Move MountingManagerTest to open-source

Summary:
Noticed this was not shared with open-source. Needs to be converted to Kotlin still.

Changelog: [Internal]

Differential Revision: D58088583

packages/react-native/ReactAndroid/src/test/java/com/facebook/react/fabric/MountingManagerTest.java

@@ -0,0 +1,84 @@
+// (c) Meta Platforms, Inc. and affiliates. Confidential and proprietary.
+
+package com.facebook.fbreact.fabric;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import com.facebook.react.ReactRootView;
+import com.facebook.react.bridge.BridgeReactContext;
+import com.facebook.react.bridge.ReactTestHelper;
+import com.facebook.react.fabric.mounting.MountingManager;
+import com.facebook.react.fabric.mounting.mountitems.MountItem;
+import com.facebook.react.internal.featureflags.ReactNativeFeatureFlagsForTests;
+import com.facebook.react.uimanager.IllegalViewOperationException;
+import com.facebook.react.uimanager.ThemedReactContext;
+import com.facebook.react.uimanager.ViewManager;
+import com.facebook.react.uimanager.ViewManagerRegistry;
+import com.facebook.react.views.view.ReactViewManager;
+import java.util.Arrays;
+import java.util.List;
+import java.util.Queue;
+import org.junit.Before;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.robolectric.RobolectricTestRunner;
+import org.robolectric.RuntimeEnvironment;
+
+/** Tests {@link FabricUIManager} */
+@RunWith(RobolectricTestRunner.class)
+public class MountingManagerTest {
+
+  private MountingManager mMountingManager;
+  private MountingManager.MountItemExecutor mMountItemExecutor;
+  private ThemedReactContext mThemedReactContext;
+  private int mNextRootTag = 1;
+
+  @Before
+  public void setUp() {
+    ReactNativeFeatureFlagsForTests.INSTANCE.setUp();
+
+    BridgeReactContext reactContext = new BridgeReactContext(RuntimeEnvironment.application);
+    reactContext.initializeWithInstance(ReactTestHelper.createMockCatalystInstance());
+    mThemedReactContext = new ThemedReactContext(reactContext, reactContext);
+    List<ViewManager> viewManagers = Arrays.<ViewManager>asList(new ReactViewManager());
+    mMountItemExecutor =
+        new MountingManager.MountItemExecutor() {
+          @Override
+          public void executeItems(Queue<MountItem> items) {
+            // no-op
+          }
+        };
+    mMountingManager =
+        new MountingManager(new ViewManagerRegistry(viewManagers), mMountItemExecutor);
+  }
+
+  @Test
+  public void addRootView() {
+    ReactRootView reactRootView = new ReactRootView(mThemedReactContext);
+    int rootReactTag = mNextRootTag++;
+    mMountingManager.startSurface(rootReactTag, mThemedReactContext, reactRootView);
+    assertThat(reactRootView.getId()).isEqualTo(rootReactTag);
+  }
+
+  @Test
+  public void unableToAddRootViewTwice() {
+    ReactRootView reactRootView = new ReactRootView(mThemedReactContext);
+    int rootReactTag = mNextRootTag++;
+    mMountingManager.startSurface(rootReactTag, mThemedReactContext, reactRootView);
+    assertThat(reactRootView.getId()).isEqualTo(rootReactTag);
+
+    // This is now a SoftException because it indicates a race condition in starting
+    // a single surface with a single View, and is concerning but not necessarily fatal.
+    // To be clear: in this case we're still guaranteed a single SurfaceMountingManager
+    // and therefore a single View involved.
+    mMountingManager.startSurface(rootReactTag, mThemedReactContext, reactRootView);
+  }
+
+  @Test(expected = IllegalViewOperationException.class)
+  public void unableToAddHandledRootView() {
+    ReactRootView reactRootView = new ReactRootView(mThemedReactContext);
+    reactRootView.setId(1234567);
+    int rootReactTag = mNextRootTag++;
+    mMountingManager.startSurface(rootReactTag, mThemedReactContext, reactRootView);
+  }
+}