feat: add watermark support.

demo/Ursa.Demo/Pages/TagInputDemo.axaml

@@ -17,6 +17,7 @@
             Margin="20"
             AllowDuplicates="True"
             Separator="-"
+            Watermark="Hello world"
             Tags="{Binding Tags}" />
         <u:TagInput
             Margin="20"

src/Ursa.Themes.Semi/Controls/TagInput.axaml

@@ -25,6 +25,11 @@
                     BorderThickness="1"
                     CornerRadius="3">
                     <Panel HorizontalAlignment="Stretch">
+                        <TextBlock
+                            Name="{x:Static u:TagInput.PART_Watermark}"
+                            Opacity="0.5"
+                            IsVisible="False"
+                            Text="{TemplateBinding Watermark}" />
                         <ItemsControl
                             Name="PART_ItemsControl"
                             HorizontalAlignment="Stretch"
@@ -42,6 +47,9 @@
                 </Border>
             </ControlTemplate>
         </Setter>
+        <Style Selector="^:empty /template/ TextBlock#PART_Watermark">
+            <Setter Property="IsVisible" Value="True"></Setter>
+        </Style>
         <Style Selector="^:pointerover /template/ Border#PART_BackgroundBorder">
             <Setter Property="Border.Background" Value="{DynamicResource TextBoxPointeroverBackground}" />
         </Style>
@@ -123,10 +131,10 @@
                             Foreground="{TemplateBinding Foreground}" />
                         <ContentPresenter
                             VerticalAlignment="{TemplateBinding VerticalContentAlignment}"
-                            FontSize="12"
-                            Foreground="{TemplateBinding Foreground}"
                             Content="{TemplateBinding Content}"
                             ContentTemplate="{TemplateBinding ContentTemplate}"
+                            FontSize="12"
+                            Foreground="{TemplateBinding Foreground}"
                             TextTrimming="CharacterEllipsis" />
                     </DockPanel>
                 </Border>

src/Ursa/Controls/TagInput/TagInput.cs

@@ -1,6 +1,7 @@
 using System.Collections;
 using System.Collections.ObjectModel;
 using System.Collections.Specialized;
+using System.ComponentModel;
 using Avalonia;
 using Avalonia.Collections;
 using Avalonia.Controls;
@@ -8,32 +9,56 @@ using Avalonia.Controls.Metadata;
 using Avalonia.Controls.Presenters;
 using Avalonia.Controls.Primitives;
 using Avalonia.Controls.Templates;
+using Avalonia.Data;
+using Avalonia.Data.Converters;
 using Avalonia.Input;
 using Avalonia.Interactivity;
 using Avalonia.Layout;
 using Avalonia.Styling;
+using Irihi.Avalonia.Shared.Common;
+using Irihi.Avalonia.Shared.Helpers;
 
 namespace Ursa.Controls;
 
 [TemplatePart(PART_ItemsControl, typeof(ItemsControl))]
+[TemplatePart(PART_Watermark, typeof(Visual))]
+[PseudoClasses(PseudoClassName.PC_Empty)]
 public class TagInput : TemplatedControl
 {
     public const string PART_ItemsControl = "PART_ItemsControl";
+    public const string PART_Watermark = "PART_Watermark";
 
     private readonly TextBox _textBox;
     private ItemsControl? _itemsControl;
-
+    private Visual? _watermark;
 
     public static readonly StyledProperty<IList<string>> TagsProperty =
         AvaloniaProperty.Register<TagInput, IList<string>>(
             nameof(Tags));
 
+    public static readonly StyledProperty<string?> WatermarkProperty = TextBox.WatermarkProperty.AddOwner<TagInput>();
+
+    public string? Watermark
+    {
+        get => GetValue(WatermarkProperty);
+        set => SetValue(WatermarkProperty, value);
+    }
+
     public IList<string> Tags
     {
         get => GetValue(TagsProperty);
         set => SetValue(TagsProperty, value);
     }
 
+    public static readonly StyledProperty<int> MaxCountProperty = AvaloniaProperty.Register<TagInput, int>(
+        nameof(MaxCount), int.MaxValue);
+
+    public int MaxCount
+    {
+        get => GetValue(MaxCountProperty);
+        set => SetValue(MaxCountProperty, value);
+    }
+
     public static readonly DirectProperty<TagInput, IList> ItemsProperty =
         AvaloniaProperty.RegisterDirect<TagInput, IList>(
             nameof(Items), o => o.Items);
@@ -150,6 +175,21 @@ public class TagInput : TemplatedControl
     {
         base.OnApplyTemplate(e);
         _itemsControl = e.NameScope.Find<ItemsControl>(PART_ItemsControl);
+        _watermark = e.NameScope.Find<Visual>(PART_Watermark);
+    }
+
+    private TextPresenter? _presenter;
+    protected override void OnLoaded(RoutedEventArgs e)
+    {
+        base.OnLoaded(e);
+        if (_watermark is null) return;
+        _presenter = _textBox.GetTemplateChildren().OfType<TextPresenter>().FirstOrDefault();
+        _presenter?.GetObservable(TextPresenter.PreeditTextProperty).Subscribe(a => CheckEmpty());
+        _textBox.GetObservable(TextBox.TextProperty).Subscribe(a => CheckEmpty());
+        if (Tags is INotifyCollectionChanged incc)
+        {
+            incc.GetWeakCollectionChangedObservable().Subscribe(a => this.CheckEmpty());
+        }
     }
 
     private void OnInputThemePropertyChanged(AvaloniaPropertyChangedEventArgs args)
@@ -160,6 +200,18 @@ public class TagInput : TemplatedControl
             _textBox.Theme = newTheme;
         }
     }
+    
+    private void CheckEmpty()
+    {
+        if(string.IsNullOrWhiteSpace(_presenter?.PreeditText) && string.IsNullOrEmpty(_textBox?.Text) && (Tags.Count==0))
+        {
+            PseudoClasses.Set(PseudoClassName.PC_Empty, true);
+        }
+        else
+        {
+            PseudoClasses.Set(PseudoClassName.PC_Empty, false);
+        }
+    }
 
     private void OnTagsPropertyChanged(AvaloniaPropertyChangedEventArgs args)
     {