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.vscode/launch.json

@@ -11,6 +11,13 @@
 			"outFiles": ["${workspaceRoot}/extension/dist/main.js"],
 			"autoAttachChildProcesses": true,
       "preLaunchTask": "npm: build - extension"
-		}
+		},
+    {
+      "name": "Attach to language server",
+      "request": "attach",
+      "type": "dart",
+      "cwd": "pkgs/sass_language_server",
+      "vmServiceUri": "${command:dart.promptForVmService}" // Prompt for the VM Service URI
+    },
 	]
 }

docs/contributing/testing-and-debugging.md

@@ -13,42 +13,65 @@ The quickest way to test the language server is to debug the language extension
 
 This will open another window of Visual Studio Code, this one running as an `[Extension Development Host]`.
 
-### Testing in isolation
+### Find the link to Dart DevTools or VM service
 
-VS Code ships with some built-in support for SCSS and CSS. To test this language server in isolation you can disable the built-in extension.
+When debugging, the client runs [`dart run --enable-vm-service`](https://github.com/sass/dart-sass-language-server/blob/main/extension/src/server.ts#L49)
+in the local `sass_language_server` package.
 
-1. Go to the Extensions tab and search for `@builtin css language features`.
-2. Click the settings icon and pick Disable from the list.
-3. Click Restart extension to turn it off.
+Use the `[Extension Development Host]` window to find the link to open Dart DevTools or to [attach the debugger](#attach-to-language-server).
 
-You should also turn off extensions like SCSS IntelliSense or Some Sass.
+1. Open a CSS, SCSS or Sass file to activate the language server.
+2. Open the Output pane (View -> Output in the menu).
+3. Choose Sass in the dropdown to the top right of the Output pane.
+4. Scroll to the top of the output.
 
-### Open the Dart DevTools
+You should see something similar to this.
 
-In this configuration, the client has run `dart run --observe` in the local `sass_language_server` package. You can now use [Dart DevTools](https://dart.dev/tools/dart-devtools) to debug the language server.
+```
+The Dart VM service is listening on http://127.0.0.1:8181/SMIxtkPzlAY=/
+The Dart DevTools debugger and profiler is available at: http://127.0.0.1:8181/SMIxtkPzlAY=/devtools/?uri=ws://127.0.0.1:8181/SMIxtkPzlAY=/ws
+```
 
-To find the link to open Dart DevTools, use the `[Extension Development Host]`.
+Click the second link to open Dart DevTools, or copy the first link to [attach a debugger](#attach-to-language-server).
 
-1. Open a CSS, SCSS or Sass file to activate the language server.
-2. Open the Output pane (View -> Output in the menu).
-3. In the dropdown in the top right, choose Sass from the list.
+![screenshot showing the output pane and the dropdown with sass selected](https://github.com/user-attachments/assets/85839d2f-4305-4fb9-aeb0-d78f435e8b7d)
+
+### Attach to language server
 
-You should see output similar to this.
+The debugger in Dart DevTools is deprecated in favor the debugger that ships with [Dart for Visual Studio Code][vscodedart].
+
+To start debugging in VS Code (provided you have the Dart extension):
+
+1. [Run the language server and extension](#run-the-language-extension-and-server) in debug mode.
+2. [Find the link to the Dart VM](#find-the-link-to-dart-devtools-or-vm-service).
+
+You should see output similar to this in the `[Extension Development Host]`.
 
 ```
 The Dart VM service is listening on http://127.0.0.1:8181/SMIxtkPzlAY=/
 The Dart DevTools debugger and profiler is available at: http://127.0.0.1:8181/SMIxtkPzlAY=/devtools/?uri=ws://127.0.0.1:8181/SMIxtkPzlAY=/ws
 ```
 
-![screenshot showing the output pane and the dropdown with sass selected](https://github.com/user-attachments/assets/85839d2f-4305-4fb9-aeb0-d78f435e8b7d)
+Copy the first link, then go back to the Run and debug window where you started the language server and extension.
+
+1. Click the Run and debug drop-down and run `Attach to language server`.
+2. Paste the link you copied and hit Enter.
+
+Your debugger should be attached, allowing you to place breakpoints and step through code.
+
+### Test in VS Code without built-in SCSS features
 
-Click the second link to open Dart DevTools.
+VS Code ships with some built-in support for SCSS and CSS. To test this language server in isolation you can disable the built-in extension.
 
-The Debugger tab has a File explorer in which you can find `package:sass_language_server`. Go to `src/language_server.dart` to find the request handlers for messages coming in from the client.
+1. Go to the Extensions tab and search for `@builtin css language features`.
+2. Click the settings icon and pick Disable from the list.
+3. Click Restart extension to turn it off.
+
+You should also turn off extensions like SCSS IntelliSense or Some Sass.
 
