Add hook to add TagHelperDirectiveDescriptors.

- Also added a test to validate that you can hook into the TagHelperDescriptorResolutionContext.

#214

src/Microsoft.AspNet.Razor/Parser/TagHelpers/AddOrRemoveTagHelperSpanVisitor.cs

@@ -32,12 +32,19 @@ namespace Microsoft.AspNet.Razor.Parser.TagHelpers
             // This will recurse through the syntax tree.
             VisitBlock(root);
 
-            var resolutionContext = new TagHelperDescriptorResolutionContext(_directiveDescriptors);
+            var resolutionContext = GetTagHelperDescriptorResolutionContext(_directiveDescriptors);
             var descriptors = _descriptorResolver.Resolve(resolutionContext);
 
             return descriptors;
         }
 
+        // Allows MVC a chance to override the TagHelperDescriptorResolutionContext
+        protected virtual TagHelperDescriptorResolutionContext GetTagHelperDescriptorResolutionContext(
+            [NotNull] IEnumerable<TagHelperDirectiveDescriptor> descriptors)
+        {
+            return new TagHelperDescriptorResolutionContext(descriptors);
+        }
+
         public override void VisitSpan(Span span)
         {
             // We're only interested in spans with an AddOrRemoveTagHelperCodeGenerator.

test/Microsoft.AspNet.Razor.Test/TagHelpers/AddOrRemoveTagHelperSpanVisitorTest.cs

@@ -68,6 +68,46 @@ namespace Microsoft.AspNet.Razor.TagHelpers
                          TagHelperDirectiveDescriptorComparer.Default);
         }
 
+        [Fact]
+        public void GetDescriptors_CanOverrideResolutionContext()
+        {
+            // Arrange
+            var resolver = new TestTagHelperDescriptorResolver();
+            var expectedInitialDirectiveDescriptors = new TagHelperDirectiveDescriptor[]
+            {
+                new TagHelperDirectiveDescriptor("one", TagHelperDirectiveType.AddTagHelper),
+                new TagHelperDirectiveDescriptor("two", TagHelperDirectiveType.RemoveTagHelper),
+                new TagHelperDirectiveDescriptor("three", TagHelperDirectiveType.RemoveTagHelper),
+            };
+            var expectedEndDirectiveDescriptors = new TagHelperDirectiveDescriptor[]
+            {
+                new TagHelperDirectiveDescriptor("custom", TagHelperDirectiveType.AddTagHelper)
+            };
+            var addOrRemoveTagHelperSpanVisitor = new CustomAddOrRemoveTagHelperSpanVisitor(
+                resolver,
+                (descriptors) =>
+                {
+                    Assert.Equal(expectedInitialDirectiveDescriptors,
+                                 descriptors,
+                                 TagHelperDirectiveDescriptorComparer.Default);
+
+                    return new TagHelperDescriptorResolutionContext(expectedEndDirectiveDescriptors);
+                });
+            var document = new MarkupBlock(
+                Factory.Code("\"one\"").AsAddTagHelper("one"),
+                Factory.Code("\"two\"").AsRemoveTagHelper("two"),
+                Factory.Code("\"three\"").AsRemoveTagHelper("three"));
+
+
+            // Act
+            addOrRemoveTagHelperSpanVisitor.GetDescriptors(document);
+
+            // Assert
+            Assert.Equal(expectedEndDirectiveDescriptors,
+                         resolver.DirectiveDescriptors,
+                         TagHelperDirectiveDescriptorComparer.Default);
+        }
+
         [Fact]
         public void GetDescriptors_LocatesAddTagHelperCodeGenerator()
         {
@@ -173,5 +213,24 @@ namespace Microsoft.AspNet.Razor.TagHelpers
                                        .CombinedHash;
             }
         }
+
+        private class CustomAddOrRemoveTagHelperSpanVisitor : AddOrRemoveTagHelperSpanVisitor
+        {
+            private Func<IEnumerable<TagHelperDirectiveDescriptor>, TagHelperDescriptorResolutionContext> _replacer;
+
+            public CustomAddOrRemoveTagHelperSpanVisitor(
+                ITagHelperDescriptorResolver descriptorResolver,
+                Func<IEnumerable<TagHelperDirectiveDescriptor>, TagHelperDescriptorResolutionContext> replacer)
+                : base(descriptorResolver)
+            {
+                _replacer = replacer;
+            }
+
+            protected override TagHelperDescriptorResolutionContext GetTagHelperDescriptorResolutionContext(
+                IEnumerable<TagHelperDirectiveDescriptor> descriptors)
+            {
+                return _replacer(descriptors);
+            }
+        }
     }
 }