The XyzMapper fields on each Handler type should be 'readonly'

[Eilon]

Description

The various Mapper fields on the various control handlers should be made 'readonly' so they don't get overwritten.

Examples:
https://github.com/dotnet/maui/blob/main/src/Core/src/Handlers/Button/ButtonHandler.cs#L19-L41
https://github.com/dotnet/maui/blob/main/src/Core/src/Handlers/Editor/EditorHandler.cs#L16-L37

We noticed this in the BlazorWebView handler (probably because I copied the code from some other handler to begin with) and it seems to make sense to mark them readonly.

Tagging @PureWeen .

Steps to Reproduce

N/A

Version with bug

Unknown/Other (please specify)

Last version that worked well

Unknown/Other

Affected platforms

I was not able test on other platforms

Affected platform versions

N/A

Did you find any workaround?

No response

Relevant log output

No response

[antonfirsov]

@elion @PureWeen shouldn't they be get-only properties instead? It feels like more appropriate C# practice.

public static IPropertyMapper<IImage, IButtonHandler> ImageButtonMapper { get; } = new PropertyMapper<IImage, IButtonHandler>()
{
	[nameof(IImage.Source)] = MapImageSource
};

[PureWeen]

Currently we're using the settable nature of these to create custom mappers inside Controls

maui/src/Controls/src/Core/HandlerImpl/Entry/Entry.cs

Line 15 in 5941e0d

EntryHandler.Mapper = ControlsEntryMapper;

If we make them readonly then we'll need to modify how we're doing this.

[Eilon]

Closing because this is by design. We are fixing the Blazor stuff where we pre-emptively changed it.