Adding style to control helper text (#2313)

Fixes #2300

Author: Keboo

MaterialDesignThemes.UITests/WPF/TextBox/TextBoxTests.cs

@@ -258,5 +258,30 @@ public async Task CharacterCount_WithMaxLengthSetAndCharacterCounterVisibilityCo
 
             recorder.Success();
         }
+
+        [Fact]
+        [Description("Issue 2300")]
+        public async Task HelperText_CanSetFontColorWithAttachedStyle()
+        {
+            await using var recorder = new TestRecorder(App);
+
+            IVisualElement grid = await LoadXaml(@"
+<Grid Margin=""30"">
+    <TextBox
+        materialDesign:HintAssist.HelperText=""Test"">
+        <materialDesign:HintAssist.HelperTextStyle>
+            <Style TargetType=""TextBlock"" BasedOn=""{StaticResource MaterialDesignHelperTextBlock}"">
+                <Setter Property=""Foreground"" Value=""Red"" />
+            </Style>
+        </materialDesign:HintAssist.HelperTextStyle>
+    </TextBox>
+</Grid>");
+            IVisualElement textBox = await grid.GetElement("/TextBox");
+            IVisualElement helperText = await textBox.GetElement("HelperTextTextBlock");
+
+            Assert.Equal(Colors.Red, await helperText.GetForegroundColor());
+
+            recorder.Success();
+        }
     }
 }

MaterialDesignThemes.Wpf/HintAssist.cs

@@ -120,5 +120,17 @@ public static void SetHelperTextFontSize(DependencyObject element, double value)
             element.SetValue(HelperTextFontSizeProperty, value);
 
         #endregion
+
+        #region AttachedProperty : HelperTextStyleProperty
+        public static readonly DependencyProperty HelperTextStyleProperty
+            = DependencyProperty.RegisterAttached("HelperTextStyle", typeof(Style), typeof(HintAssist),
+                new PropertyMetadata(null));
+
+        public static Style? GetHelperTextStyle(DependencyObject element) =>
+            (Style?)element.GetValue(HelperTextStyleProperty);
+        public static void SetHelperTextStyle(DependencyObject element, Style? value) =>
+            element.SetValue(HelperTextStyleProperty, value);
+
+        #endregion
     }
 }

MaterialDesignThemes.Wpf/Themes/MaterialDesignTheme.TextBox.xaml

@@ -29,6 +29,12 @@
             </Setter.Value>
         </Setter>
     </Style>
+
+    <Style x:Key="MaterialDesignHelperTextBlock" TargetType="TextBlock" BasedOn="{StaticResource {x:Type TextBlock}}">
+        <Setter Property="FontSize" Value="{Binding Path=(wpf:HintAssist.HelperTextFontSize), RelativeSource={RelativeSource Mode=TemplatedParent}}" />
+        <Setter Property="Opacity" Value="{Binding Path=(wpf:HintAssist.HintOpacity), RelativeSource={RelativeSource Mode=TemplatedParent}}" />
+        <Setter Property="Text" Value="{Binding Path=(wpf:HintAssist.HelperText), RelativeSource={RelativeSource Mode=TemplatedParent}}" />
+    </Style>
 
     <Style x:Key="MaterialDesignTextBoxBase" TargetType="{x:Type TextBoxBase}">
         <Setter Property="Foreground" Value="{Binding RelativeSource={RelativeSource AncestorType={x:Type FrameworkElement}}, Path=(TextElement.Foreground)}"/>
@@ -53,6 +59,7 @@
         <Setter Property="wpf:TextFieldAssist.IncludeSpellingSuggestions" Value="{Binding RelativeSource={RelativeSource Self}, Path=(SpellCheck.IsEnabled)}" />
         <Setter Property="wpf:TextFieldAssist.CharacterCounterStyle" Value="{StaticResource MaterialDesignCharacterCounterTextBlock}" />
         <Setter Property="wpf:TextFieldAssist.CharacterCounterVisibility" Value="Visible" />
+        <Setter Property="wpf:HintAssist.HelperTextStyle" Value="{StaticResource MaterialDesignHelperTextBlock}" />
         <Setter Property="Template">
             <Setter.Value>
                 <ControlTemplate TargetType="{x:Type TextBoxBase}">
@@ -249,9 +256,7 @@
                                 </Grid.ColumnDefinitions>
                                 <TextBlock
                                     x:Name="HelperTextTextBlock"
-                                    FontSize="{TemplateBinding wpf:HintAssist.HelperTextFontSize}"
-                                    Opacity="{TemplateBinding wpf:HintAssist.HintOpacity}"
-                                    Text="{TemplateBinding wpf:HintAssist.HelperText}" />
+                                    Style="{Binding Path=(wpf:HintAssist.HelperTextStyle), RelativeSource={RelativeSource TemplatedParent}}" />
                                 <Border Grid.Column="1" x:Name="CharacterCounterContainer">
                                     <TextBlock
                                         x:Name="CharacterCounterTextBlock"