Add new merge default classes tag helper

CHANGELOG.md

@@ -5,6 +5,7 @@
 - Dropped support for .NET 6
 - Dropped support for .NET 7
 - Added support for .NET 8
+- Added `MergeDefaultClassTagHelper` which will merge the `default-class` attribute with the `class` attribute
 
 ## [0.5.0](https://github.com/xt0rted/tailwindcss-tag-helpers/compare/v0.4.0...v0.5.0) - 2023-02-24
 

README.md

@@ -102,7 +102,8 @@ The naming of these attributes aligns with the comments found in the Tailwind UI
 
 The matching method can be either `Full` (default) which ensures the link path and current path are the same, or `Base` which ensures the link path starts with, or is the same as, the current path.
 
-> **Note**: Query string values are not used for either method of matching.
+> [!NOTE]
+> Query string values are not used for either method of matching.
 
 ```html
 <a
@@ -147,6 +148,37 @@ Name | Value | Description
 `default-class` | `string` | The css classes to apply if the link doesn't match the current url.
 `match` | `Full` (default) or `Base` | The method to use when matching the link to the current url.
 
+### MergeDefaultClassTagHelper
+
+The merge default class tag helper is applied by adding the `merge-classes` attribute and will merge the `default-class` attribute with the `class` attribute.
+This can be helpful when you aren't using another tag helper that works with the class lists, but still need access to the default classes of an element in your JavaScript.
+
+```html
+<div
+  merge-classes
+  class="flex flex-col"
+  default-class="bg-white text-black"
+>
+</div>
+```
+
+This tag helper target element is `*` so it can be used with any element, including web components and other tag helpers.
+
+```html
+<heroicon-solid
+  icon="AtSymbol"
+  merge-classes
+  class="h-5 w-5"
+  default-class="text-gray-400"
+/>
+```
+
+#### Attributes
+
+Name | Value | Description
+:-- | :-- | :--
+`default-class` | `string` | The classes to merge into the class attribute.
+
 ### ValidationStatusTagHelper
 
 The validation status tag helper will check the validation status of the model value passed to the `asp-for` attribute and apply one of two sets of css classes.

src/MergeClassesTagHelper.cs

@@ -0,0 +1,54 @@
+namespace Tailwind.Css.TagHelpers;
+
+using System.Text.Encodings.Web;
+
+using Microsoft.AspNetCore.Html;
+using Microsoft.AspNetCore.Mvc.TagHelpers;
+using Microsoft.AspNetCore.Razor.TagHelpers;
+using Microsoft.Extensions.Options;
+
+[HtmlTargetElement("*", Attributes = ForAttributeName)]
+public class MergeDefaultClassTagHelper : TagHelper
+{
+    protected const string ForAttributeName = "merge-classes";
+    protected const string DefaultClassAttributeName = "default-class";
+
+    private readonly TagOptions _settings;
+
+    public MergeDefaultClassTagHelper(IOptions<TagOptions> settings)
+        => _settings = settings?.Value ?? throw new ArgumentNullException(nameof(settings));
+
+    /// <summary>
+    /// The classes to merge into the main class list.
+    /// </summary>
+    [HtmlAttributeName(DefaultClassAttributeName)]
+    public string? DefaultClass { get; set; }
+
+    public override void Process(TagHelperContext context, TagHelperOutput output)
+    {
+        ArgumentNullException.ThrowIfNull(output);
+
+        var classList = Utilities.SplitClassList(DefaultClass);
+
+        if (_settings.IncludeComments)
+        {
+            output.PreElement.AppendHtmlLine("<!--");
+
+            output.PreElement.Append("  Base: ");
+            output.PreElement.AppendLine(output.Attributes.GetValue("class"));
+
+            output.PreElement.Append("  Default: ");
+            output.PreElement.AppendLine(DefaultClass ?? "");
+
+            output.PreElement.AppendHtmlLine("-->");
+        }
+
+        if (classList?.Length > 0)
+        {
+            foreach (var className in classList)
+            {
+                output.AddClass(className, HtmlEncoder.Default);
+            }
+        }
+    }
+}

test/LinkAriaCurrentStateTagHelperTests.cs

@@ -11,11 +11,9 @@ public void Should_not_throw_when_no_link_context_set()
     {
         // Given
         var context = MakeTagHelperContext(
-            tagName: "a",
-            new TagHelperAttributeList());
+            tagName: "a");
         var output = MakeTagHelperOutput(
-            tagName: "a",
-            new TagHelperAttributeList());
+            tagName: "a");
 
         var logger = A.Dummy<ILogger<LinkAriaCurrentStateTagHelper>>();
         var options = Options.Create(new TagOptions());
@@ -40,11 +38,9 @@ public void Should_set_attribute_when_enabled(LinkAriaCurrentState state, string
     {
         // Given
         var context = MakeTagHelperContext(
-            tagName: "a",
-            new TagHelperAttributeList());
+            tagName: "a");
         var output = MakeTagHelperOutput(
-            tagName: "a",
-            new TagHelperAttributeList());
+            tagName: "a");
 
         AddLinkContext(context, isMatch: true);
 
@@ -74,11 +70,9 @@ public void Should_not_emit_comments_when_setting_is_disabled()
     {
         // Given
         var context = MakeTagHelperContext(
-            tagName: "a",
-            new TagHelperAttributeList());
+            tagName: "a");
         var output = MakeTagHelperOutput(
-            tagName: "a",
-            new TagHelperAttributeList());
