[Blazor] Dispatch inbound activity handlers in Blazor Web apps

[MackinnonBuck]

Dispatch inbound activity handlers in Blazor Web apps

Fixes an issue where inbound activity handlers do not get invoked in Blazor Web apps

Description

This PR fixes the issue by:

• Building the inbound activity pipeline in Blazor Web apps just after circuit handlers are retrieved
• Invoking inbound activity handlers in web root component updates

Fixes #51934

[MackinnonBuck]

We previously didn't wait for "update" operations to complete before clearing persistent state in the first root component update batch. But it's likely preferable to wait for them to complete before returning back up the inbound activity handler pipeline, hence this change. If others think this change is problematic, we could take another approach (maybe collecting the "add" operations separately and awaiting those).

[SteveSandersonMS]

@javiercn has been the one with the most specific opinions about the semantics for persistent state clearing, so hopefully we can get his view and explanation.

[SteveSandersonMS]

This is pretty odd - it's hard to see why we have to do this here in particular. Is it just a matter of lazy initialization? If so would it be clearer still if the field was a Lazy<T> that was always initialized during the constructor, and then we wouldn't need to initialize it explicitly in other places.

[MackinnonBuck]

I agree it's a bit confusing. I believe the reason we collect the circuit handlers here is that calling _scope.GetServices<CircuitHandler>() may construct new instances of services registered as circuit handlers, and those services may access persistent component state in their constructors. So we need to be sure that persistent component state is available by the time these services get constructed. We might be able to use a Lazy<T>, but my concern is that making circuit handlers lazily and implicitly initialized might cause us to accidentally change when circuit handlers are constructed without us knowing it.

Edit: Found the existing comment explaining it:

aspnetcore/src/Components/Server/src/Circuits/CircuitFactory.cs

Lines 85 to 91 in e175853

	// In Blazor Server we have already restored the app state, so we can get the handlers from DI.
	// In Blazor Web the state is provided in the first call to UpdateRootComponents, so we need to
	// delay creating the handlers until then. Otherwise, a handler would be able to access the state
	// in the constructor for Blazor Server, but not in Blazor Web.
	var circuitHandlers = components.Count == 0 ? [] : scope.ServiceProvider.GetServices<CircuitHandler>()
	.OrderBy(h => h.Order)
	.ToArray();

[javiercn]

Yeah, this code is correct. The Blazor Server version also does the same thing.

[javiercn]

This is a bit weird. It takes a second to realize that we are calling this method and that its mutating the state from the list of pending tasks.

Is there a way we can flow the pendingTasks to HandleInboundActivityAsync, otherwise, maybe make it explcit with a class instead of a closure.

[MackinnonBuck]

I restructured things a little - let me know if you think this is still a concern!

[javiercn]

Looks good to me