 ## Debug unit tests
 
-Assuming you installed [Dart for Visual Studio Code](https://marketplace.visualstudio.com/items?itemName=Dart-Code.dart-code) you can debug individual unit tests by right-clicking the Run button in the editor gutter.
+Assuming you installed [Dart for Visual Studio Code][vscodedart] you can debug individual unit tests by right-clicking the Run button in the editor gutter.
 
 Writing a test is often faster when debugging an issue with a specific language feature, and helps improve test coverage.
 
@@ -71,3 +94,5 @@ test profile in the Run and Debug view in VS Code.
   ]
 }
 ```
+
+[vscodedart]: https://marketplace.visualstudio.com/items?itemName=Dart-Code.dart-code

extension/src/main.ts

@@ -132,61 +132,9 @@ export async function activate(context: ExtensionContext): Promise<void> {
       }
     }
   });
-
-  // TODO: Maybe worth looking into so links to built-ins resolve to something?
-  // workspace.registerFileSystemProvider(
-  //   'sass',
-  //   {
-  //     readFile(uri) {
-  //       return Uint8Array.from(
-  //         '@function hello();'.split('').map((c) => c.charCodeAt(0))
-  //       );
-  //     },
-  //     watch(uri, options) {
-  //       return Disposable.create(() => {
-  //         console.log('hello');
-  //       });
-  //     },
-  //     readDirectory(uri) {
-  //       return [];
-  //     },
-  //     stat(uri) {
-  //       return {
-  //         ctime: 0,
-  //         mtime: 0,
-  //         size: 0,
-  //         type: 1,
-  //       };
-  //     },
-  //     writeFile(uri, content, options) {
-  //       return;
-  //     },
-  //     createDirectory(uri) {
-  //       return;
-  //     },
-  //     delete(uri, options) {
-  //       return;
-  //     },
-  //     rename(oldUri, newUri, options) {
-  //       return;
-  //     },
-  //     copy(source, destination, options) {
-  //       return;
-  //     },
-  //     onDidChangeFile(e) {
-  //       return Disposable.create(() => {
-  //         console.log('hello');
-  //       });
-  //     },
-  //   },
-  //   {
-  //     isCaseSensitive: false,
-  //     isReadonly: true,
-  //   }
-  // );
 }
 
-export async function deactivate(): Promise<void> {
+export function deactivate(): Promise<void> {
   const promises: Thenable<void>[] = [];
   if (defaultClient) {
     promises.push(defaultClient.stop());

extension/src/server.ts

@@ -46,7 +46,7 @@ export async function createServerOptions(
     args: [
       'run',
       // '--pause-isolates-on-start', // Uncomment this to debug issues during startup and initial scan
-      '--observe',
+      '--enable-vm-service',
       'sass_language_server',
       '--loglevel=debug',
     ],

pkgs/sass_language_server/bin/sass_language_server.dart

@@ -1,3 +1,6 @@
+import 'dart:io';
+
+import 'package:lsp_server/lsp_server.dart';
 import 'package:sass_language_server/sass_language_server.dart';
 
 void main(List<String> arguments) async {
@@ -35,9 +38,10 @@ Logging options:
   var fileSystemProvider = LocalFileSystem();
   var server = LanguageServer();
 
+  Connection connection;
+  Socket? socket;
   if (transport == '--stdio') {
-    await server.start(
-        logLevel: logLevel, fileSystemProvider: fileSystemProvider);
+    connection = Connection(stdin, stdout);
   } else {
     // The client is the one listening to socket connections on the specified port.
     // In other words the language server is a _client_ for the socket transport.
@@ -46,11 +50,31 @@ Logging options:
     // the language server.
     var split = transport.split('=');
     int port = int.parse(split.last);
+    socket = await Socket.connect('127.0.0.1', port);
+    connection = Connection(socket, socket);
+  }
+
+  try {
+    exitCode = 1;
 
     await server.start(
-        logLevel: logLevel,
-        fileSystemProvider: fileSystemProvider,
-        transport: Transport.socket,
-        port: port);
+      connection: connection,
+      logLevel: logLevel,
+      fileSystemProvider: fileSystemProvider,
+    );
+
+    // See
+    // https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#shutdown
+    // https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#exit
+    connection.onShutdown(() async {
+      socket?.close();
+      exitCode = 0;
+    });
+
+    connection.onExit(() async {
+      exit(exitCode);
+    });
+  } on Exception catch (_) {
+    exit(1);
   }
 }