+            tagName: "a");
 
         var logger = A.Dummy<ILogger<LinkAriaCurrentStateTagHelper>>();
         var options = Options.Create(
@@ -100,11 +94,9 @@ public void Should_emit_comments_when_setting_is_enabled()
     {
         // Given
         var context = MakeTagHelperContext(
-            tagName: "a",
-            new TagHelperAttributeList());
+            tagName: "a");
         var output = MakeTagHelperOutput(
-            tagName: "a",
-            new TagHelperAttributeList());
+            tagName: "a");
 
         var logger = A.Dummy<ILogger<LinkAriaCurrentStateTagHelper>>();
         var options = Options.Create(

test/MergeDefaultClassTagHelperTests.cs

@@ -0,0 +1,149 @@
+namespace Tailwind.Css.TagHelpers;
+
+using Microsoft.AspNetCore.Html;
+using Microsoft.AspNetCore.Razor.TagHelpers;
+using Microsoft.Extensions.Options;
+
+public class MergeDefaultClassTagHelperTests : TagHelperTestBase
+{
+    [Fact]
+    public void Should_set_to_defaults_when_class_is_empty()
+    {
+        // Given
+        var context = MakeTagHelperContext(
+            tagName: "div");
+        var output = MakeTagHelperOutput(
+            tagName: "div");
+
+        var options = Options.Create(new TagOptions());
+        var helper = new MergeDefaultClassTagHelper(options)
+        {
+            DefaultClass = "bg-white text-black",
+        };
+
+        // When
+        helper.Process(context, output);
+
+        // Then
+        output.Attributes["class"].ShouldNotBeNull();
+
+        var classList = output.Attributes["class"].Value as HtmlString;
+        classList.ShouldNotBeNull();
+        classList.Value.ShouldBe("bg-white text-black");
+    }
+
+    [Fact]
+    public void Should_merge_defaults_into_existing_list()
+    {
+        // Given
+        var context = MakeTagHelperContext(
+            tagName: "div",
+            new TagHelperAttributeList
+            {
+                { "class", "flex flex-col" },
+            });
+        var output = MakeTagHelperOutput(
+            tagName: "div",
+            new TagHelperAttributeList
+            {
+                { "class", "flex flex-col" },
+            });
+
+        var options = Options.Create(new TagOptions());
+        var helper = new MergeDefaultClassTagHelper(options)
+        {
+            DefaultClass = "bg-white text-black",
+        };
+
+        // When
+        helper.Process(context, output);
+
+        // Then
+        output.Attributes["class"].ShouldNotBeNull();
+
+        var classList = output.Attributes["class"].Value as HtmlString;
+        classList.ShouldNotBeNull();
+        classList.Value.ShouldBe("flex flex-col bg-white text-black");
+    }
+
+    [Theory]
+    [InlineData(null)]
+    [InlineData("flex flex-col")]
+    public void Should_not_emit_comments_when_setting_is_disabled(string? defaultClasses)
+    {
+        // Given
+        var attributeList = new TagHelperAttributeList();
+
+        if (defaultClasses is not null)
+        {
+            attributeList.Add("class", defaultClasses);
+        }
+
+        var context = MakeTagHelperContext(
+            tagName: "div",
+            attributeList);
+        var output = MakeTagHelperOutput(
+            tagName: "div",
+            attributeList);
+
+        var options = Options.Create(
+            new TagOptions
+            {
+                IncludeComments = false,
+            });
+        var helper = new MergeDefaultClassTagHelper(options)
+        {
+            DefaultClass = "bg-white underline",
+        };
+
+        // When
+        helper.Process(context, output);
+
+        // Then
+        output.PreElement.GetContent().ShouldBeEmpty();
+    }
+
+    [Theory]
+    [InlineData(null)]
+    [InlineData("flex flex-col")]
+    public void Should_emit_comments_when_setting_is_enabled(string? defaultClasses)
+    {
+        // Given
+        var attributeList = new TagHelperAttributeList();
+
+        if (defaultClasses is not null)
+        {
+            attributeList.Add("class", defaultClasses);
+        }
+
+        var context = MakeTagHelperContext(
+            tagName: "div",
+            attributeList);
+        var output = MakeTagHelperOutput(
+            tagName: "div",
+            attributeList);
+
+        var options = Options.Create(
+            new TagOptions
+            {
+                IncludeComments = true,
+            });
+        var helper = new MergeDefaultClassTagHelper(options)
+        {
+            DefaultClass = "bg-white text-black",
+        };
+
+        // When
+        helper.Process(context, output);
+
+        // Then
+        output.PreElement.GetContent().ShouldBe(
+            $"""
+            <!--
+              Base: {defaultClasses}
+              Default: bg-white text-black
+            -->
+
+            """);
+    }
+}

test/ValidationStatusTagHelperTests.cs

@@ -13,11 +13,9 @@ public void Should_not_throw_when_for_is_null()
     {
         // Given
         var context = MakeTagHelperContext(
-            tagName: "input",
-            new TagHelperAttributeList());
+            tagName: "input");
         var output = MakeTagHelperOutput(
-            tagName: "input",
-            new TagHelperAttributeList());
+            tagName: "input");
 
         var options = Options.Create(new TagOptions());
         var helper = new ValidationStatusTagHelper(options)