[iOS] Shell flyout header on iOS now is dynamic based on the header content (#6109) fixes #3875

* fix the flyout header to size based on content opposed to a fixed height

* update storeshell

* added height slider into storeshell

* resize header if underlying measure changes

* unsubscribe

Xamarin.Forms.ControlGallery.iOS/Xamarin.Forms.ControlGallery.iOS.csproj

@@ -327,6 +327,7 @@
     <BundleResource Include="Resources\button_add%402x.png" />
     <BundleResource Include="Resources\icon_search%402x.png" />
     <BundleResource Include="Resources\icon_bookmark%402x.png" />
+    <BundleResource Include="Resources\xamarinstore.jpg" />
   </ItemGroup>
   <ItemGroup>
     <Reference Include="Microsoft.CSharp" />

Xamarin.Forms.Controls/XamStore/Controls/FlyoutHeader.xaml

@@ -1,8 +1,8 @@
-<?xml version="1.0" encoding="UTF-8"?>
+<?xml version="1.0" encoding="UTF-8"?>
 <ContentView xmlns="http://xamarin.com/schemas/2014/forms" 
 				xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
 				x:Class="Xamarin.Forms.Controls.XamStore.FlyoutHeader"
-				HeightRequest="200">
+				HeightRequest="143">
 	<ContentView.Content>
 		<Grid BackgroundColor="Black">
 			<Image Aspect="AspectFill" Source="xamarinstore.jpg" Opacity="0.6" />

Xamarin.Forms.Controls/XamStore/Views/StorePages.cs

@@ -243,6 +243,24 @@ namespace Xamarin.Forms.Controls.XamStore
 				async () => await Shell.Current.GoToAsync(navEntry.Text, true)),
 			2, 16);
 
+			var headerWidth = new Slider
+			{
+				Minimum = 0,
+				Maximum = 400,
+				Value = (Shell.Current.FlyoutHeader as VisualElement)?.HeightRequest ?? 0
+			};
+			headerWidth.ValueChanged += (_, e) =>
+			{
+				if (Shell.Current.FlyoutHeader is VisualElement ve)
+					ve.HeightRequest = e.NewValue;
+			};
+			grid.Children.Add(new Label
+			{
+				Text = "fly Header",
+				VerticalOptions = LayoutOptions.CenterAndExpand
+			}, 0, 17);
+			grid.Children.Add(headerWidth, 1, 17);
+
 			Content = new ScrollView { Content = grid };
 
 			//var listView = new ListView();

Xamarin.Forms.Platform.iOS/Renderers/ShellFlyoutContentRenderer.cs

@@ -9,7 +9,7 @@ namespace Xamarin.Forms.Platform.iOS
 	{
 		UIVisualEffectView _blurView;
 		readonly IShellContext _shellContext;
-		UIView _headerView;
+		UIContainerView _headerView;
 		ShellTableViewController _tableViewController;
 
 		public event EventHandler WillAppear;

Xamarin.Forms.Platform.iOS/Renderers/ShellTableViewController.cs

@@ -8,29 +8,28 @@ namespace Xamarin.Forms.Platform.iOS
 	public class ShellTableViewController : UITableViewController
 	{
 		readonly IShellContext _context;
-		readonly UIView _headerView;
+		readonly UIContainerView _headerView;
 		readonly ShellTableViewSource _source;
-		double _headerMax = 200;
-		double _headerMin = 44;
+		double _headerMin = 56;
 		double _headerOffset = 0;
 		double _headerSize;
 
-		public ShellTableViewController(IShellContext context, UIView headerView, Action<Element> onElementSelected)
+		public ShellTableViewController(IShellContext context, UIContainerView headerView, Action<Element> onElementSelected)
 		{
-			if (headerView == null)
-			{
-				_headerMax = 20;
-				_headerMin = 0;
-			}
-
-			_headerSize = _headerMax;
 			_context = context;
 			_headerView = headerView;
 			_source = new ShellTableViewSource(context, onElementSelected);
 			_source.ScrolledEvent += OnScrolled;
-
+			_headerView.HeaderSizeChanged += OnHeaderSizeChanged;
 			((IShellController)_context.Shell).StructureChanged += OnStructureChanged;
 		}
+		
+		void OnHeaderSizeChanged(object sender, EventArgs e)
+		{
+			_headerSize = HeaderMax;
+			TableView.ContentInset = new UIEdgeInsets((nfloat)HeaderMax + SafeAreaOffset, 0, 0, 0);
+			LayoutParallax();
+		}
 
 		void OnStructureChanged(object sender, EventArgs e)
 		{
@@ -40,16 +39,20 @@ namespace Xamarin.Forms.Platform.iOS
 
 		public void LayoutParallax()
 		{
+			if (TableView?.Superview == null)
+				return;
+
 			var parent = TableView.Superview;
 			TableView.Frame = parent.Bounds.Inset(0, SafeAreaOffset);
 			if (_headerView != null)
 			{
 				_headerView.Frame = new CGRect(0, _headerOffset + SafeAreaOffset, parent.Frame.Width, _headerSize);
 
