Fixup Flyout Template Parts (#8022)

* Fixup Flyout Template Parts

* - fix close enough comparisons

* - fix comparisons

* Update ShellFlyoutTemplatedContentRenderer.cs

* Update src/Controls/tests/DeviceTests/Elements/Shell/ShellTests.Android.cs

Co-authored-by: Rachel Kang <rachelkang@microsoft.com>

Co-authored-by: Rachel Kang <rachelkang@microsoft.com>

Author: PureWeen

src/Controls/samples/Controls.Sample.Sandbox/SandboxShell.xaml

@@ -0,0 +1,10 @@
+<Shell
+    xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
+    xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
+    x:Class="Maui.Controls.Sample.SandboxShell"
+    xmlns:local="clr-namespace:Maui.Controls.Sample"
+    x:Name="shell">
+    <FlyoutItem Title="Flyout Item Title">
+        <ShellContent ContentTemplate="{DataTemplate local:MainPage}"></ShellContent>
+    </FlyoutItem>
+</Shell>

src/Controls/samples/Controls.Sample.Sandbox/SandboxShell.xaml.cs

@@ -0,0 +1,17 @@
+using System;
+using System.Collections.ObjectModel;
+using System.Diagnostics;
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Maui;
+using Microsoft.Maui.Controls;
+
+namespace Maui.Controls.Sample
+{
+	public partial class SandboxShell : Shell
+	{
+		public SandboxShell()
+		{
+			InitializeComponent();
+		}
+	}
+}

src/Controls/src/Core/Compatibility/Handlers/Shell/Android/ContainerView.cs

@@ -3,16 +3,20 @@
 using Android.Runtime;
 using Android.Util;
 using Android.Views;
+using Microsoft.Maui.Platform;
 using AView = Android.Views.View;
 
 namespace Microsoft.Maui.Controls.Platform.Compatibility
 {
 	public class ContainerView : ViewGroup
 	{
+		int _parentTopPadding;
 		View _view;
 		ShellViewRenderer _shellContentView;
 		readonly IMauiContext _mauiContext;
-		AView PlatformView => _view?.Handler?.PlatformView as AView;
+		internal AView PlatformView => _view?.Handler?.PlatformView as AView;
+		internal Graphics.Size DesiredSize { get; private set; }
+		internal Graphics.Size MinimumSize { get; private set; }
 
 		public ContainerView(Context context, View view, IMauiContext mauiContext) : base(context)
 		{
@@ -60,6 +64,28 @@ protected override void OnLayout(bool changed, int l, int t, int r, int b)
 			((IPlatformViewHandler)View.Handler).LayoutVirtualView(l, t, r, b);
 		}
 
+		// This indicates the amount of top padding the parent wants this control to have. This
+		// allows us to adjust the height measured so that the parent container maintains a constant height.
+		//
+		// This is mainly used with ShellHeaderFlyoutBehavior.CollapseOnScroll
+		// The container added to the AppBarLayout needs to maintain a constant height otherwise the 
+		// OnOffsetChanged value passed in to IOnOffsetChangedListener will just infinitely change as the
+		// header size keeps changing. So in order to make the collapse work we just have to resize the xplat view
+		// and then shift it down in the container.
+		//
+		// In theory we could call `IView.Arrange` with the desired position in the parent but then that sets the xplat `Frame`
+		// to values that don't really make any sense from an xplat perspective. This just causes implementation details
+		// to leak up into the abstraction. It'll just be confusing that the headers position is "0,300" even though it looks
+		// like it's "0,0"
+		internal void SetParentTopPadding(int v)
+		{
+			if (_parentTopPadding == v)
+				return;
+
+			_parentTopPadding = v;
+			this.MaybeRequestLayout();
+		}
+
 		protected override void OnMeasure(int widthMeasureSpec, int heightMeasureSpec)
 		{
 			if (View == null)
@@ -77,8 +103,8 @@ protected override void OnMeasure(int widthMeasureSpec, int heightMeasureSpec)
 			var width = widthMeasureSpec.GetSize();
 			var height = heightMeasureSpec.GetSize();
 
-			var measureWidth = width > 0 ? width : MeasureSpecMode.Unspecified.MakeMeasureSpec(0);
-			var measureHeight = height > 0 ? height : MeasureSpecMode.Unspecified.MakeMeasureSpec(0);
+			var measureWidth = width > 0 ? MeasureSpecMode.Exactly.MakeMeasureSpec(width) : MeasureSpecMode.Unspecified.MakeMeasureSpec(0);
+			var measureHeight = height > 0 ? MeasureSpecMode.Exactly.MakeMeasureSpec(height) : MeasureSpecMode.Unspecified.MakeMeasureSpec(0);
 
 			double? maxHeight = null;
 
