Export ReactNativeTester (#48024)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/48024

Changelog: [internal]

Reviewed By: rshest

Differential Revision: D66599071

fbshipit-source-id: 52585e030642e5b6bd8921d2c9f9d14ee5d6ce71

packages/react-native/src/private/__tests__/ReactNativeTester.js

@@ -0,0 +1,71 @@
+/**
+ * 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
+ */
+
+import type {MixedElement} from 'react';
+
+import ReactFabric from '../../../Libraries/Renderer/shims/ReactFabric';
+
+let globalSurfaceIdCounter = 1;
+
+const nativeRuntimeScheduler = global.nativeRuntimeScheduler;
+const schedulerPriorityImmediate =
+  nativeRuntimeScheduler.unstable_ImmediatePriority;
+
+class Root {
+  #surfaceId: number;
+  #hasRendered: boolean = false;
+
+  constructor() {
+    this.#surfaceId = globalSurfaceIdCounter;
+    globalSurfaceIdCounter += 10;
+  }
+
+  render(element: MixedElement) {
+    if (!this.#hasRendered) {
+      global.$$JSTesterModuleName$$.startSurface(this.#surfaceId);
+      this.#hasRendered = true;
+    }
+
+    ReactFabric.render(element, this.#surfaceId, () => {}, true);
+  }
+
+  getMountingLogs(): Array<string> {
+    return global.$$JSTesterModuleName$$.getMountingManagerLogs(
+      this.#surfaceId,
+    );
+  }
+
+  destroy() {
+    // TODO: check for leaks.
+    global.$$JSTesterModuleName$$.stopSurface(this.#surfaceId);
+    global.$$JSTesterModuleName$$.flushMessageQueue();
+  }
+
+  // TODO: add an API to check if all surfaces were deallocated when tests are finished.
+}
+
+/*
+ * Runs a task on on the event loop. To be used together with root.render.
+ *
+ * React must run inside of event loop to ensure scheduling environment is closer to production.
+ */
+export function runTask(task: () => void) {
+  nativeRuntimeScheduler.unstable_scheduleCallback(
+    schedulerPriorityImmediate,
+    task,
+  );
+  global.$$JSTesterModuleName$$.flushMessageQueue();
+}
+
+// TODO: Add option to define surface props and pass it to startSurface
+// Surfacep rops: concurrentRoot, surfaceWidth, surfaceHeight, layoutDirection, pointScaleFactor.
+export function createRoot(): Root {
+  return new Root();
+}