LinkTagHelper.cs

// 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 System;
using System.Collections.Generic;
using System.Diagnostics;
using System.IO;
using System.Text.Encodings.Web;
using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.Html;
using Microsoft.AspNetCore.Mvc.Razor.TagHelpers;
using Microsoft.AspNetCore.Mvc.Rendering;
using Microsoft.AspNetCore.Mvc.Routing;
using Microsoft.AspNetCore.Mvc.TagHelpers.Internal;
using Microsoft.AspNetCore.Razor.TagHelpers;
using Microsoft.Extensions.Caching.Memory;

namespace Microsoft.AspNetCore.Mvc.TagHelpers
{
    /// <summary>
    /// <see cref="ITagHelper"/> implementation targeting &lt;link&gt; elements that supports fallback href paths.
    /// </summary>
    /// <remarks>
    /// The tag helper won't process for cases with just the 'href' attribute.
    /// </remarks>
    [HtmlTargetElement("link", Attributes = HrefIncludeAttributeName, TagStructure = TagStructure.WithoutEndTag)]
    [HtmlTargetElement("link", Attributes = HrefExcludeAttributeName, TagStructure = TagStructure.WithoutEndTag)]
    [HtmlTargetElement("link", Attributes = FallbackHrefAttributeName, TagStructure = TagStructure.WithoutEndTag)]
    [HtmlTargetElement("link", Attributes = FallbackHrefIncludeAttributeName, TagStructure = TagStructure.WithoutEndTag)]
    [HtmlTargetElement("link", Attributes = FallbackHrefExcludeAttributeName, TagStructure = TagStructure.WithoutEndTag)]
    [HtmlTargetElement("link", Attributes = FallbackTestClassAttributeName, TagStructure = TagStructure.WithoutEndTag)]
    [HtmlTargetElement("link", Attributes = FallbackTestPropertyAttributeName, TagStructure = TagStructure.WithoutEndTag)]
    [HtmlTargetElement("link", Attributes = FallbackTestValueAttributeName, TagStructure = TagStructure.WithoutEndTag)]
    [HtmlTargetElement("link", Attributes = AppendVersionAttributeName, TagStructure = TagStructure.WithoutEndTag)]
    public class LinkTagHelper : UrlResolutionTagHelper
    {

        private static readonly string FallbackJavaScriptResourceName =
            typeof(LinkTagHelper).Namespace + ".compiler.resources.LinkTagHelper_FallbackJavaScript.js";

        private const string HrefIncludeAttributeName = "asp-href-include";
        private const string HrefExcludeAttributeName = "asp-href-exclude";
        private const string FallbackHrefAttributeName = "asp-fallback-href";
        private const string FallbackHrefIncludeAttributeName = "asp-fallback-href-include";
        private const string FallbackHrefExcludeAttributeName = "asp-fallback-href-exclude";
        private const string FallbackTestClassAttributeName = "asp-fallback-test-class";
        private const string FallbackTestPropertyAttributeName = "asp-fallback-test-property";
        private const string FallbackTestValueAttributeName = "asp-fallback-test-value";
        private const string AppendVersionAttributeName = "asp-append-version";
        private const string HrefAttributeName = "href";
        private const string RelAttributeName = "rel";
        private static readonly Func<Mode, Mode, int> Compare = (a, b) => a - b;

        private FileVersionProvider _fileVersionProvider;

