Fabric: Simplifying the integration among RCTSurfacePresenter, RCTMountingManager, and RCTFabricSurface

Summary:
Changes:
* RCTMountingManager got two dedicated methods that designate some external view as a Surface viewport;
* The wiring between views now happens right in `start` and `stop` methods (because there is no reason to do it later); now it's symmetrical and straightforward;
* A couple of Surface legacy statuses were removed; they are not used and do not convey useful information anyway.
Changelog: [Internal] Fabric-specific internal change.

Reviewed By: sammy-SC

Differential Revision: D26095434

fbshipit-source-id: 37828e093a40a1ad22ad78f110c18a8f27781ba7

React/Base/Surface/RCTSurfaceStage.h

@@ -23,17 +23,12 @@ typedef NS_OPTIONS(NSInteger, RCTSurfaceStage) {
   RCTSurfaceStageSurfaceDidStop = 1 << 7, // Surface stopped
 
   // Most of the previously existed stages make no sense in the new architecture;
-  // now Surface exposes only three simple stages:
-  //
-  // Surface object was constructed and still valid.
-  RCTSurfaceStageInitialized = RCTSurfaceStageSurfaceDidInitialize,
-  // Surface was started.
-  RCTSurfaceStageStarted = 1 << 8,
-  // All off-main-thread work is done; we are ready to mount the UI.
-  RCTSurfaceStagePrepared = RCTSurfaceStageBridgeDidLoad | RCTSurfaceStageModuleDidLoad | RCTSurfaceStageSurfaceDidRun |
-      RCTSurfaceStageSurfaceDidInitialRendering | RCTSurfaceStageSurfaceDidInitialLayout,
-  // All main-thread work is done, the UI was mounted.
-  RCTSurfaceStageMounted = RCTSurfaceStageSurfaceDidInitialMounting,
+  // now Surface exposes only two simple stages:
+  RCTSurfaceStagePreparing = RCTSurfaceStageSurfaceDidInitialize | RCTSurfaceStageBridgeDidLoad |
+      RCTSurfaceStageModuleDidLoad,
+  RCTSurfaceStageRunning = RCTSurfaceStagePreparing | RCTSurfaceStageSurfaceDidRun |
+      RCTSurfaceStageSurfaceDidInitialRendering | RCTSurfaceStageSurfaceDidInitialLayout |
+      RCTSurfaceStageSurfaceDidInitialMounting,
 };
 
 /**

React/Fabric/Mounting/RCTMountingManager.h

@@ -26,6 +26,20 @@ NS_ASSUME_NONNULL_BEGIN
 @property (nonatomic, weak) id<RCTMountingManagerDelegate> delegate;
 @property (nonatomic, strong) RCTComponentViewRegistry *componentViewRegistry;
 
+/**
+ * Designates the view as a rendering viewport of a React Native surface.
+ * The provided view must not have any subviews, and the caller is not supposed to interact with the view hierarchy
+ * inside the provided view. The view hierarchy created by mounting infrastructure inside the provided view does not
+ * influence the intrinsic size of the view and cannot be measured using UIView/UIKit layout API.
+ * Must be called on the main thead.
+ */
+- (void)attachSurfaceToView:(UIView *)view surfaceId:(facebook::react::SurfaceId)surfaceId;
+
+/**
+ * Stops designating the view as a rendering viewport of a React Native surface.
+ */
+- (void)detachSurfaceFromView:(UIView *)view surfaceId:(facebook::react::SurfaceId)surfaceId;
+
 /**
  * Schedule a mounting transaction to be performed on the main thread.
  * Can be called from any thread.
@@ -46,7 +60,7 @@ NS_ASSUME_NONNULL_BEGIN
 
 - (void)synchronouslyUpdateViewOnUIThread:(ReactTag)reactTag
                              changedProps:(NSDictionary *)props
-                      componentDescriptor:(const facebook::react::ComponentDescriptor &)componentDescriptor;
+                      componentDescriptor:(facebook::react::ComponentDescriptor const &)componentDescriptor;
 @end
 
 NS_ASSUME_NONNULL_END

React/Fabric/Mounting/RCTMountingManager.mm

@@ -13,6 +13,7 @@
 #import <React/RCTFollyConvert.h>
 #import <React/RCTLog.h>
 #import <React/RCTUtils.h>
+#import <react/renderer/components/root/RootShadowNode.h>
 #import <react/renderer/core/LayoutableShadowNode.h>
 #import <react/renderer/core/RawProps.h>
 #import <react/renderer/debug/SystraceSection.h>
@@ -146,6 +147,29 @@ static void RCTPerformMountInstructions(
   return self;
 }
 
