Fix DatePicker not updating format when culture changes at runtime

src/Controls/tests/TestCases.HostApp/Issues/Issue9.xaml

@@ -0,0 +1,52 @@
+<?xml version="1.0" encoding="utf-8" ?>
+<ContentPage xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
+             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
+             x:Class="Maui.Controls.Sample.Issues.Issue9"
+             Title="DatePicker Culture Change Test">
+    <ScrollView>
+        <StackLayout Padding="20">
+            <Label Text="DatePicker Culture Change Test" FontSize="18" FontAttributes="Bold" Margin="0,0,0,20"/>
+
+            <Label Text="Instructions:" FontAttributes="Bold"/>
+            <Label Text="1. Note the initial date format below"/>
+            <Label Text="2. Change the culture using the buttons"/>
+            <Label Text="3. Observe that the DatePicker format updates immediately"/>
+            <Label Text="4. Try picking a new date to verify it works properly"/>
+
+            <StackLayout Orientation="Horizontal" Margin="0,20">
+                <Label Text="Current Culture:" VerticalOptions="Center"/>
+                <Label x:Name="CurrentCultureLabel" Text="{Binding CurrentCulture}" VerticalOptions="Center" FontAttributes="Bold"/>
+            </StackLayout>
+
+            <DatePicker x:Name="TestDatePicker" 
+                        Date="{Binding TestDate}" 
+                        Format="d"
+                        Margin="0,10"/>
+
+            <Label Text="Culture Selection:" FontAttributes="Bold" Margin="0,20,0,10"/>
+
+            <Button Text="English (US)" 
+                    Command="{Binding ChangeCultureCommand}" 
+                    CommandParameter="en-US"
+                    Margin="0,5"/>
+
+            <Button Text="German (Germany)" 
+                    Command="{Binding ChangeCultureCommand}" 
+                    CommandParameter="de-DE"
+                    Margin="0,5"/>
+
+            <Button Text="French (France)" 
+                    Command="{Binding ChangeCultureCommand}" 
+                    CommandParameter="fr-FR"
+                    Margin="0,5"/>
+
+            <Button Text="Japanese (Japan)" 
+                    Command="{Binding ChangeCultureCommand}" 
+                    CommandParameter="ja-JP"
+                    Margin="0,5"/>
+
+            <Label Text="Test Results:" FontAttributes="Bold" Margin="0,20,0,10"/>
+            <Label x:Name="TestResultsLabel" Text="{Binding TestResults}" BackgroundColor="LightGray" Padding="10"/>
+        </StackLayout>
+    </ScrollView>
+</ContentPage>

src/Controls/tests/TestCases.HostApp/Issues/Issue9.xaml.cs

@@ -0,0 +1,101 @@
+using System;
+using System.ComponentModel;
+using System.Globalization;
+using System.Runtime.CompilerServices;
+using System.Threading.Tasks;
+using System.Windows.Input;
+using Microsoft.Maui.Controls;
+
+namespace Maui.Controls.Sample.Issues;
+
+[Issue(IssueTracker.Github, 9, "DatePicker Does Not Update Its Format When the Culture Is Changed at Runtime",
+	PlatformAffected.All)]
+public partial class Issue9 : ContentPage, INotifyPropertyChanged
+{
+	private string _currentCulture = string.Empty;
+	private DateTime _testDate = DateTime.Today;
+	private string _testResults = "No tests run yet.";
+
+	public Issue9()
+	{
+		InitializeComponent();
+		BindingContext = this;
+
+		CurrentCulture = CultureInfo.CurrentCulture.DisplayName;
+		ChangeCultureCommand = new Command<string>(async (culture) => await ChangeCulture(culture));
+	}
+
+	public string CurrentCulture
+	{
+		get => _currentCulture;
+		set
+		{
+			_currentCulture = value;
+			OnPropertyChanged();
+		}
+	}
+
+	public DateTime TestDate
+	{
+		get => _testDate;
+		set
+		{
+			_testDate = value;
+			OnPropertyChanged();
+		}
+	}
+
+	public string TestResults
+	{
+		get => _testResults;
+		set
+		{
+			_testResults = value;
+			OnPropertyChanged();
+		}
+	}
+
+	public ICommand ChangeCultureCommand { get; }
+
+	private async Task ChangeCulture(string cultureName)
+	{
+		try
+		{
+			var previousFormat = GetCurrentDateFormat();
+
+			// Change the culture
+			var culture = new CultureInfo(cultureName);
+			CultureInfo.CurrentCulture = culture;
+			CultureInfo.CurrentUICulture = culture;
+
+			CurrentCulture = culture.DisplayName;
+
+			// Wait a moment for the culture change to propagate
+			await Task.Delay(100);
+
+			var newFormat = GetCurrentDateFormat();
+
+			TestResults = $"Culture changed to {cultureName}\n" +
+			             $"Previous format: {previousFormat}\n" +
+			             $"New format: {newFormat}\n" +
+			             $"Date example: {TestDate.ToString("d")}\n" +
+			             $"Test completed at: {DateTime.Now:HH:mm:ss}";
+		}
+		catch (Exception ex)
+		{
+			TestResults = $"Error changing culture: {ex.Message}";
+		}
+	}
+
+	private string GetCurrentDateFormat()
+	{
+		return TestDate.ToString("d", CultureInfo.CurrentCulture);
+	}
+
+	public event PropertyChangedEventHandler? PropertyChanged;
+
+	protected virtual void OnPropertyChanged([CallerMemberName] string? propertyName = null)
+	{
+		PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
+	}
+}

