Pull model: test the dispatch order the pull path keeps

./gradlew :packages:react-native:ReactAndroid:testDebugUnitTest \
  --tests com.facebook.react.fabric.FabricUIManagerPullModelTest

These pass on main and pin what any scheduling of pulls must keep:
- pre-mount items (view preallocations) queued before the dispatch run
  before the pull;
- a pull runs the mount hooks (UIManagerListener.didMountItems and
  reportMount);
- with disableIdleMountItemFrameCallbackRearmAndroid, a surface that commits
  before its root view attaches lets the frame callback idle, the attach
  schedules a frame, and that frame mounts the commit after the
  preallocation queued for the attach.

The shadow binding gains reportMount and a no-op drainPreallocateViewsQueue
so the last test can run the frame callback.

Author: bartlomiejbloniarz

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

@@ -9,15 +9,21 @@
 
 package com.facebook.react.fabric
 
+import android.os.Looper
 import com.facebook.react.ReactRootView
 import com.facebook.react.bridge.ReactApplicationContext
 import com.facebook.react.bridge.ReactTestHelper
+import com.facebook.react.bridge.UIManager
+import com.facebook.react.bridge.UIManagerListener
+import com.facebook.react.common.annotations.UnstableReactNativeAPI
 import com.facebook.react.fabric.mounting.MountItemDispatcher
 import com.facebook.react.fabric.mounting.MountingManager
 import com.facebook.react.fabric.mounting.mountitems.MountItem
 import com.facebook.react.internal.featureflags.ReactNativeFeatureFlags
 import com.facebook.react.internal.featureflags.ReactNativeFeatureFlagsDefaults
 import com.facebook.react.internal.featureflags.ReactNativeFeatureFlagsForTests
+import com.facebook.react.modules.core.ReactChoreographer
+import com.facebook.react.uimanager.GuardedFrameCallback
 import com.facebook.react.uimanager.ThemedReactContext
 import com.facebook.react.uimanager.ViewManagerRegistry
 import com.facebook.testutils.fakes.FakeBatchEventDispatchedListener
@@ -30,7 +36,9 @@ import org.junit.After
 import org.junit.Before
 import org.junit.Test
 import org.junit.runner.RunWith
+import org.mockito.kotlin.mock
 import org.robolectric.RobolectricTestRunner
+import org.robolectric.Shadows.shadowOf
 import org.robolectric.annotation.Config
 import org.robolectric.shadow.api.Shadow
 import org.robolectric.util.ReflectionHelpers
@@ -44,6 +52,7 @@ import org.robolectric.util.ReflectionHelpers
  * queued mount items synchronously; notifications posted from a background thread stay queued until
  * the next UI-thread dispatch (in production, the dispatcher's frame callback).
  */
+@OptIn(UnstableReactNativeAPI::class)
 @RunWith(RobolectricTestRunner::class)
 @Config(
     shadows =
@@ -60,6 +69,7 @@ class FabricUIManagerPullModelTest {
   private lateinit var underTest: FabricUIManager
   private lateinit var binding: FabricUIManagerBinding
   private lateinit var shadowBinding: ShadowFabricUIManagerBinding
+  private var didMountItemsCount = 0
 
   @Before
   fun setup() {
@@ -79,6 +89,21 @@ class FabricUIManagerPullModelTest {
     binding = FabricUIManagerBinding()
     shadowBinding = Shadow.extract(binding)
     underTest.setBinding(binding)
+    underTest.addUIManagerEventListener(
+        object : UIManagerListener {
+          override fun willDispatchViewUpdates(uiManager: UIManager) = Unit
+
+          override fun willMountItems(uiManager: UIManager) = Unit
+
+          override fun didMountItems(uiManager: UIManager) {
+            didMountItemsCount++
+          }
+
+          override fun didDispatchMountItems(uiManager: UIManager) = Unit
+
+          override fun didScheduleMountItems(uiManager: UIManager) = Unit
+        }
+    )
   }
 
