perf(NativeFramePresenter): [Android] Don't detach pages in the backs…

…tack

The behavior is now similar to the one from the iOS NativeFramePresenter, which keeps all pages loaded and stacked on top of each other. The behavior can be altered by using `FeatureConfiguration.NativeFramePresenter.AndroidUnloadInactivePages`, which is set to false by default.

src/Uno.UI.RuntimeTests/Tests/Windows_UI_Xaml_Controls/Given_NativeFrame.cs

@@ -0,0 +1,78 @@
+#if __ANDROID__
+#pragma warning disable CS4014 // Because this call is not awaited, execution of the current method continues before the call is completed
+
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using Private.Infrastructure;
+using System;
+using System.Collections.Generic;
+using System.Globalization;
+using System.Linq;
+using System.Text;
+using Windows.UI.Xaml.Controls;
+using Windows.UI.Xaml;
+using System.Threading.Tasks;
+using System.Diagnostics;
+
+namespace Uno.UI.RuntimeTests.Tests.Windows_UI_Xaml_Controls
+{
+	[TestClass]
+	public class Given_NativeFrame
+	{
+		[TestMethod]
+		[RunsOnUIThread]
+		public async Task When_NavigateForward()
+		{
+			var style = Windows.UI.Xaml.Application.Current.Resources["NativeDefaultFrame"] as Style;
+			Assert.IsNotNull(style);
+
+			var SUT = new Frame() {
+				Style = style
+			};
+
+			TestServices.WindowHelper.WindowContent = SUT;
+
+			int GetAllMyPages()
+				=> SUT.EnumerateAllChildren(v => v is MyPage).Count();
+
+			/// Actively waiting for pages to be stacked is
+			/// required as NativeFramePresenter.UpdateStack awaits
+			/// for animations to finish, and there's no way to determine
+			/// from the Frame PoV that the animation is finished.
+			async Task WaitForPages(int count)
+			{
+				var sw = Stopwatch.StartNew();
+
+				while(sw.Elapsed < TimeSpan.FromSeconds(5))
+				{
+					await TestServices.WindowHelper.WaitForIdle();
+
+					if (GetAllMyPages() == count)
+					{
+						break;
+					}
+				}
+
+				Assert.AreEqual(count, GetAllMyPages());
+			}
+
+			await WaitForPages(0);
+
+			SUT.Navigate(typeof(MyPage));
+
+			await WaitForPages(1);
+
+			SUT.Navigate(typeof(MyPage));
+
+			await WaitForPages(2);
+
+			SUT.GoBack();
+
+			await WaitForPages(1);
+		}
+	}
+
+	partial class MyPage : Page
+	{
+	}
+}
+#endif

src/Uno.UI/FeatureConfiguration.cs

@@ -371,6 +371,17 @@ public static class ToolTip
 			public static int ShowDuration { get; set; } = 7000;
 		}
 
+		public static class NativeFramePresenter
+		{
+#if __ANDROID__
+			/// <summary>
+			/// Determines if pages in the backstack are kept in the visual tree.
+			/// Defaults to false for performance considerations.
+			/// </summary>
+			public static bool AndroidUnloadInactivePages { get; set; } = false;
+#endif
+		}
+
 		public static class UIElement
 		{
 			/// <summary>

src/Uno.UI/NativeFramePresenter.Android.cs

@@ -130,13 +130,16 @@ private async Task UpdateStack(PageStackEntry entry, NavigationEventArgs e)
 						await newPage.AnimateAsync(GetEnterAnimation());
 						newPage.ClearAnimation();
 					}
-					if (oldPage != null)
+					if (FeatureConfiguration.NativeFramePresenter.AndroidUnloadInactivePages && oldPage != null)
 					{
 						_pageStack.Children.Remove(oldPage);
 					}
 					break;
 				case NavigationMode.Back:
-					_pageStack.Children.Insert(0, newPage);
+					if (FeatureConfiguration.NativeFramePresenter.AndroidUnloadInactivePages)
+					{
+						_pageStack.Children.Insert(0, newPage);
+					}
 					if (GetIsAnimated(oldEntry))
 					{
 						await oldPage.AnimateAsync(GetExitAnimation());
@@ -233,4 +236,4 @@ private static Animation GetExitAnimation()
 		}
 	}
 }
-#endif
+#endif