Fix NullReferenceException when displaying Toast or SnackBar

src/CommunityToolkit/Xamarin.CommunityToolkit/Extensions/PageExtension.shared.cs

@@ -10,7 +10,7 @@ namespace Xamarin.CommunityToolkit.Extensions
 {
 	public static class PageExtension
 	{
-		public static Task DisplayToastAsync(this Page page, string message, int durationMilliseconds = 3000)
+		public static async Task DisplayToastAsync(this Page page, string message, int durationMilliseconds = 3000)
 		{
 			_ = page ?? throw new ArgumentNullException(nameof(page));
 
@@ -26,11 +26,11 @@ public static Task DisplayToastAsync(this Page page, string message, int duratio
 #endif
 			};
 			var snackBar = new SnackBar();
-			snackBar.Show(page, args);
-			return args.Result.Task;
+			await snackBar.Show(page, args);
+			await args.Result.Task;
 		}
 
-		public static Task DisplayToastAsync(this Page page, ToastOptions toastOptions)
+		public static async Task DisplayToastAsync(this Page page, ToastOptions toastOptions)
 		{
 			_ = page ?? throw new ArgumentNullException(nameof(page));
 
@@ -43,11 +43,11 @@ public static Task DisplayToastAsync(this Page page, ToastOptions toastOptions)
 				BackgroundColor = arguments.BackgroundColor,
 				IsRtl = arguments.IsRtl
 			};
-			snackBar.Show(page, options);
-			return options.Result.Task;
+			await snackBar.Show(page, options);
+			await options.Result.Task;
 		}
 
-		public static Task<bool> DisplaySnackBarAsync(this Page page, string message, string actionButtonText, Func<Task> action, int durationMilliseconds = 3000)
+		public static async Task<bool> DisplaySnackBarAsync(this Page page, string message, string actionButtonText, Func<Task> action, int durationMilliseconds = 3000)
 		{
 			_ = page ?? throw new ArgumentNullException(nameof(page));
 
@@ -71,18 +71,22 @@ public static Task<bool> DisplaySnackBarAsync(this Page page, string message, st
 #endif
 			};
 			var snackBar = new SnackBar();
-			snackBar.Show(page, options);
-			return options.Result.Task;
+			await snackBar.Show(page, options);
+			var isButtonClicked = await options.Result.Task;
+
+			return isButtonClicked;
 		}
 
-		public static Task<bool> DisplaySnackBarAsync(this Page page, SnackBarOptions snackBarOptions)
+		public static async Task<bool> DisplaySnackBarAsync(this Page page, SnackBarOptions snackBarOptions)
 		{
 			_ = page ?? throw new ArgumentNullException(nameof(page));
 
 			var snackBar = new SnackBar();
-			var arguments = snackBarOptions ?? new SnackBarOptions();
-			snackBar.Show(page, arguments);
-			return arguments.Result.Task;
+			var options = snackBarOptions ?? new SnackBarOptions();
+			await snackBar.Show(page, options);
+			var isButtonClicked = await options.Result.Task;
+
+			return isButtonClicked;
 		}
 	}
 }

src/CommunityToolkit/Xamarin.CommunityToolkit/Views/Snackbar/SnackBar.android.cs

@@ -1,10 +1,11 @@
-using Xamarin.Forms;
+using System;
+using System.Threading.Tasks;
+using Xamarin.Forms;
 using Android.Graphics;
 using Android.Widget;
 using Xamarin.Forms.Platform.Android;
 using Xamarin.CommunityToolkit.UI.Views.Options;
 using Android.Util;
-using System;
 #if MONOANDROID10_0
 using AndroidSnackBar = Google.Android.Material.Snackbar.Snackbar;
 #else
@@ -15,10 +16,10 @@ namespace Xamarin.CommunityToolkit.UI.Views
 {
 	class SnackBar
 	{
-		internal void Show(Page sender, SnackBarOptions arguments)
+		internal async ValueTask Show(Page sender, SnackBarOptions arguments)
 		{
-			var view = Platform.GetRenderer(sender).View;
-			var snackBar = AndroidSnackBar.Make(view, arguments.MessageOptions.Message, (int)arguments.Duration.TotalMilliseconds);
+			var renderer = await GetRendererWithRetries(sender) ?? throw new ArgumentException("Provided page cannot be parent to SnackBar", nameof(sender));
+			var snackBar = AndroidSnackBar.Make(renderer.View, arguments.MessageOptions.Message, (int)arguments.Duration.TotalMilliseconds);
 			var snackBarView = snackBar.View;
 			if (arguments.BackgroundColor != Forms.Color.Default)
 			{
@@ -100,6 +101,20 @@ internal void Show(Page sender, SnackBarOptions arguments)
 			snackBar.Show();
 		}
 
+		/// <summary>
+		/// Tries to get renderer multiple times since it can be null while switching tabs in Shell.
+		/// See this bug for more info: https://github.com/xamarin/Xamarin.Forms/issues/13950
+		/// </summary>
+		static async Task<IVisualElementRenderer?> GetRendererWithRetries(Page page, int retryCount = 5)
+		{
+			var renderer = Platform.GetRenderer(page);
+			if (renderer != null || retryCount <= 0)
+				return renderer;
+
+			await Task.Delay(50);
+			return await GetRendererWithRetries(page, retryCount - 1);
+		}
+
 		class SnackBarCallback : AndroidSnackBar.BaseCallback
 		{
 			readonly SnackBarOptions arguments;

src/CommunityToolkit/Xamarin.CommunityToolkit/Views/Snackbar/SnackBar.gtk.cs

@@ -1,4 +1,5 @@
 using System.Linq;
+using System.Threading.Tasks;
 using System.Timers;
 using Gtk;
 using Pango;
@@ -13,7 +14,7 @@ class SnackBar
 	{
 		Timer? snackBarTimer;
 