        private static readonly ModeAttributes<Mode>[] ModeDetails = new[] {
            // Regular src with file version alone
            new ModeAttributes<Mode>(Mode.AppendVersion, new[] { AppendVersionAttributeName }),
            // Globbed Href (include only) no static href
            new ModeAttributes<Mode>(Mode.GlobbedHref, new [] { HrefIncludeAttributeName }),
            // Globbed Href (include & exclude), no static href
            new ModeAttributes<Mode>(Mode.GlobbedHref, new [] { HrefIncludeAttributeName, HrefExcludeAttributeName }),
            // Fallback with static href
            new ModeAttributes<Mode>(
                Mode.Fallback,
                new[]
                {
                    FallbackHrefAttributeName,
                    FallbackTestClassAttributeName,
                    FallbackTestPropertyAttributeName,
                    FallbackTestValueAttributeName
                }),
            // Fallback with globbed href (include only)
            new ModeAttributes<Mode>(
                Mode.Fallback,
                new[]
                {
                    FallbackHrefIncludeAttributeName,
                    FallbackTestClassAttributeName,
                    FallbackTestPropertyAttributeName,
                    FallbackTestValueAttributeName
                }),
            // Fallback with globbed href (include & exclude)
            new ModeAttributes<Mode>(
                Mode.Fallback,
                new[]
                {
                    FallbackHrefIncludeAttributeName,
                    FallbackHrefExcludeAttributeName,
                    FallbackTestClassAttributeName,
                    FallbackTestPropertyAttributeName,
                    FallbackTestValueAttributeName
                }),
        };
        private StringWriter _stringWriter;

        /// <summary>
        /// Creates a new <see cref="LinkTagHelper"/>.
        /// </summary>
        /// <param name="hostingEnvironment">The <see cref="IHostingEnvironment"/>.</param>
        /// <param name="cache">The <see cref="IMemoryCache"/>.</param>
        /// <param name="htmlEncoder">The <see cref="HtmlEncoder"/>.</param>
        /// <param name="javaScriptEncoder">The <see cref="JavaScriptEncoder"/>.</param>
        /// <param name="urlHelperFactory">The <see cref="IUrlHelperFactory"/>.</param>
        public LinkTagHelper(
            IHostingEnvironment hostingEnvironment,
            IMemoryCache cache,
            HtmlEncoder htmlEncoder,
            JavaScriptEncoder javaScriptEncoder,
            IUrlHelperFactory urlHelperFactory)
            : base(urlHelperFactory, htmlEncoder)
        {
            HostingEnvironment = hostingEnvironment;
            Cache = cache;
            JavaScriptEncoder = javaScriptEncoder;
        }

        /// <inheritdoc />
        public override int Order
        {
            get
            {
                return -1000;
            }
        }

        /// <summary>
        /// Address of the linked resource.
        /// </summary>
        /// <remarks>
        /// Passed through to the generated HTML in all cases.
        /// </remarks>
        [HtmlAttributeName(HrefAttributeName)]
        public string Href { get; set; }

        /// <summary>
        /// A comma separated list of globbed file patterns of CSS stylesheets to load.
        /// The glob patterns are assessed relative to the application's 'webroot' setting.
        /// </summary>
        [HtmlAttributeName(HrefIncludeAttributeName)]
        public string HrefInclude { get; set; }

        /// <summary>
        /// A comma separated list of globbed file patterns of CSS stylesheets to exclude from loading.
        /// The glob patterns are assessed relative to the application's 'webroot' setting.
        /// Must be used in conjunction with <see cref="HrefInclude"/>.
        /// </summary>
        [HtmlAttributeName(HrefExcludeAttributeName)]
        public string HrefExclude { get; set; }

        /// <summary>
        /// The URL of a CSS stylesheet to fallback to in the case the primary one fails.
        /// </summary>
        [HtmlAttributeName(FallbackHrefAttributeName)]
        public string FallbackHref { get; set; }

        /// <summary>
        /// Value indicating if file version should be appended to the href urls.
        /// </summary>
        /// <remarks>
        /// If <c>true</c> then a query string "v" with the encoded content of the file is added.
        /// </remarks>
        [HtmlAttributeName(AppendVersionAttributeName)]
        public bool? AppendVersion { get; set; }

        /// <summary>
        /// A comma separated list of globbed file patterns of CSS stylesheets to fallback to in the case the primary
        /// one fails.
        /// The glob patterns are assessed relative to the application's 'webroot' setting.
        /// </summary>
        [HtmlAttributeName(FallbackHrefIncludeAttributeName)]
        public string FallbackHrefInclude { get; set; }

