[Android] Small performance fixes to ListViewRenderer, PlatformSpecific IsFastScrollEnabled (#797)

* [Android] Enable fast scroll by default

* [Android] Cache count for Listview

* [Android] Add IsFastScrollEnabled AndroidSpecific and sample

* [Android] Use count cache on GetCellsFromPosition

* [Android] Fix default for platform specific IsFastScrollEnabled

* [Docs]Fix docs

* [Android] Don't used cached listCount when getting cell

2 files changed, 35 insertions, 11 deletions

diff --git a/Xamarin.Forms.Platform.Android/Renderers/ListViewAdapter.cs b/Xamarin.Forms.Platform.Android/Renderers/ListViewAdapter.cs
index 1ff11f4a..5c23fd2e 100644
--- a/Xamarin.Forms.Platform.Android/Renderers/ListViewAdapter.cs
+++ b/Xamarin.Forms.Platform.Android/Renderers/ListViewAdapter.cs
@@ -26,6 +26,7 @@ namespace Xamarin.Forms.Platform.Android
 		readonly AListView _realListView;
 		readonly Dictionary<DataTemplate, int> _templateToId = new Dictionary<DataTemplate, int>();
 		int _dataTemplateIncrementer = 2; // lets start at not 0 because
+		int _listCount = -1; // -1 we need to get count from the list
 		Cell _enabledCheckCell;
 
 		bool _fromNative;
@@ -57,22 +58,27 @@ namespace Xamarin.Forms.Platform.Android
 				MessagingCenter.Subscribe<AppCompat.Platform>(this, AppCompat.Platform.CloseContextActionsSignalName, p => CloseContextActions());
 			else
 				MessagingCenter.Subscribe<Platform>(this, Platform.CloseContextActionsSignalName, p => CloseContextActions());
+			InvalidateCount();
 		}
 
 		public override int Count
 		{
 			get
 			{
-				var templatedItems = TemplatedItemsView.TemplatedItems;
-				int count = templatedItems.Count;
-
-				if (_listView.IsGroupingEnabled)
+				if (_listCount == -1)
 				{
-					for (var i = 0; i < templatedItems.Count; i++)
-						count += templatedItems.GetGroup(i).Count;
-				}
+					var templatedItems = TemplatedItemsView.TemplatedItems;
+					int count = templatedItems.Count;
 
-				return count;
+					if (_listView.IsGroupingEnabled)
+					{
+						for (var i = 0; i < templatedItems.Count; i++)
+							count += templatedItems.GetGroup(i).Count;
+					}
+
+					_listCount = count;
+				}
+				return _listCount;
 			}
 		}
 
@@ -371,7 +377,7 @@ namespace Xamarin.Forms.Platform.Android
 			if (position < 0 || position >= Count)
 				return;
 
-			if(_lastSelected != view)
+			if (_lastSelected != view)
 				_fromNative = true;
 			Select(position, view);
 			Controller.NotifyRowTapped(position, cell);
@@ -386,11 +392,12 @@ namespace Xamarin.Forms.Platform.Android
 				return cells;
 
 			var templatedItems = TemplatedItemsView.TemplatedItems;
+			var templatedItemsCount = templatedItems.Count;
 			if (!_listView.IsGroupingEnabled)
 			{
 				for (var x = 0; x < take; x++)
 				{
-					if (position + x >= templatedItems.Count)
+					if (position + x >= templatedItemsCount)
 						return cells;
 
 					cells.Add(templatedItems[x + position]);
@@ -401,7 +408,7 @@ namespace Xamarin.Forms.Platform.Android
 
 			var i = 0;
 			var global = 0;
-			for (; i < templatedItems.Count; i++)
+			for (; i < templatedItemsCount; i++)
 			{
 				var group = templatedItems.GetGroup(i);
 
@@ -446,6 +453,7 @@ namespace Xamarin.Forms.Platform.Android
 
 		void OnDataChanged()
 		{
+			InvalidateCount();
 			if (ActionModeContext != null && !TemplatedItemsView.TemplatedItems.Contains(ActionModeContext))
 				CloseContextActions();
 
@@ -586,5 +594,10 @@ namespace Xamarin.Forms.Platform.Android
 			Row,
 			Header
 		}
+
+		void InvalidateCount()
+		{
+			_listCount = -1;
+		}
 	}
 }
\ No newline at end of file
diff --git a/Xamarin.Forms.Platform.Android/Renderers/ListViewRenderer.cs b/Xamarin.Forms.Platform.Android/Renderers/ListViewRenderer.cs
index fa83b25b..a0906915 100644
--- a/Xamarin.Forms.Platform.Android/Renderers/ListViewRenderer.cs
+++ b/Xamarin.Forms.Platform.Android/Renderers/ListViewRenderer.cs
@@ -6,6 +6,8 @@ using Android.Views;
 using AListView = Android.Widget.ListView;
 using AView = Android.Views.View;
 using Xamarin.Forms.Internals;
+using System;
+using Xamarin.Forms.PlatformConfiguration.AndroidSpecific;
 
 namespace Xamarin.Forms.Platform.Android
 {
@@ -147,6 +149,7 @@ namespace Xamarin.Forms.Platform.Android
 				UpdateHeader();
 				UpdateFooter();
 				UpdateIsSwipeToRefreshEnabled();
+				UpdateFastScrollEnabled();
 			}
 		}
 
@@ -166,6 +169,8 @@ namespace Xamarin.Forms.Platform.Android
 				UpdateIsRefreshing();
 			else if (e.PropertyName == ListView.SeparatorColorProperty.PropertyName || e.PropertyName == ListView.SeparatorVisibilityProperty.PropertyName)
 				_adapter.NotifyDataSetChanged();
+			else if (e.PropertyName == PlatformConfiguration.AndroidSpecific.ListView.IsFastScrollEnabledProperty.PropertyName)
+				UpdateFastScrollEnabled();
 		}
 
 		protected override void OnLayout(bool changed, int l, int t, int r, int b)
@@ -328,6 +333,12 @@ namespace Xamarin.Forms.Platform.Android
 				_refresh.Enabled = Element.IsPullToRefreshEnabled && (Element as IListViewController).RefreshAllowed;
 		}
 
+		void UpdateFastScrollEnabled()
+		{
+			if (Control != null)
+				Control.FastScrollEnabled = Element.OnThisPlatform().IsFastScrollEnabled();
+		}
+
 		internal class Container : ViewGroup
 		{
 			IVisualElementRenderer _child;