ComponentsWebAssemblyApplicationBuilderExtensions.cs

// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.

using System.Linq;
using System.Net.Mime;
using Microsoft.AspNetCore.Components.WebAssembly.Server;
using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.Http;
using Microsoft.AspNetCore.StaticFiles;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.FileProviders;
using Microsoft.Extensions.Primitives;
using Microsoft.Net.Http.Headers;

namespace Microsoft.AspNetCore.Builder;

/// <summary>
/// Extensions for mapping Blazor WebAssembly applications.
/// </summary>
public static class ComponentsWebAssemblyApplicationBuilderExtensions
{
    private static readonly string? s_dotnetModifiableAssemblies = GetNonEmptyEnvironmentVariableValue("DOTNET_MODIFIABLE_ASSEMBLIES");
    private static readonly string? s_aspnetcoreBrowserTools = GetNonEmptyEnvironmentVariableValue("__ASPNETCORE_BROWSER_TOOLS");

    private static string? GetNonEmptyEnvironmentVariableValue(string name)
        => Environment.GetEnvironmentVariable(name) is { Length: >0 } value ? value : null;

    /// <summary>
    /// Configures the application to serve Blazor WebAssembly framework files from the path <paramref name="pathPrefix"/>. This path must correspond to a referenced Blazor WebAssembly application project.
    /// </summary>
    /// <param name="builder">The <see cref="IApplicationBuilder"/>.</param>
    /// <param name="pathPrefix">The <see cref="Microsoft.AspNetCore.Http.PathString"/> that indicates the prefix for the Blazor WebAssembly application.</param>
    /// <returns>The <see cref="IApplicationBuilder"/></returns>
    public static IApplicationBuilder UseBlazorFrameworkFiles(this IApplicationBuilder builder, PathString pathPrefix)
    {
        ArgumentNullException.ThrowIfNull(builder);

        var webHostEnvironment = builder.ApplicationServices.GetRequiredService<IWebHostEnvironment>();

        var options = CreateStaticFilesOptions(webHostEnvironment.WebRootFileProvider);

        builder.MapWhen(ctx => ctx.Request.Path.StartsWithSegments(pathPrefix, out var rest) && rest.StartsWithSegments("/_framework") &&
        !rest.StartsWithSegments("/_framework/blazor.server.js") && !rest.StartsWithSegments("/_framework/blazor.web.js"),
        subBuilder =>
        {
            subBuilder.Use(async (context, next) =>
            {
                context.Response.Headers.Append("Blazor-Environment", webHostEnvironment.EnvironmentName);

                // DOTNET_MODIFIABLE_ASSEMBLIES is used by the runtime to initialize hot-reload specific environment variables and is configured
                // by the launching process (dotnet-watch / Visual Studio).
                // Always add the header if the environment variable is set, regardless of the kind of environment.
                if (s_dotnetModifiableAssemblies != null)
                {
                    context.Response.Headers.Append("DOTNET-MODIFIABLE-ASSEMBLIES", s_dotnetModifiableAssemblies);
                }

                // See https://github.com/dotnet/aspnetcore/issues/37357#issuecomment-941237000
                // Translate the _ASPNETCORE_BROWSER_TOOLS environment configured by the browser tools agent in to a HTTP response header.
                if (s_aspnetcoreBrowserTools != null)
                {
                    context.Response.Headers.Append("ASPNETCORE-BROWSER-TOOLS", s_aspnetcoreBrowserTools);
                }

                await next(context);
            });

            subBuilder.UseMiddleware<ContentEncodingNegotiator>();

            subBuilder.UseStaticFiles(options);
        });

        return builder;
    }

    /// <summary>
    /// Configures the application to serve Blazor WebAssembly framework files from the root path "/".
    /// </summary>
    /// <param name="applicationBuilder">The <see cref="IApplicationBuilder"/>.</param>
    /// <returns>The <see cref="IApplicationBuilder"/></returns>
    public static IApplicationBuilder UseBlazorFrameworkFiles(this IApplicationBuilder applicationBuilder) =>
        UseBlazorFrameworkFiles(applicationBuilder, default);

    private static StaticFileOptions CreateStaticFilesOptions(IFileProvider webRootFileProvider)
    {
        var options = new StaticFileOptions();
        options.FileProvider = webRootFileProvider;
        var contentTypeProvider = new FileExtensionContentTypeProvider();
        AddMapping(contentTypeProvider, ".dll", MediaTypeNames.Application.Octet);
        AddMapping(contentTypeProvider, ".webcil", MediaTypeNames.Application.Octet);
        // We unconditionally map pdbs as there will be no pdbs in the output folder for
        // release builds unless BlazorEnableDebugging is explicitly set to true.
        AddMapping(contentTypeProvider, ".pdb", MediaTypeNames.Application.Octet);
        AddMapping(contentTypeProvider, ".br", MediaTypeNames.Application.Octet);
        AddMapping(contentTypeProvider, ".dat", MediaTypeNames.Application.Octet);
        AddMapping(contentTypeProvider, ".blat", MediaTypeNames.Application.Octet);

        options.ContentTypeProvider = contentTypeProvider;

        // Static files middleware will try to use application/x-gzip as the content
        // type when serving a file with a gz extension. We need to correct that before
        // sending the file.
        options.OnPrepareResponse = fileContext =>
        {
            // At this point we mapped something from the /_framework
            fileContext.Context.Response.Headers.Append(HeaderNames.CacheControl, "no-cache");

            var requestPath = fileContext.Context.Request.Path;
            var fileExtension = Path.GetExtension(requestPath.Value);
            if (string.Equals(fileExtension, ".gz") || string.Equals(fileExtension, ".br"))
            {
                // When we are serving framework files (under _framework/ we perform content negotiation
                // on the accept encoding and replace the path with <<original>>.gz|br if we can serve gzip or brotli content
                // respectively.
                // Here we simply calculate the original content type by removing the extension and apply it
                // again.
                // When we revisit this, we should consider calculating the original content type and storing it
                // in the request along with the original target path so that we don't have to calculate it here.
                var originalPath = Path.GetFileNameWithoutExtension(requestPath.Value);
                if (originalPath != null && contentTypeProvider.TryGetContentType(originalPath, out var originalContentType))
                {
                    fileContext.Context.Response.ContentType = originalContentType;
                }
            }
        };

        return options;
    }

    private static void AddMapping(FileExtensionContentTypeProvider provider, string name, string mimeType)
    {
        if (!provider.Mappings.ContainsKey(name))
        {
            provider.Mappings.Add(name, mimeType);
        }
    }
}