-				if (_headerOffset < 0 && _headerSize + _headerOffset >= 0)
+				var headerHeight = Math.Max(_headerMin, _headerSize + _headerOffset);
+				if (_headerOffset < 0)
 				{
 					CAShapeLayer shapeLayer = new CAShapeLayer();
-					CGRect rect = new CGRect(0, _headerOffset * -1, parent.Frame.Width, _headerSize + _headerOffset);
+					CGRect rect = new CGRect(0, _headerOffset * -1, parent.Frame.Width, headerHeight);
 					var path = CGPath.FromRect(rect);
 					shapeLayer.Path = path;
 					_headerView.Layer.Mask = shapeLayer;
@@ -60,11 +63,12 @@ namespace Xamarin.Forms.Platform.iOS
 		public override void ViewDidLoad()
 		{
 			base.ViewDidLoad();
+			_headerView.MeasureIfNeeded();
 
 			TableView.SeparatorStyle = UITableViewCellSeparatorStyle.None;
 			if (Forms.IsiOS11OrNewer)
 				TableView.ContentInsetAdjustmentBehavior = UIScrollViewContentInsetAdjustmentBehavior.Never;
-			TableView.ContentInset = new UIEdgeInsets((nfloat)_headerMax + SafeAreaOffset, 0, 0, 0);
+			TableView.ContentInset = new UIEdgeInsets((nfloat)HeaderMax + SafeAreaOffset, 0, 0, 0);
 			TableView.Source = _source;
 		}
 
@@ -74,11 +78,18 @@ namespace Xamarin.Forms.Platform.iOS
 			{
 				if ((_context?.Shell as IShellController) != null)
 					((IShellController)_context.Shell).StructureChanged -= OnStructureChanged;
+
+				if(_source != null)
+					_source.ScrolledEvent -= OnScrolled;
+
+				if(_headerView != null)
+					_headerView.HeaderSizeChanged -= OnHeaderSizeChanged;
 			}
 
 			base.Dispose(disposing);
 		}
 
+
 		void OnScrolled(object sender, UIScrollView e)
 		{
 			var headerBehavior = _context.Shell.FlyoutHeaderBehavior;
@@ -87,16 +98,16 @@ namespace Xamarin.Forms.Platform.iOS
 			{
 				case FlyoutHeaderBehavior.Default:
 				case FlyoutHeaderBehavior.Fixed:
-					_headerSize = _headerMax;
+					_headerSize = HeaderMax;
 					break;
 
 				case FlyoutHeaderBehavior.Scroll:
-					_headerSize = _headerMax;
-					_headerOffset = Math.Min(0, -(_headerMax + e.ContentOffset.Y));
+					_headerSize = HeaderMax;
+					_headerOffset = Math.Min(0, -(HeaderMax + e.ContentOffset.Y));
 					break;
 
 				case FlyoutHeaderBehavior.CollapseOnScroll:
-					_headerSize = Math.Max(_headerMin, Math.Min(_headerMax, _headerMax - e.ContentOffset.Y - _headerMax));
+					_headerSize = Math.Max(_headerMin, Math.Min(HeaderMax, HeaderMax - e.ContentOffset.Y - HeaderMax));
 					break;
 			}
 
@@ -104,5 +115,6 @@ namespace Xamarin.Forms.Platform.iOS
 		}
 
 		float SafeAreaOffset => (float)Platform.SafeAreaInsetsForWindow.Top;
+		double HeaderMax => _headerView.MeasuredHeight;
 	}
 }

Xamarin.Forms.Platform.iOS/Renderers/UIContainerView.cs

@@ -1,4 +1,5 @@
-using CoreGraphics;
+using System;
+using CoreGraphics;
 using UIKit;
 
 namespace Xamarin.Forms.Platform.iOS
@@ -8,6 +9,7 @@ namespace Xamarin.Forms.Platform.iOS
 		readonly View _view;
 		IVisualElementRenderer _renderer;
 		bool _disposed;
+		internal event EventHandler HeaderSizeChanged;
 
 		public UIContainerView(View view)
 		{
@@ -17,11 +19,40 @@ namespace Xamarin.Forms.Platform.iOS
 			Platform.SetRenderer(view, _renderer);
 
 			AddSubview(_renderer.NativeView);
+			ClipsToBounds = true;
+			view.MeasureInvalidated += OnMeasureInvalidated;
+			MeasuredHeight = double.NaN;
+		}
+
+		internal double MeasuredHeight { get; private set; }
+
+		internal bool MeasureIfNeeded()
+		{
+			if (double.IsNaN(MeasuredHeight))
+			{
+				ReMeasure();
+				return true;
+			}
+			return false;
+		}
+
+		void ReMeasure()
+		{
+			var request = _view.Measure(Frame.Width, double.PositiveInfinity, MeasureFlags.IncludeMargins);
+			Layout.LayoutChildIntoBoundingRegion(_view, new Rectangle(0, 0, Frame.Width, request.Request.Height));
+			MeasuredHeight = request.Request.Height;
+			HeaderSizeChanged?.Invoke(this, EventArgs.Empty);
+		}
+
+		void OnMeasureInvalidated(object sender, System.EventArgs e)
+		{
+			ReMeasure();
 		}
 
 		public override void LayoutSubviews()
 		{
-			_view.Layout(Bounds.ToRectangle());
+			if(!MeasureIfNeeded())
+				_view.Layout(Bounds.ToRectangle());
 		}
 
 		protected override void Dispose(bool disposing)