Add helper functions for working with html controls

src/Framework/Framework/Controls/DotvvmBindableObjectHelper.cs

@@ -130,7 +130,7 @@ public static TControl SetAttribute<TControl>(this TControl control, string attr
 
             if (value is not null)
             {
-                control.Attributes[attribute] = value;
+                control.Attributes.Set(attribute, value);
             }
             else
             {
@@ -153,6 +153,62 @@ public static TControl SetAttribute<TControl>(this TControl control, string attr
             return SetAttribute(control, attribute, value?.UnwrapToObject());
         }
 
+        /// <summary> Appends a value into the specified html attribute. If the attribute already exists, the old and new values are merged. Returns <paramref name="control"/> for fluent API usage. </summary>
+        public static TControl AddAttribute<TControl>(this TControl control, string attribute, object? value)
+            where TControl : IControlWithHtmlAttributes
+        {
+            if (value is not null)
+                control.Attributes.Add(attribute, value);
+            return control;
+        }
+        /// <summary> Appends a value into the specified html attribute. If the attribute already exists, the old and new values are merged. Returns <paramref name="control"/> for fluent API usage. </summary>
+        public static TControl AddAttribute<TControl, TValue>(this TControl control, string attribute, ValueOrBinding<TValue>? value)
+            where TControl : IControlWithHtmlAttributes
+        {
+            return AddAttribute(control, attribute, value?.UnwrapToObject());
+        }
+
+        /// <summary> Appends a list of css attributes to the control. If the attributes already exist, the old and new values are merged. Returns <paramref name="control"/> for fluent API usage. </summary>
+        public static TControl AddAttributes<TControl, TValue>(this TControl control, IEnumerable<KeyValuePair<string, TValue>> attributes)
+            where TControl : IControlWithHtmlAttributes
+        {
+            foreach (var a in attributes)
+                AddAttribute(control, a.Key, a.Value);
+            return control;
+        }
+
+        /// <summary> Appends a list of css attributes to the control. If the attributes already exist, the old and new values are merged. Returns <paramref name="control"/> for fluent API usage. </summary>
+        public static TControl AddAttributes<TControl, TValue>(this TControl control, VirtualPropertyGroupDictionary<TValue> attributes)
+            where TControl : IControlWithHtmlAttributes
+        {
+            foreach (var a in attributes.RawValues)
+                AddAttribute(control, a.Key, a.Value);
+            return control;
+        }
+
+        /// <summary> Appends a css class to this control. Note that it is currently not supported if multiple bindings would have to be joined together. Returns <paramref name="control"/> for fluent API usage. </summary>
+        public static TControl AddCssClass<TControl>(this TControl control, ValueOrBinding<string> className)
+            where TControl : IControlWithHtmlAttributes
+        {
+            return AddAttribute(control, "class", className.UnwrapToObject());
+        }
+
+        /// <summary> Appends a list of css classes to this control. Returns <paramref name="control"/> for fluent API usage. </summary>
+        public static TControl AddCssClasses<TControl>(this TControl control, params string[] classes)
+            where TControl : IControlWithHtmlAttributes
+        {
+            if (classes is null || classes.Length == 0)
+                return control;
+            return AddCssClass(control, string.Join(" ", classes));
+        }
+
+        /// <summary> Adds a css inline style - the `style` attribute. Returns <paramref name="control"/> for fluent API usage. </summary>
+        public static TControl AddCssStyle<TControl>(this TControl control, string name, string styleValue)
+            where TControl : IControlWithHtmlAttributes
+        {
+            return AddAttribute(control, "style", name + ":" + styleValue);
+        }
+
         /// <summary> Sets all properties from the capability into this control. If the control does not support the capability, exception is thrown. Returns <paramref name="control"/> for fluent API usage. </summary>
         public static TControl SetCapability<TControl, TCapability>(this TControl control, [AllowNull] TCapability capability, string prefix = "")
             where TControl: DotvvmBindableObject

src/Framework/Framework/Controls/DotvvmControl.cs

@@ -206,7 +206,7 @@ protected void AddDotvvmUniqueIdAttribute()
             {
                 throw new DotvvmControlException(this, "Postback.Update can not be set on property which don't render html attributes.");
             }
-            htmlAttributes.Attributes["data-dotvvm-id"] = GetDotvvmUniqueId();
+            htmlAttributes.Attributes.Set("data-dotvvm-id", GetDotvvmUniqueId());
         }
 
         protected struct RenderState

src/Framework/Framework/Controls/HtmlGenericControl.cs

@@ -140,8 +140,6 @@ public HtmlCapability HtmlCapability
         /// </summary>
         protected virtual bool RendersHtmlTag => TagName is object;
 
-        IDictionary<string, object?> IControlWithHtmlAttributes.Attributes => this.Attributes;
-
         protected new struct RenderState
         {
             public object? Visible;

src/Framework/Framework/Controls/IControlWithHtmlAttributes.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 using System.Linq;
+using DotVVM.Framework.Binding;
 
 namespace DotVVM.Framework.Controls
 {
@@ -9,8 +10,6 @@ namespace DotVVM.Framework.Controls
     /// </summary>
     public interface IControlWithHtmlAttributes
     {
-
-        IDictionary<string, object?> Attributes { get; } 
-
+        VirtualPropertyGroupDictionary<object?> Attributes { get; } 
     }
 }

src/Tests/ControlTests/CompositeControlTests.cs

@@ -192,7 +192,7 @@ public class RepeatedButton: CompositeControl
             string additionalCssClass = "my-repeated-button"
         )
         {
-            var r = new Repeater() {
+            return new Repeater() {
                 RenderAsNamedTemplate = false,
                 WrapperTagName = wrapperTagName,
                 ItemTemplate = new DelegateTemplate(_ =>
@@ -203,10 +203,8 @@ public class RepeatedButton: CompositeControl
                 )
             }
             .SetProperty(Repeater.DataSourceProperty, dataSource)
-            .SetCapability(html);
-
-            r.Attributes.Add("class", additionalCssClass);
-            return r;
+            .SetCapability(html)
+            .AddCssClass(additionalCssClass);
         }
     }