Mac: Add ability to auto attach/detach a control

src/Eto.Mac/Forms/MacView.cs

@@ -128,6 +128,7 @@ static partial class MacView
 		public static readonly object AcceptsFirstMouse_Key = new object();
 		public static readonly object TextInputCancelled_Key = new object();
 		public static readonly object TextInputImplemented_Key = new object();
+		public static readonly object AutoAttachNative_Key = new object();
 		public static readonly IntPtr selMouseDown = Selector.GetHandle("mouseDown:");
 		public static readonly IntPtr selMouseUp = Selector.GetHandle("mouseUp:");
 		public static readonly IntPtr selMouseDragged = Selector.GetHandle("mouseDragged:");
@@ -1575,6 +1576,31 @@ public virtual void UpdateLayout()
 		{
 			ContainerControl?.Window?.LayoutIfNeeded();
 		}
+
+		public bool AutoAttachNative
+		{
+			get => Widget.Properties.Get<bool>(MacView.AutoAttachNative_Key);
+			set
+			{
+				if (Widget.Properties.TrySet(MacView.AutoAttachNative_Key, value) && value)
+				{
+					// ensure method is added to the container control's class
+					AddMethod(MacView.selViewDidMoveToWindow, MacView.TriggerViewDidMoveToWindow_Delegate, "v@:@", ContainerControl);
+				}
+			}
+		}
+
+		public virtual void OnViewDidMoveToWindow()
+		{
+			if (!AutoAttachNative)
+				return;
+
+			// ensure load/unload get called appropriately.
+			if (ContainerControl.Window == null)
+				Widget.DetachNative();
+			else
+				Widget.AttachNative();
+		}
 	}
 }
 

src/Eto.Mac/MacHelpers.cs

@@ -40,6 +40,22 @@ public static NSView ToNative(this Control control, bool attach = false)
 			}
 			return control.GetContainerView();
 		}
+
+		/// <summary>
+		/// Sets a value indicating that the control should auto attach/detach when added to a native window.
+		/// </summary>
+		/// <remarks>
+		/// This is an alternative to using AttachNative/DetachNative manually, and will automatically be called when the view
+		/// is added/removed to/from a native Window.  This ensures that both Load and UnLoad are triggered on the Eto control(s).
+		/// </remarks>
+		/// <param name="control">Control to auto attach</param>
+		/// <param name="autoAttach"><c>true</c> to auto attach, <c>false</c> otherwise.</param>
+		public static void SetAutoAttach(this Control control, bool autoAttach)
+		{
+			var handler = control.GetMacViewHandler();
+			if (handler != null)
+				handler.AutoAttachNative = true;
+		}
 
 		/// <summary>
 		/// Wraps the specified <paramref name="view"/> to an Eto control that can be used directly in Eto.Forms code.