Add optional property to PartialTagHelper (#7991)

* Add optional to PartialTagHelper

Addresses #7268

src/Microsoft.AspNetCore.Mvc.TagHelpers/PartialTagHelper.cs

@@ -22,6 +22,7 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
     {
         private const string ForAttributeName = "for";
         private const string ModelAttributeName = "model";
+        private const string OptionalAttributeName = "optional";
         private object _model;
         private bool _hasModel;
         private bool _hasFor;
@@ -74,6 +75,13 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
             }
         }
 
+        /// <summary>
+        /// When optional, executing the tag helper will no-op if the view cannot be located. 
+        /// Otherwise will throw stating the view could not be found.
+        /// </summary>
+        [HtmlAttributeName(OptionalAttributeName)]
+        public bool Optional { get; set; }
+
         /// <summary>
         /// A <see cref="ViewDataDictionary"/> to pass into the partial view.
         /// </summary>
@@ -96,16 +104,21 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
                 throw new ArgumentNullException(nameof(context));
             }
 
-            var model = ResolveModel();
+            var viewEngineResult = FindView();
-            var viewBuffer = new ViewBuffer(_viewBufferScope, Name, ViewBuffer.PartialViewPageSize);
-            using (var writer = new ViewBufferTextWriter(viewBuffer, Encoding.UTF8))
-            {
-                await RenderPartialViewAsync(writer, model);
 
-                // Reset the TagName. We don't want `partial` to render.
+            if (viewEngineResult.Success)
-                output.TagName = null;
+            {
-                output.Content.SetHtmlContent(viewBuffer);
+                var model = ResolveModel();
+                var viewBuffer = new ViewBuffer(_viewBufferScope, Name, ViewBuffer.PartialViewPageSize);
+                using (var writer = new ViewBufferTextWriter(viewBuffer, Encoding.UTF8))
+                {
+                    await RenderPartialViewAsync(writer, model, viewEngineResult.View);
+                    output.Content.SetHtmlContent(viewBuffer);
+                }
             }
+
+            // Reset the TagName. We don't want `partial` to render.
+            output.TagName = null;
         }
 
         // Internal for testing
@@ -139,7 +152,7 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
             return ViewContext.ViewData.Model;
         }
 
-        private async Task RenderPartialViewAsync(TextWriter writer, object model)
+        private ViewEngineResult FindView()
         {
             var viewEngineResult = _viewEngine.GetView(ViewContext.ExecutingFilePath, Name, isMainPage: false);
             var getViewLocations = viewEngineResult.SearchedLocations;
@@ -148,7 +161,7 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
                 viewEngineResult = _viewEngine.FindView(ViewContext, Name, isMainPage: false);
             }
 
-            if (!viewEngineResult.Success)
+            if (!viewEngineResult.Success && !Optional)
             {
                 var searchedLocations = Enumerable.Concat(getViewLocations, viewEngineResult.SearchedLocations);
                 var locations = string.Empty;
@@ -161,7 +174,11 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
                     Resources.FormatViewEngine_PartialViewNotFound(Name, locations));
             }
 
-            var view = viewEngineResult.View;
+            return viewEngineResult;
+        }
+
+        private async Task RenderPartialViewAsync(TextWriter writer, object model, IView view)
+        {
             // Determine which ViewData we should use to construct a new ViewData
             var baseViewData = ViewData ?? ViewContext.ViewData;
             var newViewData = new ViewDataDictionary<object>(baseViewData, model);

test/Microsoft.AspNetCore.Mvc.TagHelpers.Test/PartialTagHelperTest.cs

@@ -569,7 +569,7 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
         }
 
         [Fact]
-        public async Task ProcessAsync_Throws_IfGetViewAndFindReturnNotFoundResults()
+        public async Task ProcessAsync_Throws_If_NotOptional_And_GetViewAndFindReturnNotFoundResults()
         {
             // Arrange
             var bufferScope = new TestViewBufferScope();
@@ -596,6 +596,7 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
                 Name = partialName,
                 ViewContext = viewContext,
                 ViewData = viewData,
+                Optional = false
             };
             var tagHelperContext = GetTagHelperContext();
             var output = GetTagHelperOutput();
@@ -605,6 +606,47 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
                 () => tagHelper.ProcessAsync(tagHelperContext, output));
             Assert.Equal(expected, exception.Message);
         }
+        
+        [Fact]
+        public async Task ProcessAsync_IfOptional_And_ViewIsNotFound_WillNotRenderAnything()
+        {
+            // Arrange
+            var expected = string.Empty;
+            var bufferScope = new TestViewBufferScope();
+            var partialName = "_ThisViewDoesNotExists";
+            var model = new object();
+            var viewContext = GetViewContext();
+
+            var view = new Mock<IView>();
+            view.Setup(v => v.RenderAsync(It.IsAny<ViewContext>()))
+                .Callback((ViewContext v) =>
+                {
+                    v.Writer.Write(expected);
+                })
+                .Returns(Task.CompletedTask);
+
+            var viewEngine = new Mock<ICompositeViewEngine>();
+            viewEngine.Setup(v => v.GetView(It.IsAny<string>(), partialName, false))
+                .Returns(ViewEngineResult.NotFound(partialName, searchedLocations: Array.Empty<string>()));
+            viewEngine.Setup(v => v.FindView(viewContext, partialName, false))
+                .Returns(ViewEngineResult.NotFound(partialName, searchedLocations: Array.Empty<string>()));
+
+            var tagHelper = new PartialTagHelper(viewEngine.Object, bufferScope)
+            {
+                Name = partialName,
+                ViewContext = viewContext,
+                Optional = true
+            };
+            var tagHelperContext = GetTagHelperContext();
+            var output = GetTagHelperOutput();
+
+            // Act
+            await tagHelper.ProcessAsync(tagHelperContext, output);
+
+            // Assert
+            var content = HtmlContentUtilities.HtmlContentToString(output.Content, new HtmlTestEncoder());
+            Assert.Empty(content);
+        }
 
         private static ViewContext GetViewContext()
         {