Support variant for FeatureTagHelper

src/Microsoft.FeatureManagement.AspNetCore/TagHelpers/FeatureTagHelper.cs

@@ -2,6 +2,7 @@
 // Licensed under the MIT license.
 //
 using Microsoft.AspNetCore.Razor.TagHelpers;
+using System;
 using System.Collections.Generic;
 using System.Linq;
 using System.Threading.Tasks;
@@ -13,7 +14,7 @@ namespace Microsoft.FeatureManagement.Mvc.TagHelpers
     /// </summary>
     public class FeatureTagHelper : TagHelper
     {
-        private readonly IFeatureManager _featureManager;
+        private readonly IVariantFeatureManager _featureManager;
 
         /// <summary>
         /// A feature name, or comma separated list of feature names, for which the content should be rendered. By default, all specified features must be enabled to render the content, but this requirement can be controlled by the <see cref="Requirement"/> property.
@@ -30,13 +31,19 @@ public class FeatureTagHelper : TagHelper
         /// </summary>
         public bool Negate { get; set; }
 
+        /// <summary>
+        /// A variant name, or comma separated list of variant names. If any of specified variants is assigned, the content should be rendered.
+        /// If variant is specified, <see cref="Requirement"/> will be ignored. Besides, <see cref="Name"/> should be exactly one feature name.
+        /// </summary>
+        public string Variant { get; set; }
+
         /// <summary>
         /// Creates a feature tag helper.
         /// </summary>
         /// <param name="featureManager">The feature manager snapshot to use to evaluate feature state.</param>
-        public FeatureTagHelper(IFeatureManagerSnapshot featureManager)
+        public FeatureTagHelper(IVariantFeatureManagerSnapshot featureManager)
         {
-            _featureManager = featureManager;
+            _featureManager = featureManager ?? throw new ArgumentNullException(nameof(featureManager));
         }
 
         /// <summary>
@@ -52,11 +59,30 @@ public override async Task ProcessAsync(TagHelperContext context, TagHelperOutpu
 
             if (!string.IsNullOrEmpty(Name))
             {
-                IEnumerable<string> names = Name.Split(',').Select(n => n.Trim());
+                IEnumerable<string> features = Name.Split(',').Select(n => n.Trim());
+
+                if (string.IsNullOrEmpty(Variant))
+                {
+                    enabled = Requirement == RequirementType.All ?
+                        await features.All(async feature => await _featureManager.IsEnabledAsync(feature).ConfigureAwait(false)) :
+                        await features.Any(async feature => await _featureManager.IsEnabledAsync(feature).ConfigureAwait(false));
+                }
+                else
+                {
+                    if (features.Count() != 1)
+                    {
+                        throw new ArgumentException("Variant cannot be associated with multiple feature flags.", nameof(Name));
+                    }
+
+                    IEnumerable<string> variants = Variant.Split(',').Select(n => n.Trim());
+
+                    enabled = await variants.Any(
+                        async variant => {
+                            Variant assignedVariant = await _featureManager.GetVariantAsync(features.First());
 
-                enabled = Requirement == RequirementType.All ?
-                    await names.All(async n => await _featureManager.IsEnabledAsync(n).ConfigureAwait(false)) :
-                    await names.Any(async n => await _featureManager.IsEnabledAsync(n).ConfigureAwait(false));
+                            return variant == assignedVariant?.Name;    
+                        });
+                }
             }
 
             if (Negate)