[Wasm] Moves wasm reload logic out of ASP.NET Core and into the SDK (d…

…otnet#42549)

Moves the logic for setting environment variables from ASP.NET Core to the BrowserRefresh middleware in the SDK for webassembly support.

src/BuiltInTools/BrowserRefresh/BrowserRefreshMiddleware.cs

@@ -12,16 +12,27 @@ namespace Microsoft.AspNetCore.Watch.BrowserRefresh
     public class BrowserRefreshMiddleware
     {
         private static readonly MediaTypeHeaderValue _textHtmlMediaType = new("text/html");
+        private static readonly MediaTypeHeaderValue _applicationJsonMediaType = new("application/json");
+        private readonly string? _dotnetModifiableAssemblies = GetNonEmptyEnvironmentVariableValue("DOTNET_MODIFIABLE_ASSEMBLIES");
+        private readonly string? _aspnetcoreBrowserTools = GetNonEmptyEnvironmentVariableValue("__ASPNETCORE_BROWSER_TOOLS");
+
         private readonly RequestDelegate _next;
         private readonly ILogger _logger;
 
+        private static string? GetNonEmptyEnvironmentVariableValue(string name)
+            => Environment.GetEnvironmentVariable(name) is { Length: > 0 } value ? value : null;
+
         public BrowserRefreshMiddleware(RequestDelegate next, ILogger<BrowserRefreshMiddleware> logger) =>
             (_next, _logger) = (next, logger);
 
         public async Task InvokeAsync(HttpContext context)
         {
-            // We only need to support this for requests that could be initiated by a browser.
-            if (IsBrowserDocumentRequest(context))
+            if (IsWebAssemblyBootRequest(context))
+            {
+                AttachWebAssemblyHeaders(context);
+                await _next(context);
+            }
+            else if (IsBrowserDocumentRequest(context))
             {
                 // Use a custom StreamWrapper to rewrite output on Write/WriteAsync
                 using var responseStreamWrapper = new ResponseStreamWrapper(context, _logger);
@@ -59,6 +70,86 @@ public async Task InvokeAsync(HttpContext context)
             }
         }
 
+        private void AttachWebAssemblyHeaders(HttpContext context)
+        {
+            context.Response.OnStarting(() =>
+            {
+                if (!context.Response.Headers.ContainsKey("DOTNET-MODIFIABLE-ASSEMBLIES"))
+                {
+                    if(_dotnetModifiableAssemblies != null)
+                    {
+                        context.Response.Headers.Add("DOTNET-MODIFIABLE-ASSEMBLIES", _dotnetModifiableAssemblies);
+                    }
+                    else
+                    {
+                        _logger.LogDebug("DOTNET_MODIFIABLE_ASSEMBLIES environment variable is not set, likely because hot reload is not enabled. The browser refresh feature may not work as expected.");
+                    }
+                }
+                else
+                {
+                    _logger.LogDebug("DOTNET-MODIFIABLE-ASSEMBLIES header is already set.");
+                }
+
+                if (!context.Response.Headers.ContainsKey("ASPNETCORE-BROWSER-TOOLS"))
+                {
+                    if (_aspnetcoreBrowserTools != null)
+                    {
+                        context.Response.Headers.Add("ASPNETCORE-BROWSER-TOOLS", _aspnetcoreBrowserTools);
+                    }
+                    else
+                    {
+                        _logger.LogDebug("__ASPNETCORE_BROWSER_TOOLS environment variable is not set. The browser refresh feature may not work as expected.");
+                    }
+                }
+                else
+                {
+                    _logger.LogDebug("ASPNETCORE-BROWSER-TOOLS header is already set.");
+                }
+
+                return Task.CompletedTask;
+            });
+        }
+
+        internal static bool IsWebAssemblyBootRequest(HttpContext context)
+        {
+            var request = context.Request;
+            if (!HttpMethods.IsGet(request.Method))
+            {
+                return false;
+            }
+
+            if (request.Headers.TryGetValue("Sec-Fetch-Dest", out var values) &&
+                !StringValues.IsNullOrEmpty(values) &&
+                !string.Equals(values[0], "document", StringComparison.OrdinalIgnoreCase))
+            {
+                // See https://github.com/dotnet/aspnetcore/issues/37326.
+                // Only inject scripts that are destined for a browser page.
+                return false;
+            }
+
+            if (!request.Path.HasValue ||
+                !string.Equals(Path.GetFileName(request.Path.Value), "blazor.boot.json", StringComparison.OrdinalIgnoreCase))
+            {
+                return false;
+            }
+
+            var typedHeaders = request.GetTypedHeaders();
+            if (typedHeaders.Accept is not IList<MediaTypeHeaderValue> acceptHeaders)
+            {
+                return false;
+            }
+
+            for (var i = 0; i < acceptHeaders.Count; i++)
+            {
+                if (acceptHeaders[i].IsSubsetOf(_applicationJsonMediaType))
+                {
+                    return true;
+                }
+            }
+
+            return false;
+        }
+
         internal static bool IsBrowserDocumentRequest(HttpContext context)
         {
             var request = context.Request;