeBorderDarkBrush}"/>
   </Style>
   <Style Selector="TextBox:error /template/ Border#border">
+    <Setter Property="BorderBrush" Value="{StyleResource ErrorBrush}"/>
+  </Style>
+  <Style Selector="TextBox /template/ Panel#error">
+    <Setter Property="IsVisible" Value="False"/>
+  </Style>
+  <Style Selector="TextBox:error /template/ Panel#error">
+    <Setter Property="IsVisible" Value="True"/>
+  </Style>
+  <Style Selector="TextBox /template/ ToolTip">
     <Setter Property="BorderBrush" Value="Red"/>
   </Style>
 </Styles>

tests/Avalonia.Controls.UnitTests/Avalonia.Controls.UnitTests.csproj

@@ -98,7 +98,7 @@
     <Compile Include="LayoutTransformControlTests.cs" />
     <Compile Include="Presenters\ItemsPresenterTests_Virtualization_Simple.cs" />
     <Compile Include="Presenters\ItemsPresenterTests_Virtualization.cs" />
-    <Compile Include="TextBoxTests_ValidationState.cs" />
+    <Compile Include="TextBoxTests_DataValidation.cs" />
     <Compile Include="UserControlTests.cs" />
     <Compile Include="DockPanelTests.cs" />
     <Compile Include="EnumerableExtensions.cs" />

tests/Avalonia.Controls.UnitTests/TextBoxTests_ValidationState.cs → tests/Avalonia.Controls.UnitTests/TextBoxTests_DataValidation.cs

@@ -5,6 +5,7 @@ using System;
 using System.Collections;
 using System.Collections.Generic;
 using System.ComponentModel;
+using System.Linq;
 using Avalonia.Controls.Presenters;
 using Avalonia.Controls.Templates;
 using Avalonia.Data;
@@ -16,7 +17,7 @@ using Xunit;
 
 namespace Avalonia.Controls.UnitTests
 {
-    public class TextBoxTests_ValidationState
+    public class TextBoxTests_DataValidation
     {
         [Fact]
         public void Setter_Exceptions_Should_Set_Error_Pseudoclass()
@@ -40,6 +41,29 @@ namespace Avalonia.Controls.UnitTests
             }
         }
 
+        [Fact]
+        public void Setter_Exceptions_Should_Set_DataValidationErrors()
+        {
+            using (UnitTestApplication.Start(Services))
+            {
+                var target = new TextBox
+                {
+                    DataContext = new ExceptionTest(),
+                    [!TextBox.TextProperty] = new Binding(nameof(ExceptionTest.LessThan10), BindingMode.TwoWay),
+                    Template = CreateTemplate(),
+                };
+
+                target.ApplyTemplate();
+
+                Assert.Null(target.DataValidationErrors);
+                target.Text = "20";
+                Assert.Equal(1, target.DataValidationErrors.Count());
+                Assert.IsType<InvalidOperationException>(target.DataValidationErrors.Single());
+                target.Text = "1";
+                Assert.Null(target.DataValidationErrors);
+            }
+        }
+
         private static TestServices Services => TestServices.MockThreadingInterface.With(
             standardCursorFactory: Mock.Of<IStandardCursorFactory>());