Root-level cascading values 8.0

[guardrex]

Fixes #30093

Steve, for ...

something like a global “weather forecast” service that is defined in Program.cs, has some way of getting state changes from an external source of information, and exposes the current forecast as a cascaded value (which may change over time).

Do you want me to draft up something like that now, table it for later (like 24Q1), or not place coverage for that at all?

If you want me to take a 🔪 at it, I'll need some info on triggering NotifyChangedAsync from a service because the tests at ...

https://github.com/dotnet/aspnetcore/blob/f902a5b27cf2d89e555a1cf67ae796d11dab039e/src/Components/Components/test/CascadingParameterTest.cs#L571-L635

... new up the CascadingValueSource in the method. How is NotifyChangedAsync called in a service? What am I registering/injecting to call it because it apparently isn't directly related to the cascading parameter.

Also ...

to expose some information from the original HttpContext to components

Do you have an example cross-link? What I have here is fun, but I could tack on a quick real world example.

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