@@ -97,6 +123,54 @@ protected override void OnMeasure(int widthMeasureSpec, int heightMeasureSpec)
 			}
 
 			var size = _shellContentView.Measure(measureWidth, measureHeight, null, (int?)maxHeight);
+
+			if (size.Height < this.MinimumHeight)
+				size = new Graphics.Size(size.Width, this.MinimumHeight);
+
+			DesiredSize = size;
+			var newHeight = size.Height;
+
+			if (_parentTopPadding > 0)
+			{
+				newHeight -= _parentTopPadding;
+			}
+
+			if (newHeight != size.Height)
+			{
+				// Android treats zero as unspecified so we need to set the height to at least 1.
+				// If there's a view that's able to squish/stretch then passing in zero will give it's stretched size
+				// but what we want here is the squished size.
+				var minHeightMeasureSpec = 1;
+
+				if (MinimumHeight > 0)
+					minHeightMeasureSpec = MinimumHeight;
+
+				if (newHeight <= 0)
+				{
+					newHeight = minHeightMeasureSpec;
+				}
+
+				// For collapse on scroll we need to know the minimum size the view can reach so we can set the minimum size
+				// on the headers container. This is the only way to get the AppBarLayout collapse behavior to stop producing
+				// offset changes once the header has reached the minimum size it can get to
+				MinimumSize = _shellContentView.Measure(measureWidth, MeasureSpecMode.Exactly.MakeMeasureSpec(minHeightMeasureSpec), null, (int?)maxHeight);
+
+				// If the offset of the appbar layout hasn't reached the minimum size then 
+				// we use the new collapsed size to remeasure so everything is the correct size
+				if (newHeight > minHeightMeasureSpec)
+				{
+					size = _shellContentView.Measure(measureWidth, MeasureSpecMode.Exactly.MakeMeasureSpec((int)newHeight), null, (int?)maxHeight);
+				}
+				else
+				{
+					size = MinimumSize;
+				}
+			}
+			else
+			{
+				this.MinimumSize = new Graphics.Size((int)size.Width, this.MinimumHeight);
+			}
+
 			SetMeasuredDimension((int)size.Width, (int)size.Height);
 		}
 

src/Controls/src/Core/Compatibility/Handlers/Shell/Android/ShellContentView.cs

@@ -6,6 +6,7 @@
 using Google.Android.Material.AppBar;
 using Microsoft.Maui.Controls.Platform.Compatibility;
 using Microsoft.Maui.Graphics;
+using Microsoft.Maui.Platform;
 using AView = Android.Views.View;
 using LP = Android.Views.ViewGroup.LayoutParams;
 
@@ -15,7 +16,7 @@ namespace Microsoft.Maui.Controls.Platform.Compatibility
 	internal class ShellViewRenderer
 	{
 		public IViewHandler Handler { get; private set; }
-		View _view;
+		IView _view;
 		WeakReference<Context> _context;
 		readonly IMauiContext _mauiContext;
 		IView MauiView => View;
@@ -38,7 +39,7 @@ public ShellViewRenderer(Context context, View view, IMauiContext mauiContext)
 			View = view;
 		}
 
-		public View View
+		public IView View
 		{
 			get { return _view; }
 			set
@@ -49,6 +50,7 @@ public View View
 
 		public void TearDown()
 		{
+			MauiView?.Handler?.DisconnectHandler();
 			View = null;
 			Handler = null;
 			_view = null;
@@ -89,15 +91,19 @@ public Graphics.Size Measure(int widthMeasureSpec, int heightMeasureSpec, int? m
 				heightMeasureSpec = MeasureSpecMode.Unspecified.MakeMeasureSpec(0);
 
 			PlatformView.LayoutParameters = layoutParams;
+
 			return ((IPlatformViewHandler)_view.Handler).MeasureVirtualView(widthMeasureSpec, heightMeasureSpec);
 		}
 
-		public virtual void OnViewSet(View view)
+		public virtual void OnViewSet(IView view)
 		{
-			if (View != null)
+			if (view == View)
+				return;
+
+			if (View != null && PlatformView.IsAlive())
 			{
 				PlatformView.RemoveFromParent();
-				View.Handler = null;
+				View.Handler?.DisconnectHandler();
 			}
 
 			_view = view;

src/Controls/src/Core/Compatibility/Handlers/Shell/Android/ShellFlyoutRenderer.cs

@@ -124,7 +124,7 @@ void IFlyoutBehaviorObserver.OnFlyoutBehaviorChanged(FlyoutBehavior behavior)
 		int _flyoutWidthDefault;
 		double _flyoutWidth = -1;
 		double _flyoutHeight;
-		bool _flyoutFirstDrawPassFinished;
+		internal bool FlyoutFirstDrawPassFinished { get; private set; }
 		int _currentLockMode;
 		bool _disposed;
 		Brush _scrimBrush;
@@ -182,10 +182,10 @@ protected override bool DrawChild(Canvas canvas, AView child, long drawingTime)
 				canvas.DrawRect(0, 0, Width, Height, _scrimPaint);
 			}
 
-			if (!_flyoutFirstDrawPassFinished && _flyoutContent != null)
+			if (!FlyoutFirstDrawPassFinished && _flyoutContent != null)
 			{
 				if (child == _flyoutContent?.AndroidView)
-					_flyoutFirstDrawPassFinished = true;
+					FlyoutFirstDrawPassFinished = true;
 
 				if (this.IsDrawerOpen(_flyoutContent.AndroidView) != _shellContext.Shell.FlyoutIsPresented)
 				{
@@ -261,7 +261,7 @@ protected virtual void OnShellPropertyChanged(object sender, PropertyChangedEven
 
 			if (e.PropertyName == Shell.FlyoutIsPresentedProperty.PropertyName)
 			{
-				if (!_flyoutFirstDrawPassFinished)
+				if (!FlyoutFirstDrawPassFinished)
 				{
 					// if the first draw pass hasn't happened yet
 					// then calling close/open drawer really confuses