Added a test to verify incremental TagHelper discovery

test/Microsoft.AspNetCore.Razor.Design.Test/IntegrationTests/Assert.cs

@@ -100,6 +100,42 @@ namespace Microsoft.AspNetCore.Razor.Design.IntegrationTests
             throw new BuildOutputMissingException(result, match);
         }
 
+        public static void FileContains(MSBuildResult result, string filePath, string match)
+        {
+            if (result == null)
+            {
+                throw new ArgumentNullException(nameof(result));
+            }
+
+            filePath = Path.Combine(result.Project.DirectoryPath, filePath);
+            FileExists(result, filePath);
+
+            var text = File.ReadAllText(filePath);
+            if (text.Contains(match))
+            {
+                return;
+            }
+
+            throw new FileContentMissingException(result, filePath, File.ReadAllText(filePath), match);
+        }
+
+        public static void FileDoesNotContain(MSBuildResult result, string filePath, string match)
+        {
+            if (result == null)
+            {
+                throw new ArgumentNullException(nameof(result));
+            }
+
+            filePath = Path.Combine(result.Project.DirectoryPath, filePath);
+            FileExists(result, filePath);
+
+            var text = File.ReadAllText(filePath);
+            if (text.Contains(match))
+            {
+                throw new FileContentFoundException(result, filePath, File.ReadAllText(filePath), match);
+            }
+        }
+
         public static void FileContainsLine(MSBuildResult result, string filePath, string match)
         {
             if (result == null)

test/Microsoft.AspNetCore.Razor.Design.Test/IntegrationTests/RazorGenerateIntegrationTest.cs

@@ -26,6 +26,14 @@ namespace Microsoft.AspNetCore.Razor.Design.IntegrationTests
             Assert.FileExists(result, IntermediateOutputPath, "SimpleMvc.dll");
             Assert.FileDoesNotExist(result, IntermediateOutputPath, "SimpleMvc.Views.dll");
 
+            // RazorGenerate should generate correct TagHelper caches
+            Assert.FileExists(result, IntermediateOutputPath, "SimpleMvc.TagHelpers.input.cache");
+            Assert.FileExists(result, IntermediateOutputPath, "SimpleMvc.TagHelpers.output.cache");
+            Assert.FileContains(
+                result,
+                Path.Combine(IntermediateOutputPath, "SimpleMvc.TagHelpers.output.cache"),
+                @"""Name"":""SimpleMvc.SimpleTagHelper""");
+
             Assert.FileExists(result, RazorIntermediateOutputPath, "Views", "_ViewImports.cs");
             Assert.FileExists(result, RazorIntermediateOutputPath, "Views", "_ViewStart.cs");
             Assert.FileExists(result, RazorIntermediateOutputPath, "Views", "Home", "About.cs");
@@ -44,7 +52,7 @@ namespace Microsoft.AspNetCore.Razor.Design.IntegrationTests
             // Introducing a syntax error, an unclosed brace
             ReplaceContent("@{", "Views", "Home", "Index.cshtml");
 
-            var result = await DotnetMSBuild("RazorGenerate");
+            var result = await DotnetMSBuild(RazorGenerateTarget);
 
             Assert.BuildFailed(result);
 
@@ -181,6 +189,37 @@ namespace Microsoft.AspNetCore.Razor.Design.IntegrationTests
             Assert.FileDoesNotExist(result, generatedFile);
         }
 
+        [Fact(Skip = "https://github.com/aspnet/Razor/issues/2104")]
+        [InitializeTestProject("SimpleMvc")]
+        public async Task RazorGenerate_Rebuilds_IfTagHelpersChanged()
+        {
+            // Act - 1
+            var expectedTagHelperCacheContent = @"""Name"":""SimpleMvc.SimpleTagHelper""";
+            var result = await DotnetMSBuild(RazorGenerateTarget);
+            var file = Path.Combine(Project.DirectoryPath, "SimpleTagHelper.cs");
+            var tagHelperOutputCache = Path.Combine(IntermediateOutputPath, "SimpleMvc.TagHelpers.output.cache");
+            var generatedFile = Path.Combine(RazorIntermediateOutputPath, "Views", "Home", "Index.cs");
+
+            // Assert - 1
+            Assert.BuildPassed(result);
+            Assert.FileContains(result, tagHelperOutputCache, expectedTagHelperCacheContent);
+            var fileThumbPrint = GetThumbPrint(generatedFile);
+
+            // Act - 2
+            // Update the source content and build. We should expect the outputs to be regenerated.
+            // Timestamps on xplat are precise only to a second. Add a delay so we can ensure that MSBuild recognizes the
+            // file change. See https://github.com/dotnet/corefx/issues/26024
+            await Task.Delay(TimeSpan.FromSeconds(1));
+            ReplaceContent(string.Empty, file);
+            result = await DotnetMSBuild(RazorGenerateTarget);
+
+            // Assert - 2
+            Assert.BuildPassed(result);
+            Assert.FileDoesNotContain(result, tagHelperOutputCache, expectedTagHelperCacheContent);
+            var newThumbPrint = GetThumbPrint(generatedFile);
+            Assert.NotEqual(fileThumbPrint, newThumbPrint);
+        }
+
         [Fact]
         [InitializeTestProject("SimpleMvc")]
         public async Task RazorGenerate_Noops_WithNoFiles()

test/testapps/SimpleMvc/SimpleTagHelper.cs

@@ -0,0 +1,8 @@
+using Microsoft.AspNetCore.Razor.TagHelpers;
+
+namespace SimpleMvc
+{
+    public class SimpleTagHelper : TagHelper
+    {
+    }
+}