Add Mac implementation for DropDownToolItem

- Add ShowDropArrow for all platforms, and ensure it updates at runtime
- Mac has two implementations, one that uses NSSegmentedControl (10.14 and earlier) and another that uses NSMenuToolbarItem (10.15 and later)

src/Eto.Gtk/Forms/ToolBar/DropDownToolItemHandler.cs

@@ -13,24 +13,23 @@ public class DropDownToolItemHandler : ToolItemHandler<Gtk.ToggleToolButton, Dro
 		// add a drop arrow to the button text.
 
 		private Gtk.Menu dropMenu;
-
+		bool showDropArrow = true;
+
 		public DropDownToolItemHandler()
 		{
 			dropMenu = new Gtk.Menu();
 		}
-		
+
 		#region IToolBarButton Members
 
-		
+
 		#endregion
 
 		public override void CreateControl(ToolBarHandler handler, int index)
 		{
 			Gtk.Toolbar tb = handler.Control;
 
-			var buttonText = ShowDropArrow ? Text + "  ▾" : Text;
 			Control = new Gtk.ToggleToolButton();
-			Control.Label = buttonText;
 			Control.IconWidget = GtkImage;
 			Control.IsImportant = true;
 			Control.Sensitive = Enabled;
@@ -42,12 +41,30 @@ public override void CreateControl(ToolBarHandler handler, int index)
 			tb.Insert(Control, index);
 			Control.Clicked += HandleClicked;
 			dropMenu.Hidden += HandleMenuClosed;
+			SetText();
+		}
+
+		protected override void SetText()
+		{
+			if (Control != null)
+				Control.Label = ShowDropArrow ? Text + "  ▾" : Text;
 		}
 
 		/// <summary>
 		/// Gets or sets whether the drop arrow is shown on the button.
 		/// </summary>
-		public bool ShowDropArrow { get; set; } = true;
+		public bool ShowDropArrow
+		{
+			get => showDropArrow;
+			set
+			{
+				if (showDropArrow != value)
+				{
+					showDropArrow = value;
+					SetText();
+				}
+			}
+		}
 
 #if GTKCORE
 		private void HandleClicked(object sender, EventArgs e)
@@ -108,7 +125,7 @@ public void HandleClicked(object sender, EventArgs e)
 		public void AddMenu(int index, MenuItem item)
 		{
 			dropMenu.Insert((Gtk.Widget)item.ControlObject, index);
-			var handler = item.Handler as Menu.IMenuHandler; 
+			var handler = item.Handler as Menu.IMenuHandler;
 			//SetChildAccelGroup(item);
 		}
 

src/Eto.Gtk/Forms/ToolBar/ToolItemHandler.cs

@@ -10,20 +10,56 @@ public interface IToolBarItemHandler
 	}
 
 	public abstract class ToolItemHandler<TControl, TWidget> : WidgetHandler<TControl, TWidget>, ToolItem.IHandler, IToolBarItemHandler
-		where TControl: Gtk.Widget
+		where TControl: Gtk.ToolItem
 		where TWidget: ToolItem
 	{
 		bool enabled = true;
 		bool visible = true;
+		string text;
+		string toolTip;
 		Image image;
 
 		protected Gtk.Image GtkImage { get; set; }
 
 		public abstract void CreateControl(ToolBarHandler handler, int index);
 
-		public string Text { get; set; }
+		public string Text
+		{
+			get => text;
+			set
+			{
+				if (text != value)
+				{
+					text = value;
+					SetText();
+				}
+			}
+		}
+
+		public virtual string ToolTip
+		{
+			get => toolTip;
+			set 
+			{
+				if (toolTip != value)
+				{
+					toolTip = value;
+					SetToolTip();
+				}
+			}
+		}
+
+		protected virtual void SetText()
+		{
+			if (Control is Gtk.ToolButton button)
+				button.Label = Text;
+		}
 
-		public string ToolTip { get; set; }
+		protected virtual void SetToolTip()
+		{
+			if (Control is Gtk.ToolButton button)
+				button.TooltipText = ToolTip;
+		}
 
 		public Image Image
 		{

src/Eto.Mac/Eto.Mac64.csproj

@@ -45,6 +45,7 @@ You do not need to use any of the classes of this assembly (unless customizing t
     <Using Include="System.Double" Alias="nfloat" />
     <Using Include="System.Int64" Alias="nint" />
     <Using Include="System.UInt64" Alias="nuint" />
+    <Using Include="System.IntPtr" Alias="NativeHandle" />
   </ItemGroup>
 
