OnDemandRebuildableOwinPipeline.cs

namespace ClassLibrary2
{
    using System;
    using System.Reactive;
    using AppFunc = System.Func<System.Collections.Generic.IDictionary<string, object>, System.Threading.Tasks.Task>;
    using MidFunc = System.Func<
       System.Func<System.Collections.Generic.IDictionary<string, object>, System.Threading.Tasks.Task>,
       System.Func<System.Collections.Generic.IDictionary<string, object>, System.Threading.Tasks.Task>>;

    public static class RebuildablePipeline
    {
        public static MidFunc Create(IObservable<Unit> onRebuild, Func<AppFunc> createPipeline)
        {
            return next =>
            {
                var pipeline = createPipeline(); //Need a pipeline to start off
                onRebuild.Subscribe(_ => pipeline = createPipeline()); // Rebuild when signaled.
                return env => pipeline(env);
            };
        }
    }
}

namespace Tests
{
    using System;
    using System.Collections.Generic;
    using System.Reactive;
    using System.Reactive.Subjects;
    using System.Threading.Tasks;
    using ClassLibrary2;
    using Xunit;

    using AppFunc = System.Func<System.Collections.Generic.IDictionary<string, object>, System.Threading.Tasks.Task>;

    public class Class1
    {
        [Fact]
        public async Task Blah()
        {
            var subject = new Subject<Unit>();
            int i = 0;
            Func<AppFunc> createPipeline = () =>
            {
                i++;
                return env =>
                {
                    env["counter"] = i;
                    return Task.FromResult(0);
                };
            };
            var rootAppFunc = RebuildablePipeline.Create(subject, createPipeline)(_ => Task.FromResult(0));
            var env1 = new Dictionary<string, object>();

            for (int k = 1; k < 100; k++)
            {
                await rootAppFunc(env1);
                Assert.Equal(k, env1["counter"]);
                subject.OnNext(Unit.Default);
            }
        }
    }
}