+- (void)attachSurfaceToView:(UIView *)view surfaceId:(SurfaceId)surfaceId
+{
+  RCTAssertMainQueue();
+
+  RCTAssert(view.subviews.count == 0, @"The view must not have any subviews.");
+
+  RCTComponentViewDescriptor rootViewDescriptor =
+      [_componentViewRegistry dequeueComponentViewWithComponentHandle:RootShadowNode::Handle() tag:surfaceId];
+  [view addSubview:rootViewDescriptor.view];
+}
+
+- (void)detachSurfaceFromView:(UIView *)view surfaceId:(SurfaceId)surfaceId
+{
+  RCTAssertMainQueue();
+  RCTComponentViewDescriptor rootViewDescriptor = [_componentViewRegistry componentViewDescriptorWithTag:surfaceId];
+
+  [rootViewDescriptor.view removeFromSuperview];
+
+  [_componentViewRegistry enqueueComponentViewWithComponentHandle:RootShadowNode::Handle()
+                                                              tag:surfaceId
+                                          componentViewDescriptor:rootViewDescriptor];
+}
+
 - (void)scheduleTransaction:(MountingCoordinator::Shared const &)mountingCoordinator
 {
   if (RCTIsMainQueue()) {

React/Fabric/RCTSurfacePresenter.mm

@@ -381,8 +381,7 @@ static BackgroundExecutor RCTGetBackgroundExecutor()
 {
   RCTMountingManager *mountingManager = _mountingManager;
   RCTExecuteOnMainQueue(^{
-    [mountingManager.componentViewRegistry dequeueComponentViewWithComponentHandle:RootShadowNode::Handle()
-                                                                               tag:surface.rootTag];
+    [mountingManager attachSurfaceToView:surface.view surfaceId:surface.rootTag];
   });
 
   LayoutContext layoutContext = RCTGetLayoutContext(surface.viewportOffset);
@@ -402,15 +401,8 @@ static BackgroundExecutor RCTGetBackgroundExecutor()
 
   RCTMountingManager *mountingManager = _mountingManager;
   RCTExecuteOnMainQueue(^{
-    surface.view.rootView = nil;
-    RCTComponentViewDescriptor rootViewDescriptor =
-        [mountingManager.componentViewRegistry componentViewDescriptorWithTag:surface.rootTag];
-    [mountingManager.componentViewRegistry enqueueComponentViewWithComponentHandle:RootShadowNode::Handle()
-                                                                               tag:surface.rootTag
-                                                           componentViewDescriptor:rootViewDescriptor];
+    [mountingManager detachSurfaceFromView:surface.view surfaceId:surface.rootTag];
   });
-
-  [surface _unsetStage:(RCTSurfaceStagePrepared | RCTSurfaceStageMounted)];
 }
 
 - (void)_startAllSurfacesWithScheduler:(RCTScheduler *)scheduler
@@ -452,10 +444,6 @@ static BackgroundExecutor RCTGetBackgroundExecutor()
 
 - (void)schedulerDidFinishTransaction:(MountingCoordinator::Shared const &)mountingCoordinator
 {
-  RCTFabricSurface *surface = [_surfaceRegistry surfaceForRootTag:mountingCoordinator->getSurfaceId()];
-
-  [surface _setStage:RCTSurfaceStagePrepared];
-
   [_mountingManager scheduleTransaction:mountingCoordinator];
 }
 
@@ -509,17 +497,6 @@ static BackgroundExecutor RCTGetBackgroundExecutor()
 {
   RCTAssertMainQueue();
 
-  RCTFabricSurface *surface = [_surfaceRegistry surfaceForRootTag:rootTag];
-  RCTSurfaceStage stage = surface.stage;
-  if (stage & RCTSurfaceStagePrepared) {
-    // We have to progress the stage only if the preparing phase is done.
-    if ([surface _setStage:RCTSurfaceStageMounted]) {
-      auto rootComponentViewDescriptor =
-          [_mountingManager.componentViewRegistry componentViewDescriptorWithTag:rootTag];
-      surface.view.rootView = (RCTSurfaceRootView *)rootComponentViewDescriptor.view;
-    }
-  }
-
   std::shared_lock<better::shared_mutex> lock(_observerListMutex);
   for (id<RCTSurfacePresenterObserver> observer in _observers) {
     if ([observer respondsToSelector:@selector(didMountComponentsWithRootTag:)]) {

React/Fabric/Surface/RCTFabricSurface.mm

@@ -73,17 +73,17 @@ using namespace facebook::react;
 
 - (BOOL)start
 {
-  if (![self _setStage:RCTSurfaceStageStarted]) {
+  if (![self _setStage:RCTSurfaceStageRunning]) {
     return NO;
   }
-  [_surfacePresenter registerSurface:self];
 
+  [_surfacePresenter registerSurface:self];
   return YES;
 }
 
 - (BOOL)stop
 {
-  if (![self _unsetStage:RCTSurfaceStageStarted]) {
+  if (![self _unsetStage:RCTSurfaceStageRunning]) {
     return NO;
   }