   <ItemGroup>

src/Eto.Mac/Eto.XamMac2.csproj

@@ -23,6 +23,7 @@
     <Using Include="CoreFoundation" />
     <Using Include="ImageIO" />
     <Using Include="CoreText" />
+    <Using Include="System.IntPtr" Alias="NativeHandle" />
   </ItemGroup>
 
   <PropertyGroup>

src/Eto.Mac/Forms/Controls/SegmentedButtonHandler.cs

@@ -391,9 +391,9 @@ public void InsertItem(int index, SegmentedItem item)
 		static readonly IntPtr selSetShowsMenuIndicator = Selector.GetHandle("setShowsMenuIndicator:forSegment:");
 
 		// 10.13+
-		static readonly bool supportsTooltip = ObjCExtensions.InstancesRespondToSelector<NSSegmentedCell>(selSetToolTipForSegment);
-		static readonly bool supportsMenuIndicator = ObjCExtensions.InstancesRespondToSelector<NSSegmentedControl>(selSetShowsMenuIndicator);
-
+		internal static readonly bool supportsTooltip = ObjCExtensions.InstancesRespondToSelector<NSSegmentedCell>(selSetToolTipForSegment);
+		internal static readonly bool supportsMenuIndicator = ObjCExtensions.InstancesRespondToSelector<NSSegmentedControl>(selSetShowsMenuIndicator);
+		
 		public void SetItem(int index, SegmentedItem item)
 		{
 			Control.SetLabel(item.Text ?? string.Empty, index);

src/Eto.Mac/Forms/Menu/MenuHandler.cs

@@ -15,6 +15,15 @@ public interface IMenuHandler
 	public class EtoMenu : NSMenu
 	{
 		public bool WorksWhenModal { get; set; }
+
+		public EtoMenu()
+		{
+		}
+
+		public EtoMenu(NativeHandle handle)
+			: base(handle)
+		{
+		}
 	}
 
 	static class MenuHandler

src/Eto.Mac/Forms/ToolBar/DropDownToolItemHandler.cs

@@ -0,0 +1,85 @@
+using System;
+using Eto.Drawing;
+using Eto.Forms;
+
+namespace Eto.Mac.Forms.ToolBar
+{
+	public class DropDownToolItemHandler : ToolItemHandler<NSMenuToolbarItem, DropDownToolItem>, DropDownToolItem.IHandler
+	{
+		NSMenu menu;
+
+		protected override bool UseButtonStyle => false;
+		protected override bool UseAction => false;
+
+#if MACOS || XAMMAC2
+		static readonly IntPtr selInitWithItemIdentifier_Handle = Selector.GetHandle("initWithItemIdentifier:");
+
+		// constructor with identifier isn't available in the version of macOS workload we need yet..
+		class EtoMenuToolbarItem : NSMenuToolbarItem
+		{
+			public EtoMenuToolbarItem()
+			{
+			}
+
+			[Export ("initWithItemIdentifier:")]
+			public EtoMenuToolbarItem (string itemIdentifier)
+				: base (NSObjectFlag.Empty)
+			{
+				NSApplication.EnsureUIThread ();
+				if (itemIdentifier == null)
+					throw new ArgumentNullException ("itemIdentifier");
+#if USE_CFSTRING
+				var nsitemIdentifier = CFString.CreateNative(itemIdentifier);
+#else
+				var nsitemIdentifier = NSString.CreateNative(itemIdentifier);
+#endif
+
+				if (IsDirectBinding) {
+					Handle = Messaging.IntPtr_objc_msgSend_IntPtr (this.Handle, selInitWithItemIdentifier_Handle, nsitemIdentifier);
+				} else {
+					Handle = Messaging.IntPtr_objc_msgSendSuper_IntPtr (this.SuperHandle, selInitWithItemIdentifier_Handle, nsitemIdentifier);
+				}
+				NSString.ReleaseNative (nsitemIdentifier);
+
+			}
+		}
+
+		protected override NSMenuToolbarItem CreateControl() => new EtoMenuToolbarItem(Identifier);
+#else
+		protected override NSMenuToolbarItem CreateControl() => new NSMenuToolbarItem(Identifier);
+#endif
+
+		public bool ShowDropArrow
+		{
+			get => Control.ShowsIndicator;
+			set => Control.ShowsIndicator = value;
+		}
+
+		protected override void Initialize()
+		{
+			base.Initialize();
+			Control.ShowsIndicator = true;
+			menu = new NSMenu();
+			menu.AutoEnablesItems = true;
+			menu.ShowsStateColumn = true;
+			Control.Menu = menu;
+			// first item is never shown, it's the "title" of the pull down?? weird.
+			menu.InsertItem(new NSMenuItem(string.Empty), 0);
+		}
+
+		public void AddMenu(int index, MenuItem item)
+		{
+			menu.InsertItem((NSMenuItem)item.ControlObject, index + 1);
+		}
+
+		public void RemoveMenu(MenuItem item)
+		{
+			menu.RemoveItem((NSMenuItem)item.ControlObject);
+		}
+
+		public void Clear()
+		{
+			menu.RemoveAllItems();
+		}
+	}
+}