        /// <summary>
        /// A comma separated list of globbed file patterns of CSS stylesheets to exclude from the fallback list, in
        /// the case the primary one fails.
        /// The glob patterns are assessed relative to the application's 'webroot' setting.
        /// Must be used in conjunction with <see cref="FallbackHrefInclude"/>.
        /// </summary>
        [HtmlAttributeName(FallbackHrefExcludeAttributeName)]
        public string FallbackHrefExclude { get; set; }

        /// <summary>
        /// The class name defined in the stylesheet to use for the fallback test.
        /// Must be used in conjunction with <see cref="FallbackTestProperty"/> and <see cref="FallbackTestValue"/>,
        /// and either <see cref="FallbackHref"/> or <see cref="FallbackHrefInclude"/>.
        /// </summary>
        [HtmlAttributeName(FallbackTestClassAttributeName)]
        public string FallbackTestClass { get; set; }

        /// <summary>
        /// The CSS property name to use for the fallback test.
        /// Must be used in conjunction with <see cref="FallbackTestClass"/> and <see cref="FallbackTestValue"/>,
        /// and either <see cref="FallbackHref"/> or <see cref="FallbackHrefInclude"/>.
        /// </summary>
        [HtmlAttributeName(FallbackTestPropertyAttributeName)]
        public string FallbackTestProperty { get; set; }

        /// <summary>
        /// The CSS property value to use for the fallback test.
        /// Must be used in conjunction with <see cref="FallbackTestClass"/> and <see cref="FallbackTestProperty"/>,
        /// and either <see cref="FallbackHref"/> or <see cref="FallbackHrefInclude"/>.
        /// </summary>
        [HtmlAttributeName(FallbackTestValueAttributeName)]
        public string FallbackTestValue { get; set; }

        protected IHostingEnvironment HostingEnvironment { get; }

        protected IMemoryCache Cache { get; }

        protected JavaScriptEncoder JavaScriptEncoder { get; }

        // Internal for ease of use when testing.
        protected internal GlobbingUrlBuilder GlobbingUrlBuilder { get; set; }

        // Shared writer for determining the string content of a TagHelperAttribute's Value.
        private StringWriter StringWriter
        {
            get
            {
                if (_stringWriter == null)
                {
                    _stringWriter = new StringWriter();
                }

                return _stringWriter;
            }
        }

        /// <inheritdoc />
        public override void Process(TagHelperContext context, TagHelperOutput output)
        {
            if (context == null)
            {
                throw new ArgumentNullException(nameof(context));
            }

            if (output == null)
            {
                throw new ArgumentNullException(nameof(output));
            }

            // Pass through attribute that is also a well-known HTML attribute.
            if (Href != null)
            {
                output.CopyHtmlAttribute(HrefAttributeName, context);
            }

            // If there's no "href" attribute in output.Attributes this will noop.
            ProcessUrlAttribute(HrefAttributeName, output);

            // Retrieve the TagHelperOutput variation of the "href" attribute in case other TagHelpers in the
            // pipeline have touched the value. If the value is already encoded this LinkTagHelper may
            // not function properly.
            Href = output.Attributes[HrefAttributeName]?.Value as string;

            Mode mode;
            if (!AttributeMatcher.TryDetermineMode(context, ModeDetails, Compare, out mode))
            {
                // No attributes matched so we have nothing to do
                return;
            }

            if (AppendVersion == true)
            {
                EnsureFileVersionProvider();

                if (Href != null)
                {
                    var index = output.Attributes.IndexOfName(HrefAttributeName);
                    var existingAttribute = output.Attributes[index];
                    output.Attributes[index] = new TagHelperAttribute(
                        existingAttribute.Name,
                        _fileVersionProvider.AddFileVersionToPath(Href),
                        existingAttribute.ValueStyle);
                }
            }

            var builder = output.PostElement;
            builder.Clear();

            if (mode == Mode.GlobbedHref || mode == Mode.Fallback && !string.IsNullOrEmpty(HrefInclude))
            {
                BuildGlobbedLinkTags(output.Attributes, builder);
                if (string.IsNullOrEmpty(Href))
                {
                    // Only HrefInclude is specified. Don't render the original tag.
                    output.TagName = null;
                    output.Content.SetHtmlContent(HtmlString.Empty);
                }
            }

            if (mode == Mode.Fallback && HasStyleSheetLinkType(output.Attributes))
            {
                string resolvedUrl;
                if (TryResolveUrl(FallbackHref, resolvedUrl: out resolvedUrl))
                {
                    FallbackHref = resolvedUrl;
                }

