Low-level NavLink implementation

samples/StandaloneApp/Shared/NavMenu.cshtml

@@ -1,4 +1,5 @@
-<div class='main-nav'>
+@using Microsoft.AspNetCore.Blazor.Browser.Routing
+<div class='main-nav'>
     <div class='navbar navbar-inverse'>
         <div class='navbar-header'>
             <button type='button' class='navbar-toggle' data-toggle='collapse' data-target='.navbar-collapse'>
@@ -13,14 +14,14 @@
         <div class='navbar-collapse collapse'>
             <ul class='nav navbar-nav'>
                 <li>
-                    <a href='/'>
+                    <c:NavLink href=@("/")>
                         <span class='glyphicon glyphicon-home'></span> Home
-                    </a>
+                    </c:NavLink>
                 </li>
                 <li>
-                    <a href='/counter'>
+                    <c:NavLink href=@("/counter")>
                         <span class='glyphicon glyphicon-education'></span> Counter
-                    </a>
+                    </c:NavLink>
                 </li>
             </ul>
         </div>

samples/StandaloneApp/wwwroot/css/site.css

@@ -64,3 +64,22 @@
         text-overflow: ellipsis;
     }
 }
+
+/* 
+    Temporary workaround due to the <blazor-component> wrapper element. The wrapper element
+    will be eliminated in a future update, making this workaround unnecessary.
+*/
+.navbar-nav > li a {
+    color: #9d9d9d;
+    line-height: 20px;
+    position: relative;
+    display: block;
+}
+.navbar-nav > li a:hover {
+    color: #fff;
+    background-color: transparent;
+    text-decoration: none;
+}
+.navbar-nav > li a:focus {
+    text-decoration: none;
+}

src/Microsoft.AspNetCore.Blazor.Browser/Routing/NavLink.cs

@@ -0,0 +1,131 @@
+// 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.Blazor.Components;
+using Microsoft.AspNetCore.Blazor.RenderTree;
+using System;
+using System.Collections.Generic;
+
+namespace Microsoft.AspNetCore.Blazor.Browser.Routing
+{
+    // TODO: Move this into Microsoft.AspNetCore.Blazor, and use DI to break the
+    // coupling on Microsoft.AspNetCore.Blazor.Browser.Routing.UriHelper.
+    // That's because you'd use NavLink in non-browser scenarios too (e.g., prerendering).
+    // Can't do this until DI is implemented.
+
+    public class NavLink : IComponent, IDisposable
+    {
+        const string CssClassAttributeName = "class";
+        const string HrefAttributeName = "href";
+
+        private RenderHandle _renderHandle;
+        private bool _isActive;
+
+        private RenderFragment _childContent;
+        private string _cssClass;
+        private string _href;
+        private string _hrefAbsolute;
+        private IDictionary<string, string> _otherAttributes;
+
+        public void Init(RenderHandle renderHandle)
+        {
+            _renderHandle = renderHandle;
+            UriHelper.OnLocationChanged += OnLocationChanged;
+        }
+
+        public void SetParameters(ParameterCollection parameters)
+        {
+            _childContent = null;
+            _href = null;
+            _hrefAbsolute = null;
+            _cssClass = null;
+            _otherAttributes?.Clear();
+            foreach (var kvp in parameters)
+            {
+                switch (kvp.Name)
+                {
+                    case RenderTreeBuilder.ChildContent:
+                        _childContent = kvp.Value as RenderFragment;
+                        break;
+                    case CssClassAttributeName:
+                        _cssClass = kvp.Value as string;
+                        break;
+                    case HrefAttributeName:
+                        _href = kvp.Value as string;
+                        _hrefAbsolute = UriHelper.ToAbsoluteUri(_href).AbsoluteUri;
+                        break;
+                    default:
+                        if (kvp.Value != null)
+                        {
+                            if (_otherAttributes == null)
+                            {
+                                _otherAttributes = new Dictionary<string, string>();
+                            }
+                            _otherAttributes.Add(kvp.Name, kvp.Value.ToString());
+                        }
+                        break;
+                }
+            }
+
+            _isActive = UriHelper.GetAbsoluteUri().Equals(_hrefAbsolute, StringComparison.Ordinal);
+            _renderHandle.Render(Render);
+        }
+
+        public void Dispose()
+        {
+            UriHelper.OnLocationChanged -= OnLocationChanged;
+        }
+
+        private void OnLocationChanged(object sender, string e)
+        {
+            var shouldBeActiveNow = UriHelper.GetAbsoluteUri().Equals(
+                _hrefAbsolute,
+                StringComparison.Ordinal);
+
+            if (shouldBeActiveNow != _isActive)
+            {
+                _isActive = shouldBeActiveNow;
+
+                if (_renderHandle.IsInitialized)
+                {
+                    _renderHandle.Render(Render);
+                }
+            }
+        }
+
+        private void Render(RenderTreeBuilder builder)
+        {
+            builder.OpenElement(0, "a");
+
+            if (!string.IsNullOrEmpty(_href))
+            {
+                builder.AddAttribute(0, HrefAttributeName, _href);
+            }
+
+            var combinedClassValue = CombineWithSpace(_cssClass, _isActive ? "active" : null);
+            if (combinedClassValue != null)
+            {
+                builder.AddAttribute(0, CssClassAttributeName, combinedClassValue);
+            }
+
+            if (_otherAttributes != null)
+            {
+                foreach (var kvp in _otherAttributes)
+                {
+                    builder.AddAttribute(0, kvp.Key, kvp.Value);
+                }
+            }
+
+            if (_childContent != null)
+            {
+                builder.AddContent(1, _childContent);
+            }
+
+            builder.CloseElement();
+        }
+
+        private string CombineWithSpace(string str1, string str2)
+            => str1 == null ? str2
+            : (str2 == null ? str1 : $"{str1} {str2}");
+    }
+}

