问WPF ContextMenu.ItemsSource未从绑定中解析

EN

 <emsprim:SplitButton Mode="Split">
        <emsprim:SplitButton.Content>
            <Image Source="images/16x16/Full Extent 1.png"  />
        </emsprim:SplitButton.Content>
        <emsprim:SplitButton.ContextMenu>
            <ContextMenu ItemsSource="{Binding CommandGroups[ZoomToDefinedExtentsCmds]}">
                <ContextMenu.ItemContainerStyle>
                    <Style TargetType="MenuItem">                            
                        <Setter Property="Command" Value="{Binding Command}" />
                        <Setter Property="CommandParameter" Value="{Binding ViewID}" />
                        <Setter Property="Header" Value="{Binding Name}" />
                        <Setter Property="Icon" Value="{Binding Icon}" />
                    </Style>
                </ContextMenu.ItemContainerStyle>
            </ContextMenu>
        </emsprim:SplitButton.ContextMenu>        
    </emsprim:SplitButton>

<MenuItem ItemsSource="{Binding CommandGroups[ZoomToDefinedExtentsCmds]}"
        Header="Zoom To"                  
        Margin="5,1,5,0" >
        <MenuItem.ItemContainerStyle>
            <Style TargetType="MenuItem">
                <Setter Property="Command" Value="{Binding Command}" />
                <Setter Property="CommandParameter" Value="{Binding CommandParameter}" />
                <Setter Property="Header" Value="{Binding Name}" />
                <Setter Property="Icon" Value="{Binding Icon}" />
            </Style>
        </MenuItem.ItemContainerStyle>       
    </MenuItem>

using System.ComponentModel;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Controls.Primitives;
using System.Windows.Markup;
using System.Diagnostics;

namespace Controls.Dictionary.Primitives
{
    /// <summary>
    /// Implemetation of a Split Button
    /// </summary>
    [TemplatePart(Name = "PART_DropDown", Type = typeof(Button))]
    [ContentProperty("Items")]
    [DefaultProperty("Items")]
    public class SplitButton : Button
    {
        // AddOwner Dependency properties
        public static readonly DependencyProperty PlacementProperty;
        public static readonly DependencyProperty PlacementRectangleProperty;
        public static readonly DependencyProperty HorizontalOffsetProperty;
        public static readonly DependencyProperty VerticalOffsetProperty;

        /// <summary>
        /// Static Constructor
        /// </summary>
        static SplitButton()
        {
            DefaultStyleKeyProperty.OverrideMetadata(typeof(SplitButton), new FrameworkPropertyMetadata(typeof(SplitButton)));

            // AddOwner properties from the ContextMenuService class, we need callbacks from these properties
            // to update the Buttons ContextMenu properties
            PlacementProperty = ContextMenuService.PlacementProperty.AddOwner(typeof(SplitButton), new FrameworkPropertyMetadata(PlacementMode.MousePoint, OnPlacementChanged));
            PlacementRectangleProperty = ContextMenuService.PlacementRectangleProperty.AddOwner(typeof(SplitButton), new FrameworkPropertyMetadata(Rect.Empty, OnPlacementRectangleChanged));
            HorizontalOffsetProperty = ContextMenuService.HorizontalOffsetProperty.AddOwner(typeof(SplitButton), new FrameworkPropertyMetadata(0.0, OnHorizontalOffsetChanged));
            VerticalOffsetProperty = ContextMenuService.VerticalOffsetProperty.AddOwner(typeof(SplitButton), new FrameworkPropertyMetadata(0.0, OnVerticalOffsetChanged));
        }

        /*
         * Properties
         * 
        */
        /// <summary>
        /// The Split Button's Items property maps to the base classes ContextMenu.Items property
        /// </summary>
        public ItemCollection Items
        {
            get
            {
                EnsureContextMenuIsValid();
                return this.ContextMenu.Items;
            }
        }
        /*
         * Dependancy Properties & Callbacks
         * 
        */
        /// <summary>
        /// Placement of the Context menu
        /// </summary>
        public PlacementMode Placement
        {
            get { return (PlacementMode)GetValue(PlacementProperty); }
            set { SetValue(PlacementProperty, value); }
        }
        /// <summary>
        /// Placement Property changed callback, pass the value through to the buttons context menu
        /// </summary>
        private static void OnPlacementChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            SplitButton s = d as SplitButton;
            if (s == null) return;

            s.EnsureContextMenuIsValid();
            s.ContextMenu.Placement = (PlacementMode)e.NewValue;
        }


