DebuggerSessionObserver: JavaScript API (#46043)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/46043

# Changelog: [Internal]

Defined JavaScript interface, which can be used by other modules, such as LogBox, which will be migrated in the next diff

Reviewed By: robhogan

Differential Revision: D61301333

fbshipit-source-id: 63bb8581b893d0fdcb36e1fa16d243f1a5b08ac4

packages/react-native/src/private/fusebox/FuseboxSessionObserver.js

@@ -0,0 +1,42 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @flow strict
+ * @format
+ * @oncall react_native
+ */
+
+class FuseboxSessionObserver {
+  #hasNativeSupport: boolean;
+
+  constructor() {
+    this.#hasNativeSupport = global.hasOwnProperty(
+      '__DEBUGGER_SESSION_OBSERVER__',
+    );
+  }
+
+  hasActiveSession(): boolean {
+    if (!this.#hasNativeSupport) {
+      return false;
+    }
+
+    return global.__DEBUGGER_SESSION_OBSERVER__.hasActiveSession;
+  }
+
+  subscribe(callback: (status: boolean) => void): () => void {
+    if (!this.#hasNativeSupport) {
+      return () => {};
+    }
+
+    global.__DEBUGGER_SESSION_OBSERVER__.subscribers.add(callback);
+    return () => {
+      global.__DEBUGGER_SESSION_OBSERVER__.subscribers.delete(callback);
+    };
+  }
+}
+
+const observerInstance: FuseboxSessionObserver = new FuseboxSessionObserver();
+export default observerInstance;