SuggestionBox control

SuggestionBox.cs

    public class SuggestionBox : Control
    {
        static SuggestionBox() { DefaultStyleKeyProperty.OverrideMetadata(typeof(SuggestionBox), new FrameworkPropertyMetadata(typeof(SuggestionBox))); }

        public static DependencyProperty SelectedItemProperty =
            DependencyProperty.Register("SelectedItem", typeof(string), typeof(SuggestionBox), new FrameworkPropertyMetadata("", FrameworkPropertyMetadataOptions.BindsTwoWayByDefault, (d, bv) => (d as SuggestionBox).OnSelectedItemChanged()));

        public string SelectedItem
        {
            get { return (string)GetValue(SelectedItemProperty); }
            set { SetValue(SelectedItemProperty, value); }
        }

        public static DependencyProperty ItemsSourceProperty =
            DependencyProperty.Register("ItemsSource", typeof(IEnumerable<string>), typeof(SuggestionBox));

        public IEnumerable<string> ItemsSource
        {
            get { return (IEnumerable<string>)GetValue(ItemsSourceProperty); }
            set { SetValue(ItemsSourceProperty, value); }
        }

        public static DependencyProperty SelectedBackgroundBrushProperty =
            DependencyProperty.Register("SelectedBackgroundBrush", typeof(Brush), typeof(SuggestionBox));

        public Brush SelectedBackgroundBrush
        {
            get { return (Brush)GetValue(SelectedBackgroundBrushProperty); }
            set { SetValue(SelectedBackgroundBrushProperty, value); }
        }

        public static DependencyProperty SelectOnFocusLossProperty =
            DependencyProperty.Register("SelectOnFocusLoss", typeof(bool), typeof(SuggestionBox), new FrameworkPropertyMetadata(false));

        public bool SelectOnFocusLoss
        {
            get { return (bool)GetValue(SelectOnFocusLossProperty); }
            set { SetValue(SelectOnFocusLossProperty, value); }
        }

        private TextBox PART_TextBox;
        private ItemsControl PART_ItemsControl;
        private Popup PART_Popup;
        public IReadOnlyList<string> CurrentSuggestions { get; private set; }
        public IReadOnlyList<string> FilteredSuggestions { get; private set; }
        public int SelectedIndex { get; private set; }

        public SuggestionBox()
        {
            CurrentSuggestions = new string[] { };
            SelectedIndex = 0;
        }

        private void UpdateSuggestions()
        {
            if (CurrentSuggestions.Any(s => object.ReferenceEquals(s, SelectedItem)))
            {
                if (PART_Popup != null) PART_Popup.IsOpen = false;
            }
            else
            {
                if (PART_Popup != null) PART_Popup.IsOpen = true;

                FilteredSuggestions = CurrentSuggestions.Where(s => s.ToLower().Contains(SelectedItem.ToLower())).ToList();
                SelectedIndex = Math.Min(SelectedIndex, FilteredSuggestions.Count() - 1);
                if (PART_ItemsControl != null) PART_ItemsControl.ItemsSource = FilteredSuggestions.Select((s, i) => new SuggestionBoxItem
                {
                    Text = s,
                    Background = (i == SelectedIndex) ? SelectedBackgroundBrush : null,
                    OnMouseDown = new DelegateCommand<SuggestionBoxItem>(OnItemClicked)
                });
            }
        }

        public override void OnApplyTemplate()
        {
            base.OnApplyTemplate();

            PART_TextBox = Template.FindName(nameof(PART_TextBox), this) as TextBox;
            PART_TextBox.TextChanged += OnTextChanged;

            PART_ItemsControl = Template.FindName(nameof(PART_ItemsControl), this) as ItemsControl;

            PART_Popup = Template.FindName(nameof(PART_Popup), this) as Popup;
        }

        private void OnTextChanged(object sender, TextChangedEventArgs e)
        {
            SelectedItem = PART_TextBox.Text;
        }

        private void OnItemClicked(SuggestionBoxItem s)
        {
            SelectedItem = s.Text;
        }

        private void OnSelectedItemChanged()
        {
            UpdateSuggestions();
        }

        protected override void OnGotKeyboardFocus(KeyboardFocusChangedEventArgs e)
        {
            CurrentSuggestions = ItemsSource.ToList();
            SelectedIndex = 0;
            UpdateSuggestions();
        }
        public void SetGotFocus() => OnGotKeyboardFocus(null);

        protected override void OnLostKeyboardFocus(KeyboardFocusChangedEventArgs e)
        {
            if (PART_Popup != null) PART_Popup.IsOpen = false;

            if (SelectOnFocusLoss)
            {
                SelectedItem = FilteredSuggestions.ElementAtOrDefault(SelectedIndex) ?? SelectedItem;
            }
        }
        public void SetLostFocus() => OnLostKeyboardFocus(null);

        protected override void OnPreviewKeyDown(KeyEventArgs e)
        {
            base.OnPreviewKeyDown(e);

            if (e.Key == Key.Down && SelectedIndex < CurrentSuggestions.Count - 1)
            {
                SelectedIndex++;
                UpdateSuggestions();
            }
            else if (e.Key == Key.Up && SelectedIndex > 0)
            {
                SelectedIndex--;
                UpdateSuggestions();
            }
            else if (e.Key == Key.Enter)
            {
                SelectedItem = FilteredSuggestions.ElementAtOrDefault(SelectedIndex) ?? SelectedItem;
            }
        }
    }

SuggestionBox.xaml

<ResourceDictionary x:Class="The.WPF.Controls.SuggestionBoxStyle" x:ClassModifier="internal"
                    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    xmlns:controls="clr-namespace:The.WPF.Controls">
    <Style TargetType="{x:Type controls:SuggestionBox}">
        <Setter Property="IsTabStop" Value="False"/>
        <Setter Property="SelectedBackgroundBrush" Value="{DynamicResource {x:Static SystemColors.HighlightBrushKey}}"/>
        <Setter Property="Background" Value="{DynamicResource {x:Static SystemColors.ControlBrushKey}}"/>
        <Setter Property="BorderThickness" Value="1,0,1,1"/>
        <Setter Property="BorderBrush" Value="Black"/>
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="{x:Type controls:SuggestionBox}">
                    <StackPanel Orientation="Vertical">
                        <TextBox x:Name="PART_TextBox" Text="{TemplateBinding SelectedItem}"/>

                        <Popup x:Name="PART_Popup" IsOpen="False" Visibility="Collapsed" AllowsTransparency="True" Width="{Binding ActualWidth,ElementName=PART_TextBox}">
                            <ItemsControl x:Name="PART_ItemsControl" IsTabStop="False" Background="{TemplateBinding Background}" BorderThickness="{TemplateBinding BorderThickness}" BorderBrush="{TemplateBinding BorderBrush}">
                                <ItemsControl.ItemTemplate>
                                    <DataTemplate>
                                        <Grid MouseDown="Item_MouseDown" Tag="{Binding OnMouseDown}" Height="22">
                                            <Border Background="{Binding Background}" Opacity="0.25"/>
                                            <TextBlock Text="{Binding Text}" VerticalAlignment="Center"/>
                                        </Grid>
                                    </DataTemplate>
                                </ItemsControl.ItemTemplate>
                            </ItemsControl>
                        </Popup>
                    </StackPanel>

                    <ControlTemplate.Triggers>
                        <Trigger Property="IsKeyboardFocusWithin" Value="False">
                            <Setter TargetName="PART_ItemsControl" Property="BorderThickness" Value="0"/>
                        </Trigger>
                    </ControlTemplate.Triggers>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>
</ResourceDictionary>