Fix Singleton Pattern for LocalizationResourceManager (#919)

* Update LocalizationResourceManager.shared.cs

* Implement Lazy Loading for Thread Safety

* Fix Namespace

* Removed changes from main branch

* Revert Commit

* Update LocalizationResourceManager.shared.cs

* Fix Unit Tests

* Add Collection to LocalizationResourceManagerTests

The `Collection` attribute prevents these tests from running in parallel https://xunit.net/docs/running-tests-in-parallel.html

Author: TheCodeTraveler

src/CommunityToolkit/Xamarin.CommunityToolkit.UnitTests/Helpers/LocalizationResourceManagerTests/LocalizationResourceManagerTests.cs

@@ -4,20 +4,18 @@
 using Xamarin.CommunityToolkit.UnitTests.Mocks;
 using Xunit;
 
-namespace Xamarin.CommunityToolkit.UnitTests.Helpers.WeakEventManagerTests
+namespace Xamarin.CommunityToolkit.UnitTests.Helpers.LocalizationResourceManagerTests
 {
+	[Collection(nameof(LocalizationResourceManager))]
 	public class LocalizationResourceManagerTests
 	{
 		public LocalizationResourceManagerTests()
 		{
 			resourceManager = new MockResourceManager();
-#pragma warning disable CS0618 // Type or member is obsolete
-			localizationManager = new LocalizationResourceManager();
-#pragma warning restore CS0618 // Type or member is obsolete
 			localizationManager.Init(resourceManager, initialCulture);
 		}
 
-		readonly LocalizationResourceManager localizationManager;
+		readonly LocalizationResourceManager localizationManager = LocalizationResourceManager.Current;
 		readonly CultureInfo initialCulture = CultureInfo.InvariantCulture;
 		readonly ResourceManager resourceManager;
 
@@ -49,15 +47,15 @@ public void LocalizationResourceManager_PropertyChanged_Triggered()
 		{
 			// Arrange
 			var culture2 = new CultureInfo("en");
-			localizationManager.SetCulture(culture2);
+			localizationManager.CurrentCulture = culture2;
 			CultureInfo changedCulture = null;
 			localizationManager.PropertyChanged += (s, e) => changedCulture = localizationManager.CurrentCulture;
 
 			// Act, Assert
 			localizationManager.Init(resourceManager, initialCulture);
 			Assert.Equal(initialCulture, changedCulture);
 
-			localizationManager.SetCulture(culture2);
+			localizationManager.CurrentCulture = culture2;
 			Assert.Equal(culture2, changedCulture);
 		}
 	}

src/CommunityToolkit/Xamarin.CommunityToolkit.UnitTests/Helpers/LocalizedStringTests/LocalizedStringTests.cs

@@ -7,20 +7,19 @@
 
 namespace Xamarin.CommunityToolkit.UnitTests.Helpers.LocalizedStringTests
 {
+	[Collection(nameof(LocalizationResourceManager))]
 	public class LocalizedStringTests
 	{
 		public LocalizedStringTests()
 		{
 			resourceManager = new MockResourceManager();
-#pragma warning disable CS0618 // Type or member is obsolete
-			localizationManager = new LocalizationResourceManager();
-#pragma warning restore CS0618 // Type or member is obsolete
 			localizationManager.Init(resourceManager, initialCulture);
 		}
 
-		readonly LocalizationResourceManager localizationManager;
+		readonly LocalizationResourceManager localizationManager = LocalizationResourceManager.Current;
 		readonly CultureInfo initialCulture = CultureInfo.InvariantCulture;
 		readonly ResourceManager resourceManager;
+
 		LocalizedString localizedString;
 
 		[Fact]

src/CommunityToolkit/Xamarin.CommunityToolkit/Helpers/LocalizationResourceManager.shared.cs

@@ -10,16 +10,14 @@ namespace Xamarin.CommunityToolkit.Helpers
 #if !NETSTANDARD1_0
 	public class LocalizationResourceManager : ObservableObject
 	{
-#pragma warning disable CS0618 // Type or member is obsolete
-		public static LocalizationResourceManager Current { get; } = new LocalizationResourceManager();
-#pragma warning restore CS0618 // Type or member is obsolete
+		static readonly Lazy<LocalizationResourceManager> currentHolder = new Lazy<LocalizationResourceManager>(() => new LocalizationResourceManager());
+
+		public static LocalizationResourceManager Current => currentHolder.Value;
 
 		ResourceManager resourceManager;
 		CultureInfo currentCulture = Thread.CurrentThread.CurrentUICulture;
 
-		[Obsolete("Please use the Current property instead of creating a new instance of this class")]
-		[EditorBrowsable(EditorBrowsableState.Never)]
-		public LocalizationResourceManager()
+		LocalizationResourceManager()
 		{
 		}