Fix DatePicker CharacterSpacing property not working on Windows platform

[Copilot]

Description

The CharacterSpacing property on DatePicker was not working on the Windows platform. While this property worked correctly on other platforms like Android and iOS, it had no visible effect on Windows.

Root Cause

The issue occurred because:

• On Android: MauiDatePicker inherits from AppCompatEditText (which extends TextView), so it automatically gets the CharacterSpacing property support through existing TextViewExtensions.UpdateCharacterSpacing().

• On Windows: CalendarDatePicker is a composite control that doesn't have a direct CharacterSpacing property like text controls do. The existing implementation tried to set platformDatePicker.CharacterSpacing = datePicker.CharacterSpacing.ToEm() but CalendarDatePicker doesn't expose this property.

Solution

Modified the Windows-specific UpdateCharacterSpacing method in DatePickerExtensions.cs to:

1. Traverse the visual tree of the CalendarDatePicker to find internal TextBlock elements
2. Apply character spacing to each TextBlock using the existing CharacterSpacing.ToEm() conversion
3. Handle loading state using the OnLoaded pattern to ensure the visual tree is available

public static void UpdateCharacterSpacing(this CalendarDatePicker platformDatePicker, IDatePicker datePicker)
{
    var characterSpacing = datePicker.CharacterSpacing;
    
    if (platformDatePicker.IsLoaded)
    {
        ApplyCharacterSpacingToTextBlocks(platformDatePicker, characterSpacing);
    }
    else
    {
        platformDatePicker.OnLoaded(() => ApplyCharacterSpacingToTextBlocks(platformDatePicker, characterSpacing));
    }
}

static void ApplyCharacterSpacingToTextBlocks(CalendarDatePicker platformDatePicker, double characterSpacing)
{
    var textBlocks = platformDatePicker.GetChildren<Microsoft.UI.Xaml.Controls.TextBlock>();
    var characterSpacingEm = characterSpacing.ToEm();
    
    foreach (var textBlock in textBlocks)
    {
        if (textBlock != null)
        {
            textBlock.CharacterSpacing = characterSpacingEm;
        }
    }
}

Testing

• Added comprehensive test cases in DatePickerHandlerTests.Windows.cs to verify character spacing is correctly applied
• Created Issue30066 test case that reproduces the original problem and validates the fix
• The fix handles both initial character spacing and dynamic updates

Before/After

Before: Setting CharacterSpacing = 10 on a DatePicker had no visual effect on Windows.

After: Character spacing is properly applied to the DatePicker text display on Windows, matching the behavior on other platforms.

Fixes #30066.

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[sheiksyedm]

@copilot Can you directly access the placeholder TextBlock element in the CalendarDatePicker like below?
platformDatePicker.GetDescendantByName("DateText");

[jsuarezruiz]

Like in other parts of the framework, can use the extension method

maui/src/Core/src/Platform/Windows/FrameworkElementExtensions.cs

Line 48 in 7884b9a

internal static T? GetDescendantByName<T>(this DependencyObject parent, string elementName) where T : DependencyObject

[jsuarezruiz]

if (textBlock is not null)

[Copilot]

Updated to use if (textBlock is not null) pattern matching in commit ba3447c.

[jsuarezruiz]

Could you create another test to validate updates? CharacterSpacingUpdatesCorrectly passing different CharacterSpacing values.

[Copilot]

Added CharacterSpacingUpdatesCorrectly test that validates character spacing updates with different values (5.0 to 15.0) in commit ba3447c.

[jsuarezruiz]

Could create an extension method to convert back from Em. In the extension method, include comments explaining the details.

[Copilot]

Created FromEm extension method in CharacterSpacingExtensions.cs with comprehensive XML documentation explaining the conversion details. Updated the test to use this method instead of manual calculation in commit ba3447c.

[jsuarezruiz]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 3 pipeline(s).

[jsuarezruiz]

The added FromEm method is not part of the publicAPIs, could you add it?

C:\a\_work\1\s\src\Core\src\Platform\Windows\CharacterSpacingExtensions.cs(23,24): error RS0016: Symbol 'FromEm' is not part of the declared public API (https://github.com/dotnet/roslyn-analyzers/blob/main/src/PublicApiAnalyzers/PublicApiAnalyzers.Help.md) [C:\a\_work\1\s\src\Core\src\Core.csproj::TargetFramework=net9.0-windows10.0.19041.0]
C:\a\_work\1\s\src\Core\src\Platform\Windows\CharacterSpacingExtensions.cs(23,24): error RS0016: Symbol 'FromEm' is not part of the declared public API (https://github.com/dotnet/roslyn-analyzers/blob/main/src/PublicApiAnalyzers/PublicApiAnalyzers.Help.md) [C:\a\_work\1\s\src\Core\src\Core.csproj::TargetFramework=net9.0-windows10.0.20348.0]
    2 Error(s)

[Copilot]

The added FromEm method is not part of the publicAPIs, could you add it?

C:\a\_work\1\s\src\Core\src\Platform\Windows\CharacterSpacingExtensions.cs(23,24): error RS0016: Symbol 'FromEm' is not p...

Added FromEm method to Windows PublicAPI unshipped declarations in commit a921d93.