Add RenderTreeBuilder extension methods for "prevent default" and "stop bubbling"

src/Components/Web/src/Web/RenderTreeBuilderExtensions.cs

@@ -0,0 +1,53 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using Microsoft.AspNetCore.Components.Rendering;
+using Microsoft.AspNetCore.Components.RenderTree;
+
+namespace Microsoft.AspNetCore.Components.Web
+{
+    /// <summary>
+    /// Provides methods for building a collection of <see cref="RenderTreeFrame"/> entries.
+    /// </summary>
+    public static class RenderTreeBuilderExtensions
+    {
+        // The "prevent default" and "stop bubbling" flags behave like attributes, in that:
+        // - you can have multiple of them on a given element (for separate events)
+        // - you can add and remove them dynamically
+        // - they are independent of other attributes (e.g., you can "stop bubbling" of a given
+        //   event type on an element that doesn't itself have a handler for that event)
+        // As such, they are represented as attributes to give the right diffing behavior.
+        //
+        // As a private implementation detail, their internal representation is magic-named
+        // attributes. This may change in the future. If we add support for multiple-same
+        // -named-attributes-per-element (#14365), then we will probably also declare a new
+        // AttributeType concept, and have specific attribute types for these flags, and
+        // the "name" can simply be the name of the event being modified.
+
+        /// <summary>
+        /// Appends a frame representing an instruction to prevent the default action
+        /// for a specified event.
+        /// </summary>
+        /// <param name="builder">The <see cref="RenderTreeBuilder"/>.</param>
+        /// <param name="sequence">An integer that represents the position of the instruction in the source code.</param>
+        /// <param name="eventName">The name of the event to be affected.</param>
+        /// <param name="value">True if the default action is to be prevented, otherwise false.</param>
+        public static void AddEventPreventDefaultAttribute(this RenderTreeBuilder builder, int sequence, string eventName, bool value)
+        {
+            builder.AddAttribute(sequence, $"__internal_preventDefault_{eventName}", value);
+        }
+
+        /// <summary>
+        /// Appends a frame representing an instruction to stop the specified event from
+        /// bubbling up beyond the current element.
+        /// </summary>
+        /// <param name="builder">The <see cref="RenderTreeBuilder"/>.</param>
+        /// <param name="sequence">An integer that represents the position of the instruction in the source code.</param>
+        /// <param name="eventName">The name of the event to be affected.</param>
+        /// <param name="value">True if bubbling should stop bubbling here, otherwise false.</param>
+        public static void AddEventStopBubblingAttribute(this RenderTreeBuilder builder, int sequence, string eventName, bool value)
+        {
+            builder.AddAttribute(sequence, $"__internal_stopBubbling_{eventName}", value);
+        }
+    }
+}