                BuildFallbackBlock(output.Attributes, builder);
            }
        }

        private void BuildGlobbedLinkTags(TagHelperAttributeList attributes, TagHelperContent builder)
        {
            EnsureGlobbingUrlBuilder();

            // Build a <link /> tag for each matched href.
            var urls = GlobbingUrlBuilder.BuildUrlList(null, HrefInclude, HrefExclude);
            for (var i = 0; i < urls.Count; i++)
            {
                var url = urls[i];

                // "url" values come from bound attributes and globbing. Must always be non-null.
                Debug.Assert(url != null);

                if (string.Equals(Href, url, StringComparison.OrdinalIgnoreCase))
                {
                    // Don't build duplicate link tag for the original href url.
                    continue;
                }

                BuildLinkTag(url, attributes, builder);
            }
        }

        private void BuildFallbackBlock(TagHelperAttributeList attributes, TagHelperContent builder)
        {
            EnsureGlobbingUrlBuilder();
            var fallbackHrefs = GlobbingUrlBuilder.BuildUrlList(
                FallbackHref,
                FallbackHrefInclude,
                FallbackHrefExclude);

            if (fallbackHrefs.Count == 0)
            {
                return;
            }

            builder.AppendHtml(HtmlString.NewLine);

            // Build the <meta /> tag that's used to test for the presence of the stylesheet
            builder
                .AppendHtml("<meta name=\"x-stylesheet-fallback-test\" content=\"\" class=\"")
                .Append(FallbackTestClass)
                .AppendHtml("\" />");

            // Build the <script /> tag that checks the effective style of <meta /> tag above and renders the extra
            // <link /> tag to load the fallback stylesheet if the test CSS property value is found to be false,
            // indicating that the primary stylesheet failed to load.
            // GetEmbeddedJavaScript returns JavaScript to which we add '"{0}","{1}",{2});'
            builder
                .AppendHtml("<script>")
                .AppendHtml(JavaScriptResources.GetEmbeddedJavaScript(FallbackJavaScriptResourceName))
                .AppendHtml("\"")
                .AppendHtml(JavaScriptEncoder.Encode(FallbackTestProperty))
                .AppendHtml("\",\"")
                .AppendHtml(JavaScriptEncoder.Encode(FallbackTestValue))
                .AppendHtml("\",");

            AppendFallbackHrefs(builder, fallbackHrefs);

            builder.AppendHtml(", \"");

            // Perf: Avoid allocating enumerator
            for (var i = 0; i < attributes.Count; i++)
            {
                var attribute = attributes[i];
                if (string.Equals(attribute.Name, HrefAttributeName, StringComparison.OrdinalIgnoreCase))
                {
                    continue;
                }

                attribute.WriteTo(StringWriter, HtmlEncoder);
                StringWriter.Write(' ');
            }

            var stringBuilder = StringWriter.GetStringBuilder();
            var scriptTags = stringBuilder.ToString();
            stringBuilder.Clear();
            var encodedScriptTags = JavaScriptEncoder.Encode(scriptTags);
            builder.AppendHtml(encodedScriptTags);

            builder.AppendHtml("\");</script>");
        }

        private bool HasStyleSheetLinkType(TagHelperAttributeList attributes)
        {
            TagHelperAttribute relAttribute;
            if (!attributes.TryGetAttribute(RelAttributeName, out relAttribute) ||
                relAttribute.Value == null)
            {
                return false;
            }

            var attributeValue = relAttribute.Value;
            var contentValue = attributeValue as IHtmlContent;
            var stringValue = attributeValue as string;
            if (contentValue != null)
            {
                contentValue.WriteTo(StringWriter, HtmlEncoder);
                stringValue = StringWriter.ToString();