src/Microsoft.AspNetCore.Blazor.Browser/Routing/UriHelper.cs

@@ -17,6 +17,8 @@ namespace Microsoft.AspNetCore.Blazor.Browser.Routing
     {
         static readonly string _functionPrefix = typeof(UriHelper).FullName;
         static string _currentAbsoluteUri;
+        static string _baseUriString;
+        static Uri _baseUri;
 
         /// <summary>
         /// An event that fires when the navigation location has changed.
@@ -38,9 +40,20 @@ namespace Microsoft.AspNetCore.Blazor.Browser.Routing
         /// <returns>The URI prefix.</returns>
         public static string GetBaseUriPrefix()
         {
-            var baseUri = RegisteredFunction.InvokeUnmarshalled<string>(
+            EnsureBaseUriPopulated();
-                $"{_functionPrefix}.getBaseURI");
+            return _baseUriString;
-            return ToBaseUriPrefix(baseUri);
+        }
+
+        private static void EnsureBaseUriPopulated()
+        {
+            // The <base href> is fixed for the lifetime of the page, so just cache it
+            if (_baseUriString == null)
+            {
+                var baseUri = RegisteredFunction.InvokeUnmarshalled<string>(
+                    $"{_functionPrefix}.getBaseURI");
+                _baseUriString = ToBaseUriPrefix(baseUri);
+                _baseUri = new Uri(_baseUriString);
+            }
         }
 
         /// <summary>
@@ -58,6 +71,17 @@ namespace Microsoft.AspNetCore.Blazor.Browser.Routing
             return _currentAbsoluteUri;
         }
 
+        /// <summary>
+        /// Converts a relative URI into an absolute one.
+        /// </summary>
+        /// <param name="relativeUri">The relative URI.</param>
+        /// <returns>The absolute URI.</returns>
+        public static Uri ToAbsoluteUri(string relativeUri)
+        {
+            EnsureBaseUriPopulated();
+            return new Uri(_baseUri, relativeUri);
+        }
+
         /// <summary>
         /// Given a base URI prefix (e.g., one previously returned by <see cref="GetBaseUriPrefix"/>),
         /// converts an absolute URI into one relative to the base URI prefix.

src/Microsoft.AspNetCore.Blazor/RenderTree/RenderTreeBuilder.cs

@@ -26,7 +26,7 @@ namespace Microsoft.AspNetCore.Blazor.RenderTree
         /// <summary>
         /// The reserved parameter name used for supplying child content.
         /// </summary>
-        public static string ChildContent = nameof(ChildContent);
+        public const string ChildContent = nameof(ChildContent);
 
         /// <summary>
         /// Constructs an instance of <see cref="RenderTreeBuilder"/>.