setup global for fusebox runtime

packages/react-native/src/private/fusebox/.npmignore

@@ -0,0 +1 @@
+assets

packages/react-native/src/private/fusebox/README.md

@@ -0,0 +1,12 @@
+# Fusebox Runtime
+
+## What is Fusebox?
+"Fusebox" is the internal codename for the new React Native debugger stack based on Chrome DevTools.
+
+## Architecture for React DevTools communication
+
+### Frontend to Backend communication
+![Frontend to backend communication diagram](./assets/frontend-to-backend.excalidraw-embedded.png)
+
+### Backend to Frontend communication
+![Backend to frontend communication diagram](./assets/backend-to-frontend.excalidraw-embedded.png)

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

@@ -0,0 +1,108 @@
+/**
+ * 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-local
+ * @format
+ * @oncall react_native
+ */
+
+type JSONValue =
+  | string
+  | number
+  | boolean
+  | null
+  | {[key: string]: JSONValue}
+  | Array<JSONValue>;
+type DomainName = 'react-devtools';
+
+class EventScope<T> {
+  #listeners: Set<(T) => void> = new Set();
+
+  addEventListener(listener: T => void): void {
+    this.#listeners.add(listener);
+  }
+
+  removeEventListener(listener: T => void): void {
+    this.#listeners.delete(listener);
+  }
+
+  emit(value: T): void {
+    // Assuming that listeners won't throw.
+    for (const listener of this.#listeners) {
+      listener(value);
+    }
+  }
+}
+
+class Domain {
+  name: DomainName;
+  onMessage: EventScope<JSONValue>;
+
+  constructor(name: DomainName) {
+    if (global[FuseboxReactDevToolsDispatcher.BINDING_NAME] == null) {
+      throw new Error(
+        `Could not create domain ${name}: receiving end doesn't exist`,
+      );
+    }
+
+    this.name = name;
+    this.onMessage = new EventScope<JSONValue>();
+  }
+
+  sendMessage(message: JSONValue) {
+    const messageWithDomain = {domain: this.name, message};
+    const serializedMessageWithDomain = JSON.stringify(messageWithDomain);
+
+    global[FuseboxReactDevToolsDispatcher.BINDING_NAME](
+      serializedMessageWithDomain,
+    );
+  }
+}
+
+class FuseboxReactDevToolsDispatcher {
+  static #domainNameToDomainMap: Map<DomainName, Domain> = new Map();
+
+  // Referenced and initialized from Chrome DevTools frontend.
+  static BINDING_NAME: string = '__CHROME_DEVTOOLS_FRONTEND_BINDING__';
+  static onDomainInitialization: EventScope<Domain> = new EventScope<Domain>();
+
+  // Should be private, referenced from Chrome DevTools frontend only.
+  static initializeDomain(domainName: DomainName): Domain {
+    const domain = new Domain(domainName);
+
+    this.#domainNameToDomainMap.set(domainName, domain);
+    this.onDomainInitialization.emit(domain);
+
+    return domain;
+  }
+
+  // Should be private, referenced from Chrome DevTools frontend only.
+  static sendMessage(domainName: DomainName, message: string): void {
+    const domain = this.#domainNameToDomainMap.get(domainName);
+    if (domain == null) {
+      throw new Error(
+        `Could not send message to ${domainName}: domain doesn't exist`,
+      );
+    }
+
+    try {
+      const parsedMessage = JSON.parse(message);
+      domain.onMessage.emit(parsedMessage);
+    } catch (err) {
+      console.error(
+        `Error while trying to send a message to domain ${domainName}:`,
+        err,
+      );
+    }
+  }
+}
+
+Object.defineProperty(global, '__FUSEBOX_REACT_DEVTOOLS_DISPATCHER__', {
+  value: FuseboxReactDevToolsDispatcher,
+  configurable: false,
+  enumerable: false,
+  writable: false,
+});