   @After
@@ -238,4 +263,79 @@ class FabricUIManagerPullModelTest {
 
     assertThat(mountedRevisions(appliedBatches)).containsExactly(1, 2, 3)
   }
+
+  @Test
+  fun preallocationQueuedBeforeDispatch_runsBeforeThePull() {
+    val log = mutableListOf<String>()
+    val surface = PulledSurface(11, log)
+    shadowBinding.onPull = { surface.pull() }
+
+    surface.commit()
+    runOnBackgroundThread {
+      dispatcher.addPreAllocateMountItem(recordingMountItem(11, "preallocate", log))
+    }
+    dispatcher.tryDispatchMountItems()
+
+    assertThat(log).containsExactly("preallocate", "0->1")
+  }
+
+  @Test
+  fun pull_runsMountHooks() {
+    val surface = PulledSurface(11, mutableListOf())
+    shadowBinding.onPull = { surface.pull() }
+
+    surface.commit()
+    dispatcher.tryDispatchMountItems()
+    shadowOf(Looper.getMainLooper()).idle()
+
+    assertThat(didMountItemsCount).isEqualTo(1)
+    assertThat(shadowBinding.reportedMountSurfaceIds).containsExactly(11)
+  }
+
+  @Test
+  fun pullWaitingForRootAttach_isMountedByTheFrameAfterTheAttach() {
+    // With the idle frame callback, nothing may keep frames running while the root is detached, so
+    // the attach must schedule the frame that mounts the surface.
+    ReactNativeFeatureFlags.dangerouslyReset()
+    ReactNativeFeatureFlags.override(
+        object : ReactNativeFeatureFlagsDefaults() {
+          override fun enableMountingCoordinatorPullModelAndroid(): Boolean = true
+
+          override fun disableIdleMountItemFrameCallbackRearmAndroid(): Boolean = true
+        },
+    )
+    val originalChoreographer = ReactChoreographer.overrideInstanceForTest(mock())
+    try {
+      val frameCallback: GuardedFrameCallback =
+          ReflectionHelpers.getField(underTest, "mDispatchUIFrameCallback")
+      ReflectionHelpers.setField(frameCallback, "mShouldSchedule", true)
+      fun isFrameScheduled(): Boolean = ReflectionHelpers.getField(frameCallback, "mIsScheduled")
+      val surfaceId = 12
+      val themedReactContext =
+          ThemedReactContext(reactContext, reactContext, "TestModule", surfaceId)
+      mountingManager.startSurface(surfaceId, themedReactContext, null)
+      val log = mutableListOf<String>()
+      val surface = PulledSurface(surfaceId, log)
+      shadowBinding.onPull = { surface.pull() }
+
+      surface.commit()
+      runOnBackgroundThread {
+        dispatcher.addPreAllocateMountItem(recordingMountItem(surfaceId, "preallocate", log))
+      }
+      frameCallback.doFrame(System.nanoTime())
+
+      assertThat(log).isEmpty()
+      assertThat(isFrameScheduled()).isFalse()
+
+      mountingManager.attachRootView(surfaceId, ReactRootView(reactContext), themedReactContext)
+
+      assertThat(isFrameScheduled()).isTrue()
+
+      frameCallback.doFrame(System.nanoTime())
+
+      assertThat(log).containsExactly("preallocate", "0->1")
+    } finally {
+      ReactChoreographer.overrideInstanceForTest(originalChoreographer)
+    }
+  }
 }

packages/react-native/ReactAndroid/src/test/java/com/facebook/testutils/shadows/ShadowFabricUIManagerBinding.kt

@@ -18,6 +18,7 @@ import org.robolectric.annotation.Implements
 @Implements(className = "com.facebook.react.fabric.FabricUIManagerBinding")
 class ShadowFabricUIManagerBinding {
   val pulledSurfaceIds: MutableList<Int> = mutableListOf()
+  val reportedMountSurfaceIds: MutableList<Int> = mutableListOf()
   /** Stands in for the C++ pull and mount. */
   var onPull: (surfaceId: Int) -> Unit = {}
 
@@ -26,4 +27,11 @@ class ShadowFabricUIManagerBinding {
     pulledSurfaceIds.add(surfaceId)
     onPull(surfaceId)
   }
+
+  @Implementation
+  fun reportMount(surfaceId: Int) {
+    reportedMountSurfaceIds.add(surfaceId)
+  }
+
+  @Implementation fun drainPreallocateViewsQueue() = Unit
 }