Add Core flags feature; add flag verification for CollectionView feat…

…ures

Xamarin.Forms.ControlGallery.Android/FormsAppCompatActivity.cs

@@ -41,11 +41,11 @@ protected override void OnCreate(Bundle bundle)
 			base.OnCreate(bundle);
 
 #if TEST_EXPERIMENTAL_RENDERERS
-			Forms.SetFlags("FastRenderers_Experimental", "CollectionView_Experimental");
+			Forms.SetFlags("FastRenderers_Experimental");
 #else
 			// Fake_Flag is here so we can test for flag initialization issues
 			// CollectionView lets us test CollectionView stuff until it's officially released
-			Forms.SetFlags("Fake_Flag", "CollectionView_Experimental"); 
+			Forms.SetFlags("Fake_Flag"); 
 #endif
 
 			Forms.Init(this, bundle);

Xamarin.Forms.ControlGallery.WindowsUniversal/App.xaml.cs

@@ -65,8 +65,6 @@ protected override void OnLaunched(LaunchActivatedEventArgs e)
 
                 rootFrame.NavigationFailed += OnNavigationFailed;
 
-				Forms.SetFlags("CollectionView_Experimental");
-
 				Forms.Init (e);
 				//FormsMaps.Init (Controls.App.Config["UWPMapsAuthKey"]);
 

Xamarin.Forms.ControlGallery.iOS/AppDelegate.cs

@@ -160,8 +160,6 @@ public override bool FinishedLaunching(UIApplication uiApplication, NSDictionary
 
 			Xamarin.Calabash.Start();
 
-			Forms.SetFlags("CollectionView_Experimental");
-
 			Forms.Init();
 			FormsMaps.Init();
 			Forms.ViewInitialized += (object sender, ViewInitializedEventArgs e) =>

Xamarin.Forms.Controls/App.cs

@@ -1,5 +1,7 @@
 using System;
 using System.Collections.Generic;
+using System.ComponentModel;
+using System.Diagnostics;
 using System.IO;
 using System.Reflection;
 using System.Threading.Tasks;
@@ -31,6 +33,9 @@ public App()
 		{
 			_testCloudService = DependencyService.Get<ITestCloudService>();
 
+			// Allow use of CollectionView in Control Gallery
+			SetFlags("CollectionView_Experimental");
+
 			SetMainPage(CreateDefaultMainPage());
 
 			//TestMainPageSwitches();

Xamarin.Forms.Core/Application.cs

@@ -7,6 +7,7 @@
 using Xamarin.Forms.Internals;
 using Xamarin.Forms.Platform;
 using System.Diagnostics;
+using System.Linq;
 
 namespace Xamarin.Forms
 {
@@ -51,6 +52,17 @@ public static bool LogWarningsToApplicationOutput
 			}
 		}
 
+		static IReadOnlyList<string> s_flags;
+#if NETSTANDARD1_0
+		public static IReadOnlyList<string> Flags => s_flags ?? (s_flags = new ReadOnlyCollection<string>(new List<string>()));
+#else
+		public static IReadOnlyList<string> Flags => s_flags ?? (s_flags = new List<string>().AsReadOnly());
+#endif	
+
+
+		static bool MainPageSet { get; set; }
+
+
 		public Application()
 		{
 			var f = false;
@@ -120,6 +132,7 @@ public Page MainPage
 				}
 
 				_mainPage = value;
+				MainPageSet = true;
 
 				if (_mainPage != null)
 				{
@@ -206,6 +219,19 @@ public ResourceDictionary Resources
 
 		public event EventHandler<Page> PageDisappearing;
 
+		public static void SetFlags(params string[] flags)
+		{
+			if (MainPageSet)
+			{
+				throw new InvalidOperationException($"{nameof(SetFlags)} must be called before MainPage is set");
+			}
+
+#if NETSTANDARD1_0
+			s_flags = new ReadOnlyCollection<string>(flags.ToList());
+#else
+			s_flags = flags.ToList().AsReadOnly();
+#endif
+		}
 
 		async void SaveProperties()
 		{

Xamarin.Forms.Core/Items/CollectionView.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections;
 using System.Collections.Generic;
+using System.Linq;
 using System.Text;
 using Xamarin.Forms.Platform;
 
