C#实现的下拉多选框，下拉多选树，多级节点

[ToolboxItem(true)]
    public partial class DropDownTreeViewControl : RadControl
    {
        public DropDownTreeViewElement TreeViewElement { get; private set; }

        public RadTreeView TreeViewControl
        {
            get
            {
                return this.TreeViewElement.TreeViewControl;
            }
        }

        protected override void CreateChildItems(RadElement parent)
        {
            this.AllowShowFocusCues = true;

            base.CreateChildItems(parent);

            this.TreeViewElement = new DropDownTreeViewElement();

            parent.Children.Add(TreeViewElement);

        }

        protected override void OnEnter(EventArgs e)
        {
            base.OnEnter(e);
            this.TreeViewElement.Focus();
        }

        protected override void OnBindingContextChanged(EventArgs e)
        {
            base.OnBindingContextChanged(e);

            this.TreeViewControl.BindingContext = this.BindingContext;
        }

        public class DropDownTreeViewElement : LightVisualElement
        {
            private readonly Color BG_COLOR = Color.White;
            private readonly Color BORDER_COLOR = Color.LightBlue;
            private readonly Color ARROW_BORDER_COLOR = Color.LightGray;
            private readonly Color ARROW_NORMAL_BG_COLOR = Color.White;
            private readonly Color ARROW_MOUSE_OVER_BG_COLOR = Color.LightYellow;
            private readonly Color ARROW_PRESSED_BG_COLOR = Color.DarkOrange;

            private readonly int NORMAL_BORDER_WIDTH = 1;
            private readonly int FOCUS_BORDER_WIDTH = 2;

            private RadArrowButtonElement arrow;
            private PopupForm popup;
            private bool isInnerCallHide;

            public bool IsPopupOpen { get; private set; }
            public RadTreeView TreeViewControl
            {
                get
                {
                    return this.popup.TreeView;
                }
            }

            protected override void InitializeFields()
            {
                base.InitializeFields();

                // style
                this.DrawBorder = true;
                this.BorderBoxStyle = BorderBoxStyle.SingleBorder;
                this.BorderGradientStyle = GradientStyles.Solid;
                this.BorderColor = BORDER_COLOR;
                this.DrawFill = true;
                this.NumberOfColors = 1;
                this.GradientStyle = GradientStyles.Solid;
                this.BackColor = BG_COLOR;
                this.StretchHorizontally = true;
                this.StretchVertically = true;
            }

            protected override void CreateChildElements()
            {
                base.CreateChildElements();

                // arrow
                this.CreateArrow();

                // popup
                this.CreatePopup();

                this.Children.Add(arrow);
            }

            private void CreatePopup()
            {
                this.popup = new PopupForm(this);
                this.popup.PopupClosing += Popup_PopupClosing;
                this.popup.PopupClosed += Popup_PopupClosed;
            }

            private void Popup_PopupClosing(object sender, RadPopupClosingEventArgs args)
            {
                // mouse postion in control-bounds prevent
                if (args.CloseReason == RadPopupCloseReason.Mouse)
                {
                    var boundsSc = RectangleToScreen(this.Bounds);
                    if (boundsSc.Contains(MousePosition))
                    {
                        args.Cancel = true;
                    }
                }
            }

            private void Popup_PopupClosed(object sender, RadPopupClosedEventArgs args)
            {
                if (isInnerCallHide)
                {
                    return;
                }
                this.IsPopupOpen = false;
                this.SwitchArrowState(false);
            }

            private void CreateArrow()
            {
                this.arrow = new RadArrowButtonElement()
                {
                    ClickMode = ClickMode.Press,
                    MinSize = new Size(SystemInformation.VerticalScrollBarWidth,
                                    RadArrowButtonElement.RadArrowButtonDefaultSize.Height),
                    StretchHorizontally = false,
                    StretchVertically = true,
                    Margin = new System.Windows.Forms.Padding(2),
                };

                arrow.Fill.NumberOfColors = 1;
                arrow.Fill.BackColor = ARROW_NORMAL_BG_COLOR;
                arrow.Border.BoxStyle = BorderBoxStyle.SingleBorder;
                arrow.Border.GradientStyle = GradientStyles.Solid;
                arrow.Border.ForeColor = ARROW_BORDER_COLOR;