src/Eto.Mac/Forms/ToolBar/DropDownToolItemPreCatalinaHandler.cs

@@ -0,0 +1,84 @@
+using System;
+using Eto.Drawing;
+using Eto.Forms;
+using Eto.Mac.Forms.Controls;
+using Eto.Mac.Forms.Menu;
+
+namespace Eto.Mac.Forms.ToolBar
+{
+	/// <summary>
+	/// Drop down handler pre-catalina.  This could probably be done a little better.
+	/// </summary>
+	public class DropDownToolItemPreCatalinaHandler : ToolItemHandler<NSToolbarItem, DropDownToolItem>, DropDownToolItem.IHandler
+	{
+		ContextMenu contextMenu;
+		MenuSegmentedItem menuItem;
+		SegmentedButton segmentedButton;
+		bool showDropArrow = true;
+		protected override bool UseButtonStyle => false;
+
+		protected override bool IsButton => true;
+
+		protected override bool UseAction => false;
+
+		public bool ShowDropArrow
+		{
+			get => showDropArrow;
+			set
+			{
+				showDropArrow = value;
+				if (SegmentedButtonHandler.supportsMenuIndicator)
+				{
+					var nssegmentedControl = SegmentedButtonHandler.GetControl(segmentedButton);
+					nssegmentedControl.SetShowsMenuIndicator(value, 0);
+				}
+			}
+		}
+
+		protected override void Initialize()
+		{
+			contextMenu = new ContextMenu();
+
+			menuItem = new MenuSegmentedItem { Menu = contextMenu };
+
+			segmentedButton = new SegmentedButton();
+			segmentedButton.Items.Add(menuItem);
+
+			Control.View = segmentedButton.ToNative(true);
+
+			base.Initialize();
+		}
+
+		protected override NSMenuItem CreateMenuFormRepresentation()
+		{
+			var menu = base.CreateMenuFormRepresentation();
+			menu.Submenu = contextMenu.ToNS();
+			return menu;
+		}
+
+		public void AddMenu(int index, MenuItem item)
+		{
+			contextMenu.Items.Insert(index, item);
+		}
+
+		public void RemoveMenu(MenuItem item)
+		{
+			contextMenu.Items.Remove(item);
+		}
+
+		public void Clear()
+		{
+			contextMenu.Items.Clear();
+		}
+
+		protected override void SetImage()
+		{
+			base.SetImage();
+			if (Image is Bitmap bmp)
+				menuItem.Image = bmp.WithSize(ImageSize, ImageSize);
+			else if (Image is Icon icon)
+				menuItem.Image = icon.WithSize(ImageSize, ImageSize);
+		}
+
+	}
+}

src/Eto.Mac/Forms/ToolBar/SeparatorToolItemHandler.cs

@@ -13,6 +13,8 @@ public class SeparatorToolItemHandler : ToolItemHandler<NSToolbarItem, Separator
 
 		protected override bool IsButton => false;
 
+		protected override bool UseButtonStyle => false;
+
 		public override string Identifier
 		{
 			get

src/Eto.Mac/Forms/ToolBar/ToolBarHandler.cs

@@ -271,7 +271,7 @@ internal int GetIndex(ToolItem item, bool forInsert = false, bool checkVisible =
 				else if (curitem.Visible)
 				{
 					var nativeItem = nativeItems[idx];
-					if (nativeItem.Identifier == GetIdentifier(curitem))
+					if (nativeItem.Identifier == GetIdentifier(curitem) || object.Equals(nativeItem, curitem.ControlObject))
 						idx++;
 				}
 			}