src/Controls/tests/TestCases.Shared.Tests/Tests/Issues/Issue9.cs

@@ -0,0 +1,63 @@
+using NUnit.Framework;
+using UITest.Appium;
+using UITest.Core;
+
+namespace Microsoft.Maui.TestCases.Tests.Issues;
+
+public class Issue9 : _IssuesUITest
+{
+	public Issue9(TestDevice testDevice) : base(testDevice)
+	{
+	}
+
+	public override string Issue => "DatePicker Does Not Update Its Format When the Culture Is Changed at Runtime";
+
+	[Test]
+	[Category(UITestCategories.DatePicker)]
+	public void DatePickerFormatUpdatesWhenCultureChanges()
+	{
+		App.WaitForElement("TestDatePicker");
+
+		// Get the initial date format
+		var initialDateFormat = GetDatePickerText();
+
+		// Change culture to German
+		App.Tap("German (Germany)");
+		App.WaitForElement("TestDatePicker");
+
+		// Wait for culture change to propagate
+		App.WaitForNoElement("No tests run yet.", timeout: TimeSpan.FromSeconds(5));
+
+		// Get the new date format after culture change
+		var germanDateFormat = GetDatePickerText();
+
+		// The formats should be different
+		Assert.That(germanDateFormat, Is.Not.EqualTo(initialDateFormat), 
+			"DatePicker format should change when culture changes");
+
+		// Change culture to French
+		App.Tap("French (France)");
+		App.WaitForElement("TestDatePicker");
+
+		// Wait for culture change to propagate
+		System.Threading.Thread.Sleep(500);
+
+		// Get the French date format
+		var frenchDateFormat = GetDatePickerText();
+
+		// The French format should be different from German
+		Assert.That(frenchDateFormat, Is.Not.EqualTo(germanDateFormat), 
+			"DatePicker format should change when culture changes to French");
+
+		// Verify we can still interact with the DatePicker
+		App.Tap("TestDatePicker");
+		// On some platforms, this opens a date picker dialog
+		// The test passes if no exception is thrown
+	}
+
+	private string GetDatePickerText()
+	{
+		var datePicker = App.FindElement("TestDatePicker");
+		return datePicker.GetText();
+	}
+}