        /// <summary>
        /// PlacementRectangle of the Context menu
        /// </summary>
        public Rect PlacementRectangle
        {
            get { return (Rect)GetValue(PlacementRectangleProperty); }
            set { SetValue(PlacementRectangleProperty, value); }
        }
        /// <summary>
        /// PlacementRectangle Property changed callback, pass the value through to the buttons context menu
        /// </summary>
        private static void OnPlacementRectangleChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            SplitButton s = d as SplitButton;
            if (s == null) return;
            s.EnsureContextMenuIsValid();
            s.ContextMenu.PlacementRectangle = (Rect)e.NewValue;
        }


        /// <summary>
        /// HorizontalOffset of the Context menu
        /// </summary>
        public double HorizontalOffset
        {
            get { return (double)GetValue(HorizontalOffsetProperty); }
            set { SetValue(HorizontalOffsetProperty, value); }
        }
        /// <summary>
        /// HorizontalOffset Property changed callback, pass the value through to the buttons context menu
        /// </summary>
        private static void OnHorizontalOffsetChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            SplitButton s = d as SplitButton;
            if (s == null) return;

            s.EnsureContextMenuIsValid();
            s.ContextMenu.HorizontalOffset = (double)e.NewValue;
        }


        /// <summary>
        /// VerticalOffset of the Context menu
        /// </summary>
        public double VerticalOffset
        {
            get { return (double)GetValue(VerticalOffsetProperty); }
            set { SetValue(VerticalOffsetProperty, value); }
        }
        /// <summary>
        /// VerticalOffset Property changed callback, pass the value through to the buttons context menu
        /// </summary>
        private static void OnVerticalOffsetChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            SplitButton s = d as SplitButton;
            if (s == null) return;

            s.EnsureContextMenuIsValid();
            s.ContextMenu.VerticalOffset = (double)e.NewValue;
        }

        /// <summary>
        /// Defines the Mode of operation of the Button
        /// </summary>
        /// <remarks>
        ///     The SplitButton two Modes are
        ///     Split (default),    - the button has two parts, a normal button and a dropdown which exposes the ContextMenu
        ///     Dropdown            - the button acts like a combobox, clicking anywhere on the button opens the Context Menu
        /// </remarks>
        public SplitButtonMode Mode
        {
            get { return (SplitButtonMode)GetValue(ModeProperty); }
            set { SetValue(ModeProperty, value); }
        }
        public static readonly DependencyProperty ModeProperty = DependencyProperty.Register("Mode", typeof(SplitButtonMode), typeof(SplitButton), new FrameworkPropertyMetadata(SplitButtonMode.Split));

        /*
         * Methods
         * 
        */
        /// <summary>
        /// OnApplyTemplate override, set up the click event for the dropdown if present in the template
        /// </summary>
        public override void OnApplyTemplate()
        {
            base.OnApplyTemplate();

            // set up the event handlers
            ButtonBase dropDown = this.Template.FindName("PART_DropDown", this) as ButtonBase;
            if (dropDown != null)
                dropDown.Click += DoDropdownClick;

        }

        /// <summary>
        /// Make sure the Context menu is not null
        /// </summary>
        private void EnsureContextMenuIsValid()
        {
            if (ContextMenu == null)
                ContextMenu = new ContextMenu();
        }

        /*
         * Events
         * 
        */
        /// <summary>
        /// Event Handler for the Drop Down Button's Click event
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        void DoDropdownClick(object sender, RoutedEventArgs e)
        {
            if (Mode == SplitButtonMode.Dropdown)
                return;

            if (ContextMenu == null || ContextMenu.HasItems == false) return;

            ContextMenu.PlacementTarget = this;
            ContextMenu.IsOpen = true;

            e.Handled = true;
        }
    }
}