Improvements to DynamicRouteValueTransformer

Summary

3.1 Introduced DynamicRouteValueTransformer - a feature that allows you to use a custom endpoint to dynamically select a controller-action or a razor page.

This is useful when you want to create a slug route that accesses the database to choose where to go, or when you're layering another framework on top of MVC (OData).

Now that we've had a chance to get some feedback on it's clear that we're missing a few features that ought to be there.

Context

The way that DynamicRouteValueTransformer is hooked up, there's no way to pass additional context to it. DynamicRouteValueTransformer comes from DI.

endpoints.MapDynamicControllerRoute<MyTransformer>("...");

If you have multiple endpoints that use the same transformer type, you can't pass any data to them, because they come from DI. You effectively need a distinct type for each instance, which isn't a solution. Both OData and Orchard hit this issue.

Filtering

You might also want to do some filtering based on the results of your transformer. MVC resolves the matching endpoints based on the set of route values returned by a transformer, but the transformer can't do any filtering, or see the endpoints that resulted.

See: https://github.com/dotnet/aspnetcore/blob/master/src/Mvc/Mvc.Core/src/Routing/DynamicControllerEndpointMatcherPolicy.cs#L121

Writing a separate policy to handle this doesn't work well. The transformer has context that is lost when you get past this phase:

A transformer could filter just the endpoints it produced.
A transformer could store context across method calls if it is transient

A policy that runs later can't see which endpoints came from the transformer, and can't store context because policies are singletons.

Ordering

The methods we provide for registering transformers don't support providing an Order. That's a fairly important thing when considering that transformers will be interleaved with other MVC endpoints.

However MapDynamicControllerRoute doesn't follow the same ordering rules as MapControllerRoute - it always uses order 0.

aspnetcore/src/Mvc/Mvc.Core/src/Builder/ControllerEndpointRouteBuilderExtensions.cs

Line 481 in 6ce8a87

endpoints.Map(

We should consider a breaking change to make MapDynamicControllerRoute follow the ordering rules of MapControllerRoute and also allow fine-grained control over the order value.

Fixes

We should add some functionality like the following:

public abstract class DynamicRouteValueTransformer
{
        // Will be set by the framework when initialized. This needs to come with the guidance
        // to use Transient DI lifetime.
        public object State { get; set; }

        public abstract ValueTask<RouteValueDictionary> TransformAsync(HttpContext httpContext, RouteValueDictionary values);

        // Will be called when a non-zero amount of endpoints is found by MVC after calling
        // TransformAsync. They must return a new list of endpoints to perform filtering, or can
        // return the original list to skip.
        // The route values will be the ones returned from TransformAsync
        public virtual ValueTask<IReadOnlyList<Endpoint>> FilterEndpointsAsync(HttpContext httpContext, IReadOnlyList<Endpoint> endpoints, RouteValueDictionary values);
}

And the ability to pass in state:

endpoints.MapDynamicControllerRoute<MyTransformer>("...", state: new MyObject());

Oh i would really need this feature too. But for pages
endpoints.MapDynamicPageRoute<MyTransformer>(...);

@rynowak - will the following scenario be covered with these enhancements?

Localization using a PageRouteModelConvention ie:
Template = AttributeRouteModel.CombineTemplates("{culture}", selector.AttributeRouteModel.Template)

Using a DynamicRouteValueTransformer as pages defined in headless CMS:
endpoints.MapDynamicPageRoute<CustomRouteValueTransformer>("{**url}");

Gives:
InvalidOperationException: Using ExpandEndpoint requires that the replaced endpoint have a unique priority. The following endpoints were found with the same priority

Not sure if relevant but I notice there are routes set up other than the DynamicPageRoute even though I have commented out:
endpoints.MapRazorPages();

Minimal repro:
https://github.com/kccsf/DynamicRouteIssue

Can log new issue if outside the scope of the above enhancements.

Is there a workaround?

TIA
Steve

Linking this here in case this can be resoled as part of these improvements: #16965