Force iOS 13 to use Full Screen for Modal

- Once we are on xCode 11 and can account for swipe dismiss we can make this better

Author: PureWeen

Xamarin.Forms.Core/PlatformConfiguration/iOSSpecific/Page.cs

@@ -131,7 +131,7 @@ public static IPlatformElementConfiguration<iOS, FormsElement> SetSafeAreaInsets
 		}
 
 		public static readonly BindableProperty ModalPresentationStyleProperty =
-			BindableProperty.Create(nameof(ModalPresentationStyle), typeof(UIModalPresentationStyle), typeof(Page), UIModalPresentationStyle.FullScreen);
+			BindableProperty.Create(nameof(ModalPresentationStyle), typeof(UIModalPresentationStyle), typeof(Page), UIModalPresentationStyle.Automatic);
 
 		public static UIModalPresentationStyle ModalPresentationStyle(this IPlatformElementConfiguration<iOS, FormsElement> config)
 		{

Xamarin.Forms.Core/PlatformConfiguration/iOSSpecific/UIModalPresentationStyle.cs

@@ -3,6 +3,7 @@
 	public enum UIModalPresentationStyle
 	{
 		FullScreen,
-		FormSheet
+		FormSheet,
+		Automatic
 	}
 }

Xamarin.Forms.Platform.iOS/Extensions/Extensions.cs

@@ -1,3 +1,4 @@
+using System;
 using UIKit;
 using Xamarin.Forms.Internals;
 
@@ -74,6 +75,21 @@ public static void ApplyKeyboard(this IUITextInputTraits textInput, Keyboard key
 			}
 		}
 
+		internal static UIModalPresentationStyle ToNativeModalPresentationStyle(this PlatformConfiguration.iOSSpecific.UIModalPresentationStyle style)
+		{
+			switch (style)
+			{
+				case PlatformConfiguration.iOSSpecific.UIModalPresentationStyle.FormSheet:
+					return UIModalPresentationStyle.FormSheet;
+				case PlatformConfiguration.iOSSpecific.UIModalPresentationStyle.FullScreen:
+					return UIModalPresentationStyle.FullScreen;
+				case PlatformConfiguration.iOSSpecific.UIModalPresentationStyle.Automatic:
+					return UIModalPresentationStyle.Automatic;
+				default:
+					throw new ArgumentOutOfRangeException(nameof(style));
+			}
+		}
+
 		internal static UIReturnKeyType ToUIReturnKeyType(this ReturnType returnType)
 		{
 			switch (returnType)

Xamarin.Forms.Platform.iOS/Forms.cs

@@ -37,6 +37,7 @@ public static class Forms
 		static bool? s_isiOS10OrNewer;
 		static bool? s_isiOS11OrNewer;
 		static bool? s_respondsTosetNeedsUpdateOfHomeIndicatorAutoHidden;
+		static bool? s_isiOS13OrNewer;
 #endif
 
 #if __MOBILE__
@@ -80,6 +81,16 @@ internal static bool RespondsToSetNeedsUpdateOfHomeIndicatorAutoHidden
 				return s_respondsTosetNeedsUpdateOfHomeIndicatorAutoHidden.Value;
 			}
 		}
+
+		internal static bool IsiOS13OrNewer
+		{
+			get
+			{
+				if (!s_isiOS13OrNewer.HasValue)
+					s_isiOS13OrNewer = UIDevice.CurrentDevice.CheckSystemVersion(13, 0);
+				return s_isiOS13OrNewer.Value;
+			}
+		}
 #endif
 
 		static IReadOnlyList<string> s_flags;

Xamarin.Forms.Platform.iOS/ModalWrapper.cs

@@ -14,8 +14,16 @@ internal ModalWrapper(IVisualElementRenderer modal)
 			_modal = modal;
 
 			var elementConfiguration = modal.Element as IElementConfiguration<Page>;
-			if (elementConfiguration?.On<PlatformConfiguration.iOS>().ModalPresentationStyle() == PlatformConfiguration.iOSSpecific.UIModalPresentationStyle.FormSheet)
-				ModalPresentationStyle = UIKit.UIModalPresentationStyle.FormSheet;
+			var modalPresentationStyle = elementConfiguration?.On<PlatformConfiguration.iOS>()?.ModalPresentationStyle() ?? PlatformConfiguration.iOSSpecific.UIModalPresentationStyle.Automatic;
+
+			if (modalPresentationStyle != PlatformConfiguration.iOSSpecific.UIModalPresentationStyle.Automatic)
+			{
+				ModalPresentationStyle = modalPresentationStyle.ToNativeModalPresentationStyle();
+			}
+			else if(Forms.IsiOS13OrNewer)
+			{
+				ModalPresentationStyle = UIKit.UIModalPresentationStyle.FullScreen;
+			}
 
 			View.BackgroundColor = UIColor.White;
 			View.AddSubview(modal.ViewController.View);