@@ -9,5 +10,9 @@ namespace Xamarin.Forms
 	[RenderWith(typeof(_CollectionViewRenderer))]
 	public class CollectionView : ItemsView
 	{
+		public CollectionView()
+		{
+			Flags.VerifyCollectionView(constructorHint: nameof(CollectionView));
+		}
 	}
 }

Xamarin.Forms.Core/Items/ItemsControl.cs

@@ -17,6 +17,7 @@ public abstract class ItemsLayout : BindableObject, IItemsLayout
 
 		protected ItemsLayout(ItemsLayoutOrientation orientation)
 		{
+			Flags.VerifyCollectionView(constructorHint: nameof(ItemsLayout));
 			Orientation = orientation;
 		}
 
@@ -96,9 +97,7 @@ public enum SnapPointsAlignment
 	public enum SnapPointsType
 	{
 		None,
-		Optional,
 		Mandatory,
-		OptionalSingle,
-		MandatorySingle,
+		MandatorySingle
 	}
 }

Xamarin.Forms.Core/Items/ItemsView.cs

@@ -12,11 +12,13 @@ public static class PropertyChangedEventArgsExtensions
 	{
 		public static bool Is(this PropertyChangedEventArgs args, BindableProperty property)
 		{
+			Flags.VerifyCollectionView();
 			return args.PropertyName == property.PropertyName;
 		}
 
 		public static bool IsOneOf(this PropertyChangedEventArgs args, params BindableProperty[] properties)
 		{
+			Flags.VerifyCollectionView();
 			for (int n = 0; n < properties.Length; n++)
 			{
 				if (args.PropertyName == properties[n].PropertyName)
@@ -34,6 +36,7 @@ public class CarouselView : ItemsView
 	{
 		public CarouselView()
 		{
+			Flags.VerifyCollectionView(constructorHint: nameof(CarouselView));
 			ItemsLayout = new ListItemsLayout(ItemsLayoutOrientation.Horizontal)
 			{
 				SnapPointsType = SnapPointsType.MandatorySingle,
@@ -44,6 +47,11 @@ public CarouselView()
 
 	public class ItemsView : View
 	{
+		protected internal ItemsView()
+		{
+			Flags.VerifyCollectionView(constructorHint: nameof(ItemsView));
+		}
+
 		// TODO hartez 2018/06/24 11:37:00 Give DisplayMemberPath some thought	
 
 		public static readonly BindableProperty EmptyViewProperty =

Xamarin.Forms.Core/Items/ScrollToRequestEventArgs.cs

@@ -18,6 +18,8 @@ public class ScrollToRequestEventArgs : EventArgs
 		public ScrollToRequestEventArgs(int index, int groupIndex, 
 			ScrollToPosition scrollToPosition, bool isAnimated)
 		{
+			Flags.VerifyCollectionView(nameof(ScrollToRequestEventArgs));
+
 			Mode = ScrollToMode.Position;
 
 			Index = index;
@@ -29,6 +31,8 @@ public class ScrollToRequestEventArgs : EventArgs
 		public ScrollToRequestEventArgs(object item, object group, 
 			ScrollToPosition scrollToPosition, bool isAnimated)
 		{
+			Flags.VerifyCollectionView(nameof(ScrollToRequestEventArgs));
+
 			Mode = ScrollToMode.Element;
 
 			Item = item;

Xamarin.Forms.Platform.Android/CollectionView/CarouselViewRenderer.cs

@@ -10,13 +10,6 @@ public class CarouselViewRenderer : ItemsViewRenderer
 
 		public CarouselViewRenderer(Context context) : base(context)
 		{
-			if (!Forms.Flags.Contains(Flags.CollectionViewExperimental))
-			{
-				var collectionViewFlagError = 
-					$"To use CarouselView on this platform, you must opt-in by calling " 
-					+ $"Forms.SetFlags(\"{Flags.CollectionViewExperimental}\") before Forms.Init().";
-				throw new InvalidOperationException(collectionViewFlagError);
-			}
 		}
 
 		protected override void UpdateItemsSource()

