Added support for TokenSelectionMode in TokenizingTextBox

Microsoft.Toolkit.Uwp.SampleApp/SamplePages/TokenizingTextBox/TokenizingTextBoxXaml.bind

@@ -39,7 +39,8 @@
           MaxHeight="104"
           HorizontalAlignment="Stretch"
           TextMemberPath="Text"
-          TokenDelimiter=",">
+          TokenDelimiter=","
+          TokenSelectionMode="Single">
         <controls:TokenizingTextBox.SuggestedItemTemplate>
           <DataTemplate>
             <StackPanel Orientation="Horizontal">
@@ -75,6 +76,7 @@
             QueryIcon="{ui:SymbolIconSource Symbol=Find}"
             TextMemberPath="Text"
             TokenDelimiter=","
+            TokenSelectionMode="Multiple"
             IsItemClickEnabled="True"
             TokenItemTemplate="{StaticResource EmailTokenTemplate}">
       </controls:TokenizingTextBox>

Microsoft.Toolkit.Uwp.UI.Controls.Input/TokenizingTextBox/TokenSelectionMode.cs

@@ -0,0 +1,28 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Microsoft.Toolkit.Uwp.UI.Controls
+{
+    /// <summary>
+    /// Indicates how tokens are selected in the <see cref="TokenizingTextBox"/>.
+    /// </summary>
+    public enum TokenSelectionMode
+    {
+        /// <summary>
+        /// Only one token can be selected at a time. A new token should replace the active selection.
+        /// </summary>
+        Single,
+
+        /// <summary>
+        /// Multiple tokens can be selected at a time.
+        /// </summary>
+        Multiple,
+    }
+}

Microsoft.Toolkit.Uwp.UI.Controls.Input/TokenizingTextBox/TokenizingTextBox.Properties.cs

@@ -157,6 +157,26 @@ namespace Microsoft.Toolkit.Uwp.UI.Controls
             typeof(TokenizingTextBox),
             new PropertyMetadata(false));
 
+        /// <summary>
+        /// Identifies the <see cref="TokenSelectionMode"/> property.
+        /// </summary>
+        public static readonly DependencyProperty TokenSelectionModeProperty = DependencyProperty.Register(
+            nameof(TokenSelectionMode),
+            typeof(TokenSelectionMode),
+            typeof(TokenizingTextBox),
+            new PropertyMetadata(TokenSelectionMode.Multiple, OnTokenSelectionModeChanged));
+
+        private static void OnTokenSelectionModeChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
+        {
+            if (d is TokenizingTextBox ttb && e.NewValue is TokenSelectionMode newTokenSelectionMode && newTokenSelectionMode == TokenSelectionMode.Single)
+            {
+                while (ttb.Items.Count > 1)
+                {
+                    ttb.Items.RemoveAt(ttb.Items.Count - 1);
+                }
+            }
+        }
+
         /// <summary>
         /// Gets or sets the Style for the contained AutoSuggestBox template part.
         /// </summary>
@@ -303,5 +323,16 @@ namespace Microsoft.Toolkit.Uwp.UI.Controls
                 return PrepareSelectionForClipboard();
             }
         }
+
+        /// <summary>
+        /// Gets or sets how the control should display tokens.
+        /// <see cref="TokenSelectionMode.Multiple"/> is the default. Multiple tokens can be selected at a time.
+        /// <see cref="TokenSelectionMode.Single"/> indicates that only one token can be present in the control at a time.
+        /// </summary>
+        public TokenSelectionMode TokenSelectionMode
+        {
+            get => (TokenSelectionMode)GetValue(TokenSelectionModeProperty);
+            set => SetValue(TokenSelectionModeProperty, value);
+        }
     }
 }

Microsoft.Toolkit.Uwp.UI.Controls.Input/TokenizingTextBox/TokenizingTextBox.cs

@@ -448,6 +448,19 @@ namespace Microsoft.Toolkit.Uwp.UI.Controls
                 }
             }
 
+            if (TokenSelectionMode == TokenSelectionMode.Single)
+            {
+                // Remove any existing tokens.
+                for (var i = _innerItemsSource.Count - 1; i >= 0; --i)
+                {
+                    var item = _innerItemsSource[i];
+                    if (item is not ITokenStringContainer)
+                    {
+                        _innerItemsSource.Remove(item);
+                    }
+                }
+            }
+
             // If we've been typing in the last box, just add this to the end of our collection
             if (atEnd == true || _currentTextEdit == _lastTextEdit)
             {