feat(vscode): define singleton DartFrogDeamon (#917)

extensions/vscode/src/daemon/dart-frog-daemon.ts

@@ -0,0 +1,18 @@
+/**
+ * The Dart Frog daemon is a long-running process that is responsible for
+ * managing a single or multiple Dart Frog projects simultaneously.
+ *
+ * @see {@link https://dartfrog.vgv.dev/docs/advanced/daemon Dart Frog deamon documentation }
+ */
+export class DartFrogDaemon {
+  private static _instance: DartFrogDaemon;
+
+  /**
+   * A singleton instance of the Dart Frog daemon.
+   *
+   * A Dart Frog daemon can manage multiple Dart Frog projects simultaneously.
+   */
+  public static get instance() {
+    return this._instance || (this._instance = new this());
+  }
+}

extensions/vscode/src/daemon/index.ts

@@ -0,0 +1 @@
+export * from "./dart-frog-daemon";

extensions/vscode/src/test/suite/daemon/dart-frog-daemon.test.ts

@@ -0,0 +1,11 @@
+import assert = require("assert");
+import { DartFrogDaemon } from "../../../daemon";
+
+suite("DartFrogDaemon", () => {
+  test("instance retrieves a singleton", () => {
+    const dartFrogDaemon = DartFrogDaemon.instance;
+    const dartFrogDaemon2 = DartFrogDaemon.instance;
+
+    assert.equal(dartFrogDaemon, dartFrogDaemon2);
+  });
+});