                arrow.RadPropertyChanged += Arrow_RadPropertyChanged;
                arrow.Click += Arrow_Click;
            }

            private void Arrow_Click(object sender, EventArgs e)
            {
                if (this.IsPopupOpen)
                {
                    this.IsPopupOpen = false;
                    this.SwitchArrowState(false);
                    this.HidePopup();
                }
                else
                {
                    this.IsPopupOpen = true;
                    this.SwitchArrowState(true);
                    this.ShowPopup();
                }
            }

            private void HidePopup()
            {
                this.isInnerCallHide = true;
                this.popup.Hide();
                this.isInnerCallHide = false;
            }

            private void ShowPopup()
            {
                this.popup.Width = this.Bounds.Width;
                this.popup.Height = 250;
                this.popup.ShowPopup(this.RectangleToScreen(this.ControlBoundingRectangle));
            }

            private void SwitchArrowState(bool isPressed)
            {
                this.arrow.Fill.BackColor = isPressed ? ARROW_PRESSED_BG_COLOR :
                    (arrow.IsMouseOver ? ARROW_MOUSE_OVER_BG_COLOR : ARROW_NORMAL_BG_COLOR);
            }

            protected override void OnPropertyChanged(RadPropertyChangedEventArgs e)
            {
                if (e.Property == ContainsFocusProperty)
                {
                    var isFocus = (bool)e.NewValue;
                    this.BorderWidth = isFocus ? FOCUS_BORDER_WIDTH : NORMAL_BORDER_WIDTH;
                }

                base.OnPropertyChanged(e);
            }

            protected override SizeF ArrangeOverride(SizeF finalSize)
            {
                base.ArrangeOverride(finalSize);

                // arrow on right side
                float width = this.arrow.DesiredSize.Width;
                float x = this.RightToLeft ? 0f : (finalSize.Width - width);
                RectangleF finalRect = new RectangleF(x, 0f, width, finalSize.Height);
                this.arrow.Arrange(finalRect);

                return finalSize;
            }

            private void Arrow_RadPropertyChanged(object sender, RadPropertyChangedEventArgs e)
            {
                if (e.Property == RadArrowButtonElement.IsMouseOverProperty)
                {
                    if (this.IsPopupOpen)
                    {
                        return;
                    }

                    var arrow = sender as RadArrowButtonElement;
                    var isMouseOver = (bool)e.NewValue;

                    arrow.Fill.BackColor = isMouseOver ? ARROW_MOUSE_OVER_BG_COLOR : ARROW_NORMAL_BG_COLOR;
                }
            }

        }

        public class PopupForm : RadSizablePopupControl
        {
            private HostTreeView tv;

            public PopupForm(RadItem owner)
                : base(owner)
            {
                this.SizingMode = SizingMode.UpDownAndRightBottom;
                this.VerticalAlignmentCorrectionMode = AlignmentCorrectionMode.SnapToOuterEdges;
            }

            public RadTreeView TreeView
            {
                get
                {
                    return this.tv.TreeView;
                }
            }

            protected override void CreateChildItems(RadElement parent)
            {
                base.CreateChildItems(parent);

                this.tv = new HostTreeView();
                this.tv.TreeView.Focusable = false;
                this.tv.TreeView.CheckBoxes = true;
                this.SizingGripDockLayout.Children.Add(tv);
            }


            public override bool OnMouseWheel(Control target, int delta)
            {
                if (delta < 0)
                {
                    this.tv.TreeView.VScrollBar.PerformSmallIncrement(1);
                }
                else
                {
                    this.tv.TreeView.VScrollBar.PerformSmallDecrement(1);
                }

                return true;
            }
        }

        public class HostTreeView : Telerik.WinControls.RadHostItem
        {
            public HostTreeView()
                : base(new RadTreeView())
            {

            }

            public RadTreeView TreeView
            {
                get
                {
                    return this.HostedControl as RadTreeView;
                }
            }
        }
    }