                // Reset writer
                StringWriter.GetStringBuilder().Clear();
            }
            else if (stringValue == null)
            {
                stringValue = attributeValue.ToString();
            }

            var hasRelStylesheet = string.Equals("stylesheet", stringValue, StringComparison.Ordinal);

            return hasRelStylesheet;
        }

        private void AppendFallbackHrefs(TagHelperContent builder, IReadOnlyList<string> fallbackHrefs)
        {
            builder.AppendHtml("[");
            var firstAdded = false;
            // Perf: Avoid allocating enumerator
            for (var i = 0; i < fallbackHrefs.Count; i++)
            {
                if (firstAdded)
                {
                    builder.AppendHtml(",\"");
                }
                else
                {
                    builder.AppendHtml("\"");
                    firstAdded = true;
                }

                // fallbackHrefs come from bound attributes (a C# context) and globbing. Must always be non-null.
                Debug.Assert(fallbackHrefs[i] != null);

                var valueToWrite = fallbackHrefs[i];
                if (AppendVersion == true)
                {
                    valueToWrite = _fileVersionProvider.AddFileVersionToPath(fallbackHrefs[i]);
                }

                // Must HTML-encode the href attribute value to ensure the written <link/> element is valid. Must also
                // JavaScript-encode that value to ensure the doc.write() statement is valid.
                valueToWrite = HtmlEncoder.Encode(valueToWrite);
                valueToWrite = JavaScriptEncoder.Encode(valueToWrite);

                builder.AppendHtml(valueToWrite);
                builder.AppendHtml("\"");
            }
            builder.AppendHtml("]");
        }

        private void EnsureGlobbingUrlBuilder()
        {
            if (GlobbingUrlBuilder == null)
            {
                GlobbingUrlBuilder = new GlobbingUrlBuilder(
                    HostingEnvironment.WebRootFileProvider,
                    Cache,
                    ViewContext.HttpContext.Request.PathBase);
            }
        }

        private void EnsureFileVersionProvider()
        {
            if (_fileVersionProvider == null)
            {
                _fileVersionProvider = new FileVersionProvider(
                    HostingEnvironment.WebRootFileProvider,
                    Cache,
                    ViewContext.HttpContext.Request.PathBase);
            }
        }

        private void BuildLinkTag(string href, TagHelperAttributeList attributes, TagHelperContent builder)
        {
            builder.AppendHtml("<link ");

            var addHref = true;

            // Perf: Avoid allocating enumerator
            for (var i = 0; i < attributes.Count; i++)
            {
                var attribute = attributes[i];

                if (string.Equals(attribute.Name, HrefAttributeName, StringComparison.OrdinalIgnoreCase))
                {
                    addHref = false;

                    AppendVersionedHref(attribute.Name, href, builder);
                }
                else
                {
                    attribute.CopyTo(builder);
                    builder.AppendHtml(" ");
                }
            }

            if (addHref)
            {
                AppendVersionedHref(HrefAttributeName, href, builder);
            }

            builder.AppendHtml("/>");
        }

        private void AppendVersionedHref(string hrefName, string hrefValue, TagHelperContent builder)
        {
            if (AppendVersion == true)
            {
                hrefValue = _fileVersionProvider.AddFileVersionToPath(hrefValue);
            }

            builder
                .AppendHtml(hrefName)
                .AppendHtml("=\"")
                .Append(hrefValue)
                .AppendHtml("\" ");
        }

        private enum Mode
        {
            /// <summary>
            /// Just adding a file version for the generated urls.
            /// </summary>
            AppendVersion = 0,

            /// <summary>
            /// Just performing file globbing search for the href, rendering a separate &lt;link&gt; for each match.
            /// </summary>
            GlobbedHref = 1,

            /// <summary>
            /// Rendering a fallback block if primary stylesheet fails to load. Will also do globbing for both the
            /// primary and fallback hrefs if the appropriate properties are set.
            /// </summary>
            Fallback = 2,
        }
    }
}