fix: Produce diagnostics only after user stops typing

This commit creates a timer to keep track of the last change made by the
user, and sends the diagnostics only after user stops typing.
Currently, it triggers for every change, and slows down the entire
operation.
A default delay of 200ms is consistent with TypeScript.

PR closes #432

Author: Keen Yee Liau

server/src/session.ts

@@ -38,6 +38,7 @@ const EMPTY_RANGE = lsp.Range.create(0, 0, 0, 0);
 export class Session {
   private readonly connection: lsp.IConnection;
   private readonly projectService: ProjectService;
+  private diagnosticsTimeout: NodeJS.Timeout|null = null;
 
   constructor(options: SessionOptions) {
     // Create a connection for the server. The connection uses Node's IPC as a transport.
@@ -91,16 +92,38 @@ export class Session {
       case ts.server.ProjectsUpdatedInBackgroundEvent:
         // ProjectsUpdatedInBackgroundEvent is sent whenever diagnostics are
         // requested via project.refreshDiagnostics()
-        this.refreshDiagnostics(event.data.openFiles);
+        this.triggerDiagnostics(event.data.openFiles);
         break;
     }
   }
 
   /**
-   * Retrieve Angular diagnostics for the specified `openFiles`.
+   * Retrieve Angular diagnostics for the specified `openFiles` after a specific
+   * `delay`, or renew the request if there's already a pending one.
    * @param openFiles
+   * @param delay time to wait before sending request (milliseconds)
    */
-  private refreshDiagnostics(openFiles: string[]) {
+  private triggerDiagnostics(openFiles: string[], delay: number = 200) {
+    // Do not immediately send a diagnostics request. Send only after user has
+    // stopped typing after the specified delay.
+    if (this.diagnosticsTimeout) {
+      // If there's an existing timeout, cancel it
+      clearTimeout(this.diagnosticsTimeout);
+    }
+    // Set a new timeout
+    this.diagnosticsTimeout = setTimeout(() => {
+      this.diagnosticsTimeout = null;  // clear the timeout
+      this.sendPendingDiagnostics(openFiles);
+      // Default delay is 200ms, consistent with TypeScript. See
+      // https://github.com/microsoft/vscode/blob/7b944a16f52843b44cede123dd43ae36c0405dfd/extensions/typescript-language-features/src/features/bufferSyncSupport.ts#L493)
+    }, delay);
+  }
+
+  /**
+   * Execute diagnostics request for each of the specified `openFiles`.
+   * @param openFiles
+   */
+  private sendPendingDiagnostics(openFiles: string[]) {
     for (const fileName of openFiles) {
       const scriptInfo = this.projectService.getScriptInfo(fileName);
       if (!scriptInfo) {