Xamarin.Forms.Platform.Android/CollectionView/ItemsViewRenderer.cs

@@ -32,14 +32,6 @@ public class ItemsViewRenderer : RecyclerView, IVisualElementRenderer, IEffectCo
 
 		public ItemsViewRenderer(Context context) : base(context)
 		{
-			if (!Forms.Flags.Contains(Flags.CollectionViewExperimental))
-			{
-				var collectionViewFlagError = 
-					$"To use CollectionView on this platform, you must opt-in by calling " 
-					+ $"Forms.SetFlags(\"{Flags.CollectionViewExperimental}\") before Forms.Init().";
-				throw new InvalidOperationException(collectionViewFlagError);
-			}
-
 			_automationPropertiesProvider = new AutomationPropertiesProvider(this);
 			_effectControlProvider = new EffectControlProvider(this);
 		}

Xamarin.Forms.Platform.Android/Flags.cs

@@ -3,6 +3,5 @@ namespace Xamarin.Forms
 	internal static class Flags
 	{
 		internal const string FastRenderersExperimental = "FastRenderers_Experimental";
-		internal const string CollectionViewExperimental = "CollectionView_Experimental";
 	}
 }

Xamarin.Forms.Platform.UAP/CollectionViewRenderer.cs

@@ -21,17 +21,6 @@ public class CollectionViewRenderer : ViewRenderer<CollectionView, ItemsControl>
 
 		protected ItemsControl ItemsControl { get; private set; }
 
-		public CollectionViewRenderer()
-		{
-			if (!Forms.Flags.Contains(Flags.CollectionViewExperimental))
-			{
-				var collectionViewFlagError = 
-					$"To use CollectionView on this platform, you must opt-in by calling " 
-					+ $"Forms.SetFlags(\"{Flags.CollectionViewExperimental}\") before Forms.Init().";
-				throw new InvalidOperationException(collectionViewFlagError);
-			}
-		}
-
 		protected override void OnElementChanged(ElementChangedEventArgs<CollectionView> args)
 		{
 			base.OnElementChanged(args);

Xamarin.Forms.Platform.UAP/Flags.cs

@@ -2,6 +2,5 @@
 {
 	internal static class Flags
 	{
-		internal const string CollectionViewExperimental = "CollectionView_Experimental";
 	}
 }

Xamarin.Forms.Platform.iOS/CollectionView/CollectionViewRenderer.cs

@@ -7,16 +7,6 @@ namespace Xamarin.Forms.Platform.iOS
 {
 	public class CarouselViewRenderer
 	{
-		public CarouselViewRenderer()
-		{
-			if (!Forms.Flags.Contains(Flags.CollectionViewExperimental))
-			{
-				var collectionViewFlagError = 
-					$"To use CarouselView on this platform, you must opt-in by calling " 
-					+ $"Forms.SetFlags(\"{Flags.CollectionViewExperimental}\") before Forms.Init().";
-				throw new InvalidOperationException(collectionViewFlagError);
-			}
-		}
 	}
 
 	// TODO hartez 2018/05/30 08:58:42 This follows the same basic scheme as RecyclerView.Adapter; you should be able to reuse the same wrapper class for the IEnumerable	
@@ -27,17 +17,6 @@ public class CollectionViewRenderer : ViewRenderer<CollectionView, UIView>
 		ItemsViewLayout _layout;
 		bool _disposed;
 
-		public CollectionViewRenderer()
-		{
-			if (!Forms.Flags.Contains(Flags.CollectionViewExperimental))
-			{
-				var collectionViewFlagError = 
-					$"To use CollectionView on this platform, you must opt-in by calling " 
-					+ $"Forms.SetFlags(\"{Flags.CollectionViewExperimental}\") before Forms.Init().";
-				throw new InvalidOperationException(collectionViewFlagError);
-			}
-		}
-
 		public override UIViewController ViewController => _collectionViewController;
 
 		public override SizeRequest GetDesiredSize(double widthConstraint, double heightConstraint)

Xamarin.Forms.Platform.iOS/Flags.cs

@@ -2,6 +2,5 @@
 {
 	internal static class Flags
 	{
-		internal const string CollectionViewExperimental = "CollectionView_Experimental";
 	}
 }