files tree made better.

src/Yavsc/Extensions/AppBuilderExtensions.cs

@@ -0,0 +1,39 @@
+using System;
+using Microsoft.AspNet.Builder;
+using Microsoft.AspNet.Http;
+
+namespace Yavsc.Extensions {
+    public static class AppBuilderExtensions {
+        public static IApplicationBuilder UseWhen(this IApplicationBuilder app,
+            Func<HttpContext, bool> condition, Action<IApplicationBuilder> configuration) {
+            if (app == null) {
+                throw new ArgumentNullException(nameof(app));
+            }
+
+            if (condition == null) {
+                throw new ArgumentNullException(nameof(condition));
+            }
+
+            if (configuration == null) {
+                throw new ArgumentNullException(nameof(configuration));
+            }
+
+            var builder = app.New();
+            configuration(builder);
+
+            return app.Use(next => {
+                builder.Run(next);
+
+                var branch = builder.Build();
+
+                return context => {
+                    if (condition(context)) {
+                        return branch(context);
+                    }
+
+                    return next(context);
+                };
+            });
+        }
+    }
+}