Add support for filter factory to minimal APIs

[captainsafia]

Background and Motivation

To support providing users with the ability to generate endpoint filters that target route handlers based on their method signature, we are adding support for a new "filter factory" API that allows users to access the MethodInfo associated with a route handler when constructing their filter.

Proposed API

Introduce new RouteHandlerFilterDelegate type and replace instances of Func<RouteHandlerFilterContext, ValueTask<object?> with it.

+ namespace Microsoft.AspNetCore.Http;

+ public delegate ValueTask<object?> RouteHandlerFilterDelegate(RouteHandlerFilterContext context);

public interface IRouteHandlerFilter
{
- ValueTask<object?> InvokeAsync(RouteHandlerFilterContext context, Func<RouteHandlerFilterContext, ValueTask<object?>> next);
+ ValueTask<object?> InvokeAsync(RouteHandlerFilterContext context, RouteHandlerFilterDelegate next)
}

Instead of taking a list of IRouteHandlerFilters directly, the RequestDelegateFactory now takes in a list of delegates representing the factory in the generation.

public sealed class RequestDelegateFactoryOptions
{
-   public IReadOnlyList<IRouteHandlerFilter>? RouteHandlerFilters { get; init; }
+   public IReadOnlyList<Func<MethodInfo, RouteHandlerFilterDelegate, RouteHandlerFilterDelegate>>? RouteHandlerFilterFactories { get; init; }
}

And finally, we add a new extension method to that takes the filter factory delegates directly.

namespace Microsoft.AspNetCore.Builder;

public static class RouteHandlerFilterExtensions
{
+   public static RouteHandlerBuilder AddFilter(this RouteHandlerBuilder builder, Func<MethodInfo, RouteHandlerFilterDelegate, RouteHandlerFilterDelegate> filterFactory);
}

Usage Examples

app.MapGet("/foo/{id}", (int id) => ...)
  .AddFilter(
    (methodInfo, next) =>
      {
          var doubleTrouble = methodInfo.GetParameters().Length == 2;
          return async (context) => {
            if (doubleTrouble) ..
           }
      }));

Thank you for submitting this for API review. This will be reviewed by @dotnet/aspnet-api-review at the next meeting of the ASP.NET Core API Review group. Please ensure you take a look at the API review process documentation and ensure that:

• The PR contains changes to the reference-assembly that describe the API change. Or, you have included a snippet of reference-assembly-style code that illustrates the API change.
• The PR describes the impact to users, both positive (useful new APIs) and negative (breaking changes).
• Someone is assigned to "champion" this change in the meeting, and they understand the impact and design of the change.

[halter73]

• Let's include endpoint metadata along with MethodInfo in a RouteHandlerContext.
• RouteHandlerFilterContext -> RouteHandlerInvocationContext.

public sealed class RouteHandlerContext
{
    public RouteHandlerContext(MethodInfo methodInfo, EndpointMetadataCollection endpointMetadata);
    public MethodInfo MethodInfo { get; }
    public EndpointMetadataCollection EndpointMetadata { get; }
}
public sealed class RouteHandlerInvocationContext
{
    public RouteHandlerInvocationContext(HttpContext httpContext, params object[] parameters);
    public HttpContext HttpContext { get; }
    // Keep as IList instead of IReadOnlyList to allow replacement
    public IList<object?> Parameters { get; }
}
namespace Microsoft.AspNetCore.Builder;

public static class RouteHandlerFilterExtensions
{
+   public static RouteHandlerBuilder AddFilter(this RouteHandlerBuilder builder, Func<RouteHandlerContext, RouteHandlerFilterDelegate, RouteHandlerFilterDelegate> filterFactory);
}
public sealed class RequestDelegateFactoryOptions
{
-   public IReadOnlyList<IRouteHandlerFilter>? RouteHandlerFilters { get; init; }
+   public IReadOnlyList<Func<RouteHandlerContext, RouteHandlerFilterDelegate, RouteHandlerFilterDelegate>>? RouteHandlerFilterFactories { get; init; }
}

This API approved with the above changes.