src/Core/src/CultureTracker.cs

@@ -0,0 +1,88 @@
+using System;
+using System.Collections.Concurrent;
+using System.Globalization;
+
+namespace Microsoft.Maui
+{
+	/// <summary>
+	/// Provides culture change detection functionality for MAUI controls.
+	/// </summary>
+	internal static class CultureTracker
+	{
+		static CultureInfo? s_currentCulture;
+		static readonly ConcurrentDictionary<WeakReference, Action> s_subscribers = new();
+
+		/// <summary>
+		/// Checks if the culture has changed since the last call and notifies subscribers if it has.
+		/// </summary>
+		public static void CheckForCultureChanges()
+		{
+			var currentCulture = CultureInfo.CurrentCulture;
+
+			if (s_currentCulture == null || !s_currentCulture.Equals(currentCulture))
+			{
+				s_currentCulture = currentCulture;
+				NotifyCultureChanged();
+			}
+		}
+
+		/// <summary>
+		/// Subscribes an object to culture change notifications.
+		/// </summary>
+		/// <param name="subscriber">The object to subscribe</param>
+		/// <param name="action">The action to invoke when culture changes</param>
+		public static void Subscribe(object subscriber, Action action)
+		{
+			var weakRef = new WeakReference(subscriber);
+			s_subscribers.TryAdd(weakRef, action);
+		}
+
+		/// <summary>
+		/// Unsubscribes an object from culture change notifications.
+		/// </summary>
+		/// <param name="subscriber">The object to unsubscribe</param>
+		public static void Unsubscribe(object subscriber)
+		{
+			// Find and remove the weak reference
+			foreach (var kvp in s_subscribers)
+			{
+				if (kvp.Key.IsAlive && ReferenceEquals(kvp.Key.Target, subscriber))
+				{
+					s_subscribers.TryRemove(kvp.Key, out _);
+					break;
+				}
+			}
+		}
+
+		static void NotifyCultureChanged()
+		{
+			// Clean up dead references and notify live ones
+			var deadRefs = new System.Collections.Generic.List<WeakReference>();
+
+			foreach (var kvp in s_subscribers)
+			{
+				if (!kvp.Key.IsAlive)
+				{
+					deadRefs.Add(kvp.Key);
+				}
+				else
+				{
+					try
+					{
+						kvp.Value?.Invoke();
+					}
+					catch
+					{
+						// Ignore exceptions from subscribers to prevent one bad subscriber from affecting others
+					}
+				}
+			}
+
+			// Clean up dead references
+			foreach (var deadRef in deadRefs)
+			{
+				s_subscribers.TryRemove(deadRef, out _);
+			}
+		}
+	}
+}

src/Core/src/Handlers/DatePicker/DatePickerHandler.Android.cs

@@ -33,6 +33,9 @@ protected override void ConnectHandler(MauiDatePicker platformView)
 			platformView.ViewAttachedToWindow += OnViewAttachedToWindow;
 			platformView.ViewDetachedFromWindow += OnViewDetachedFromWindow;
 
+			// Subscribe to culture changes
+			CultureTracker.Subscribe(this, OnCultureChanged);
+
 			if (platformView.IsAttachedToWindow)
 				OnViewAttachedToWindow();
 		}
@@ -61,6 +64,9 @@ protected override void DisconnectHandler(MauiDatePicker platformView)
 			platformView.ViewDetachedFromWindow -= OnViewDetachedFromWindow;
 			OnViewDetachedFromWindow();
 
+			// Unsubscribe from culture changes
+			CultureTracker.Unsubscribe(this);
+
 			base.DisconnectHandler(platformView);
 		}
 
@@ -164,5 +170,14 @@ void OnMainDisplayInfoChanged(object? sender, DisplayInfoChangedEventArgs e)
 				ShowPickerDialog(currentDialog.DatePicker.Year, currentDialog.DatePicker.Month, currentDialog.DatePicker.DayOfMonth);
 			}
 		}
+
+		void OnCultureChanged()
+		{
+			// Refresh the date format when culture changes
+			if (PlatformView != null && VirtualView != null)
+			{
+				PlatformView.UpdateDate(VirtualView);
+			}
+		}
 	}
 }

src/Core/src/Handlers/DatePicker/DatePickerHandler.Windows.cs

@@ -11,11 +11,17 @@ public partial class DatePickerHandler : ViewHandler<IDatePicker, CalendarDatePi
 		protected override void ConnectHandler(CalendarDatePicker platformView)
 		{
 			platformView.DateChanged += DateChanged;
+
+			// Subscribe to culture changes
+			CultureTracker.Subscribe(this, OnCultureChanged);
 		}
 
 		protected override void DisconnectHandler(CalendarDatePicker platformView)
 		{
 			platformView.DateChanged -= DateChanged;
+
+			// Unsubscribe from culture changes
+			CultureTracker.Unsubscribe(this);
 		}
 
 		public static partial void MapFormat(IDatePickerHandler handler, IDatePicker datePicker)
@@ -85,5 +91,14 @@ public static partial void MapBackground(IDatePickerHandler handler, IDatePicker
 		{
 			handler.PlatformView?.UpdateBackground(datePicker);
 		}
+
+		void OnCultureChanged()
+		{
+			// Refresh the date format when culture changes
+			if (PlatformView != null && VirtualView != null)
+			{
+				PlatformView.UpdateDate(VirtualView);
+			}
+		}
 	}
 }