add auto-select functionality to ComboBox control

src/Avalonia.Controls/ComboBox.cs

@@ -10,6 +10,7 @@ using Avalonia.Interactivity;
 using Avalonia.Layout;
 using Avalonia.LogicalTree;
 using Avalonia.Media;
+using Avalonia.Threading;
 using Avalonia.VisualTree;
 
 namespace Avalonia.Controls
@@ -76,6 +77,14 @@ namespace Avalonia.Controls
         public static readonly StyledProperty<VerticalAlignment> VerticalContentAlignmentProperty =
             ContentControl.VerticalContentAlignmentProperty.AddOwner<ComboBox>();
 
+        /// <summary>
+        /// Defines the <see cref="IsAutoSelectEnabled"/> property.
+        /// </summary>
+        public static readonly StyledProperty<bool> IsAutoSelectEnabledProperty =
+            AvaloniaProperty.Register<ComboBox, bool>(nameof(IsAutoSelectEnabled));
+
+        private string _autoSelectTerm = string.Empty;
+        private DispatcherTimer _autoSelectTimer;
         private bool _isDropDownOpen;
         private Popup _popup;
         private object _selectionBoxItem;
@@ -164,6 +173,17 @@ namespace Avalonia.Controls
             set { SetValue(VerticalContentAlignmentProperty, value); }
         }
 
+        /// <summary>
+        /// Gets or sets value indicating whether auto-select is currently enabled.
+        /// If <c>true</c>, the control will try to find then select matched <see cref="ComboBoxItem"/>
+        /// based on current keyboard inputs.
+        /// </summary>
+        public bool IsAutoSelectEnabled
+        {
+            get { return GetValue(IsAutoSelectEnabledProperty); }
+            set { SetValue(IsAutoSelectEnabledProperty, value); }
+        }
+
         /// <inheritdoc/>
         protected override IItemContainerGenerator CreateItemContainerGenerator()
         {
@@ -229,6 +249,32 @@ namespace Avalonia.Controls
             }
         }
 
+        /// <inheritdoc />
+        protected override void OnTextInput(TextInputEventArgs e)
+        {
+            if (!IsAutoSelectEnabled || e.Handled)
+                return;
+
+            StopAutoSelectTimer();
+
+            _autoSelectTerm += e.Text;
+
+            bool match(ItemContainerInfo info) => 
+                info.ContainerControl is IContentControl control &&
+                control.Content?.ToString()?.StartsWith(_autoSelectTerm, StringComparison.OrdinalIgnoreCase) == true;
+
+            var info = ItemContainerGenerator.Containers.FirstOrDefault(match);
+
+            if (info != null)
+            {
+                SelectedIndex = info.Index;
+            }
+
+            StartAutoSelectTimer();
+
+            e.Handled = true;
+        }
+
         /// <inheritdoc/>
         protected override void OnPointerWheelChanged(PointerWheelEventArgs e)
         {
@@ -426,5 +472,31 @@ namespace Avalonia.Controls
 
             SelectedIndex = prev;
         }
+
+        private void StartAutoSelectTimer()
+        {
+            _autoSelectTimer = new DispatcherTimer { Interval = TimeSpan.FromSeconds(1) };
+            _autoSelectTimer.Tick += AutoSelectTimer_Tick;
+            _autoSelectTimer.Start();
+        }
+
+        private void StopAutoSelectTimer()
+        {
+            if (_autoSelectTimer == null)
+            {
+                return;
+            }
+
+            _autoSelectTimer.Stop();
+            _autoSelectTimer.Tick -= AutoSelectTimer_Tick;
+
+            _autoSelectTimer = null;
+        }
+
+        private void AutoSelectTimer_Tick(object sender, EventArgs e)
+        {
+            _autoSelectTerm = string.Empty;
+            StopAutoSelectTimer();
+        }
     }
 }

tests/Avalonia.Controls.UnitTests/ComboBoxTests.cs

@@ -1,7 +1,9 @@
+using System.Linq;
 using Avalonia.Controls.Presenters;
 using Avalonia.Controls.Primitives;
 using Avalonia.Controls.Shapes;
 using Avalonia.Controls.Templates;
+using Avalonia.Input;
 using Avalonia.LogicalTree;
 using Avalonia.Media;
 using Avalonia.UnitTests;
@@ -137,5 +139,40 @@ namespace Avalonia.Controls.UnitTests
                 Assert.True(other.IsFocused);
             }
         }
+
+        [Theory]
+        [InlineData(-1, 2, "c", "A item", "B item", "C item")]
+        [InlineData(0, 1, "b", "A item", "B item", "C item")]
+        [InlineData(2, 2, "x", "A item", "B item", "C item")]
+        public void AutoSelect_Should_Have_Expected_SelectedIndex(
+            int initialSelectedIndex,
+            int expectedSelectedIndex,
+            string searchTerm,
+            params string[] items)
+        {
+            using (UnitTestApplication.Start(TestServices.MockThreadingInterface))
+            {
+                var target = new ComboBox
+                {
+                    IsAutoSelectEnabled = true,
+                    Template = GetTemplate(),                    
+                    Items = items.Select(x => new ComboBoxItem { Content = x })
+                };
+
+                target.ApplyTemplate();
+                target.Presenter.ApplyTemplate();
+                target.SelectedIndex = initialSelectedIndex;
+
+                var args = new TextInputEventArgs
+                {
+                    Text = searchTerm,
+                    RoutedEvent = InputElement.TextInputEvent
+                };
+
+                target.RaiseEvent(args);
+
+                Assert.Equal(expectedSelectedIndex, target.SelectedIndex);
+            }
+        }
     }
 }