1 files changed, 123 insertions, 0 deletions

diff --git a/Xamarin.Forms.Core/TemplateUtilities.cs b/Xamarin.Forms.Core/TemplateUtilities.cs
new file mode 100644
index 00000000..0599e747
--- /dev/null
+++ b/Xamarin.Forms.Core/TemplateUtilities.cs
@@ -0,0 +1,123 @@
+using System;
+using System.Collections.Generic;
+using System.Collections.ObjectModel;
+using System.Threading.Tasks;
+
+namespace Xamarin.Forms
+{
+	internal static class TemplateUtilities
+	{
+		public static async Task<Element> FindTemplatedParentAsync(Element element)
+		{
+			if (element.RealParent is Application)
+				return null;
+
+			var skipCount = 0;
+			element = await GetRealParentAsync(element);
+			while (!Application.IsApplicationOrNull(element))
+			{
+				var controlTemplated = element as IControlTemplated;
+				if (controlTemplated?.ControlTemplate != null)
+				{
+					if (skipCount == 0)
+						return element;
+					skipCount--;
+				}
+				if (element is ContentPresenter)
+					skipCount++;
+				element = await GetRealParentAsync(element);
+			}
+
+			return null;
+		}
+
+		public static Task<Element> GetRealParentAsync(Element element)
+		{
+			Element parent = element.RealParent;
+			if (parent is Application)
+				return Task.FromResult<Element>(null);
+
+			if (parent != null)
+				return Task.FromResult(parent);
+
+			var tcs = new TaskCompletionSource<Element>();
+			EventHandler handler = null;
+			handler = (sender, args) =>
+			{
+				tcs.TrySetResult(element.RealParent);
+				element.ParentSet -= handler;
+			};
+			element.ParentSet += handler;
+
+			return tcs.Task;
+		}
+
+		public static void OnContentChanged(BindableObject bindable, object oldValue, object newValue)
+		{
+			var self = (IControlTemplated)bindable;
+			var newElement = (Element)newValue;
+			if (self.ControlTemplate == null)
+			{
+				for (var i = 0; i < self.InternalChildren.Count; i++)
+				{
+					self.InternalChildren.Remove(self.InternalChildren[i]);
+				}
+
+				if (newValue != null)
+					self.InternalChildren.Add(newElement);
+			}
+			else
+			{
+				if (newElement != null)
+				{
+					BindableObject.SetInheritedBindingContext(newElement, bindable.BindingContext);
+				}
+			}
+		}
+
+		public static void OnControlTemplateChanged(BindableObject bindable, object oldValue, object newValue)
+		{
+			var self = (IControlTemplated)bindable;
+
+			// First make sure any old ContentPresenters are no longer bound up. This MUST be
+			// done before we attempt to make the new template.
+			if (oldValue != null)
+			{
+				var queue = new Queue<Element>(16);
+				queue.Enqueue((Element)self);
+
+				while (queue.Count > 0)
+				{
+					ReadOnlyCollection<Element> children = queue.Dequeue().LogicalChildren;
+					for (var i = 0; i < children.Count; i++)
+					{
+						Element child = children[i];
+						var controlTemplated = child as IControlTemplated;
+
+						var presenter = child as ContentPresenter;
+						if (presenter != null)
+							presenter.Clear();
+						else if (controlTemplated == null || controlTemplated.ControlTemplate == null)
+							queue.Enqueue(child);
+					}
+				}
+			}
+
+			// Now remove all remnants of any other children just to be sure
+			for (var i = 0; i < self.InternalChildren.Count; i++)
+			{
+				self.InternalChildren.Remove(self.InternalChildren[i]);
+			}
+
+			ControlTemplate template = self.ControlTemplate;
+			var content = template.CreateContent() as View;
+
+			if (content == null)
+			{
+				throw new NotSupportedException("ControlTemplate must return a type derived from View.");
+			}
+
+			self.InternalChildren.Add(content);
+		}
+	}
+}
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