Reduce overhead of pushing existing navigation stack (#672)

* Make StackCopy less awkward

* Clean up comment

* Update docs

* Update docs

* Replace SecondToLast with an arbitrarily deep Peek method

* Update docs

* Handle negative depths in Peek()

2 files changed, 16 insertions, 7 deletions

diff --git a/Xamarin.Forms.Core/INavigationPageController.cs b/Xamarin.Forms.Core/INavigationPageController.cs
index eddbe750..d4ae8675 100644
--- a/Xamarin.Forms.Core/INavigationPageController.cs
+++ b/Xamarin.Forms.Core/INavigationPageController.cs
@@ -7,7 +7,9 @@ namespace Xamarin.Forms
 {
 	public interface INavigationPageController
 	{
-		Stack<Page> StackCopy { get; }
+		Page Peek(int depth);
+
+		IEnumerable<Page> Pages { get; }
 
 		int StackDepth { get; }
 
diff --git a/Xamarin.Forms.Core/NavigationPage.cs b/Xamarin.Forms.Core/NavigationPage.cs
index 7b393b9c..fdafec06 100644
--- a/Xamarin.Forms.Core/NavigationPage.cs
+++ b/Xamarin.Forms.Core/NavigationPage.cs
@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using System.Collections.ObjectModel;
 using System.Linq;
 using System.Threading.Tasks;
 using Xamarin.Forms.Internals;
@@ -61,17 +62,23 @@ namespace Xamarin.Forms
 
 		internal Task CurrentNavigationTask { get; set; }
 
-		Stack<Page> INavigationPageController.StackCopy
+		Page INavigationPageController.Peek(int depth)
 		{
-			get
+			if (depth < 0)
 			{
-				var result = new Stack<Page>(PageController.InternalChildren.Count);
-				foreach (Page page in PageController.InternalChildren)
-					result.Push(page);
-				return result;
+				return null;
 			}
+
+			if (PageController.InternalChildren.Count <= depth)
+			{
+				return null;
+			}
+
+			return (Page)PageController.InternalChildren[PageController.InternalChildren.Count - depth - 1];
 		}
 
+		IEnumerable<Page> INavigationPageController.Pages => PageController.InternalChildren.Cast<Page>();
+
 		int INavigationPageController.StackDepth
 		{
 			get { return PageController.InternalChildren.Count; }