-		public void Show(Page page, SnackBarOptions arguments)
+		public ValueTask Show(Page page, SnackBarOptions arguments)
 		{
 			var mainWindow = (Platform.GetRenderer(page).Container.Child as Forms.Platform.GTK.Controls.Page)?.Children[0] as VBox;
 			var snackBarLayout = GetSnackBarLayout(mainWindow, arguments);
@@ -29,6 +30,7 @@ public void Show(Page page, SnackBarOptions arguments)
 			};
 
 			snackBarTimer.Start();
+			return default;
 		}
 
 		HBox GetSnackBarLayout(Container? container, SnackBarOptions arguments)

src/CommunityToolkit/Xamarin.CommunityToolkit/Views/Snackbar/SnackBar.ios.macos.cs

@@ -17,7 +17,7 @@ namespace Xamarin.CommunityToolkit.UI.Views
 {
 	class SnackBar
 	{
-		internal void Show(Page sender, SnackBarOptions arguments)
+		internal ValueTask Show(Page sender, SnackBarOptions arguments)
 		{
 			var snackBar = NativeSnackBar.MakeSnackBar(arguments.MessageOptions.Message)
 							.SetDuration(arguments.Duration.TotalMilliseconds)
@@ -124,6 +124,8 @@ internal void Show(Page sender, SnackBarOptions arguments)
 			}
 
 			snackBar.Show();
+
+			return default;
 		}
 	}
 }

src/CommunityToolkit/Xamarin.CommunityToolkit/Views/Snackbar/SnackBar.netstandard.tvos.watchos.cs

@@ -1,11 +1,12 @@
 using System;
+using System.Threading.Tasks;
 using Xamarin.CommunityToolkit.UI.Views.Options;
 using Xamarin.Forms;
 
 namespace Xamarin.CommunityToolkit.UI.Views
 {
 	class SnackBar
 	{
-		internal void Show(Page sender, SnackBarOptions arguments) => throw new PlatformNotSupportedException();
+		internal ValueTask Show(Page sender, SnackBarOptions arguments) => throw new PlatformNotSupportedException();
 	}
 }

src/CommunityToolkit/Xamarin.CommunityToolkit/Views/Snackbar/SnackBar.tizen.cs

@@ -1,13 +1,14 @@
 using System;
 using System.Linq;
+using System.Threading.Tasks;
 using Xamarin.CommunityToolkit.UI.Views.Options;
 using EButton = ElmSharp.Button;
 
 namespace Xamarin.CommunityToolkit.UI.Views
 {
 	class SnackBar
 	{
-		internal void Show(Forms.Page sender, SnackBarOptions arguments)
+		internal ValueTask Show(Forms.Page sender, SnackBarOptions arguments)
 		{
 			var snackBarDialog =
 				Forms.Platform.Tizen.Native.Dialog.CreateDialog(Forms.Forms.NativeParent,
@@ -36,11 +37,11 @@ internal void Show(Forms.Page sender, SnackBarOptions arguments)
 			}
 
 			snackBarDialog.TimedOut += (s, evt) => DismissSnackBar();
-
 			snackBarDialog.BackButtonPressed += (s, evt) => DismissSnackBar();
-
 			snackBarDialog.Show();
 
+			return default;
+
 			void DismissSnackBar()
 			{
 				snackBarDialog.Dismiss();

src/CommunityToolkit/Xamarin.CommunityToolkit/Views/Snackbar/SnackBar.uwp.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Threading.Tasks;
 using Windows.UI.Xaml;
 using Windows.UI.Xaml.Controls;
 using Windows.UI.Xaml.Media;
@@ -34,7 +35,7 @@ class SnackBar
 			return null;
 		}
 
-		internal void Show(Forms.Page page, SnackBarOptions arguments)
+		internal ValueTask Show(Forms.Page page, SnackBarOptions arguments)
 		{
 			var snackBarLayout = new SnackBarLayout(arguments);
 			var pageControl = Platform.GetRenderer(page).ContainerElement.Parent;
@@ -59,6 +60,7 @@ internal void Show(Forms.Page page, SnackBarOptions arguments)
 			grid.RowDefinitions.Add(snackBarRow);
 			grid.Children.Add(snackBarLayout);
 			Grid.SetRow(snackBarLayout, grid.RowDefinitions.Count - 1);
+			return default;
 		}
 	}
 }

src/CommunityToolkit/Xamarin.CommunityToolkit/Views/Snackbar/SnackBar.wpf.cs

@@ -1,4 +1,5 @@
-using System.Windows.Forms;
+using System.Threading.Tasks;
+using System.Windows.Forms;
 using Xamarin.CommunityToolkit.UI.Views.Helpers;
 using Xamarin.CommunityToolkit.UI.Views.Options;
 using Xamarin.Forms;
@@ -11,7 +12,7 @@ class SnackBar
 	{
 		Timer? snackBarTimer;
 
-		internal void Show(Page page, SnackBarOptions arguments)
+		internal ValueTask Show(Page page, SnackBarOptions arguments)
 		{
 			var formsAppBar = System.Windows.Application.Current.MainWindow.FindChild<FormsAppBar>("PART_BottomAppBar");
 			var currentContent = formsAppBar.Content;
@@ -31,6 +32,7 @@ internal void Show(Page page, SnackBarOptions arguments)
 			};
 			snackBarTimer.Start();
 			formsAppBar.Content = snackBar;
+			return default;
 		}
 	}
 }