Add abstraction for Theme (dark/light) switching

[nickrandolph]

Provide an abstraction where the theme (light or dark mode) of the app can be switched (and persisted)

[francoistanguay]

If it can be persisted, I think we also need a mode that uses the "System" instead of a Light/Dark override... More like Default/Light/Dark where Default could clear the stored override or something

[francoistanguay]

Unclear how Default makes sense as a Value at the public api level though. It's mostly something internal. Although if you override, you want to be able to clear the override, so we need to come up with some form of API for it all.

[arqueror]

Proposed interface:

	public interface IThemeService
	{
	        bool UseSystemTheme {get; set;}
		bool IsDarkMode{ get; }
		Task SetThemeAsync(bool darkMode);
		Task SetThemeAsync();
	}

[arqueror]

Updated interface proposal:

	public interface IThemeService
	{
		bool IsDark { get; }
		DesiredTheme Theme { get; }
		Task SetThemeAsync(DesiredTheme theme = DesiredTheme.System);
		event EventHandler<DesiredTheme> DesiredThemeChanged;
	}

[dansiegel]

Perhaps I've missed something here, but this issue seems to be missing a the context of the SDK we're already working with. We already have:

public enum ApplicationTheme
{
    Light,
    Dark
}

public enum ElementTheme
{
    Default,
    Light,
    Dark
}

Why exactly do we need to introduce yet another enum that replicates what we already have to work with? This whole thing seems like it can be simplified to:

public interface IThemeService
{
    // Use ApplicationTheme here because it will either be
    // Light or Dark
    ApplicationTheme CurrentTheme { get; }

    // Use ElementTheme here because we may want to set Default
    // to revert to the OS preference for Light/Dark
    Task SetThemeAsync(ElementTheme theme);

    event EventHandler<ApplicationTheme> ThemeChanged;
}

I don't really see a reason why we would want to have a boolean for IsDark when we have an Enum that has 2 choices

[francoistanguay]

First, I think they have different meanings .... Second, they are in a WinUI-defined assembly and IThemeService is meant to be used in a presentation layer that is UI-tech agnostic...

You could argue that we could easily cast one into the other where applicable, but am unclear what we're using in our SystemThemeHelper under the hood...

[francoistanguay]

But I'd have no issue converting Light/Dark into an enum in Uno Extensions instead of beingn a boolean if it makes it more symmetric/cleaner

[dansiegel]

Yeah but it should still be built against either WinUI/UWP which both of them have those enums so we shouldn't have an issue using them.