Fabric: Introducing RCTMountingTransactionObserving protocol

Summary:
See the comment in RCTMountingTransactionObserving first.

I think we have to add this to the iOS mounting layer to be able reasonably easy implement things like:

* MovableNavigationBar: seems, currently we don't handle the situation when the container was mounted first and the nested scroll view second.
* TTI component: It does not have access to telemetry.

The protocol is meant to replace `RCTSurfacePresenterObserver`.

Changelog: [Internal] Fabric-specific change.

Reviewed By: mdvacca

Differential Revision: D16270107

fbshipit-source-id: 2d4bdb7d0092cc214cc433fc633e41e58f6677df

React/Fabric/Mounting/RCTMountingTransactionObserving.h

@@ -0,0 +1,64 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+#import <UIKit/UIKit.h>
+
+#import <react/mounting/MountingTransactionMetadata.h>
+
+NS_ASSUME_NONNULL_BEGIN
+
+/*
+ * # Achtung!
+ * Remember, with great power comes great responsibility.
+ * Observers of this protocol are being called several times on every single mount transaction. Any thoughtless or
+ * suboptimal implementation of this protocol will slow down the whole app. Please, be responsible.
+ *
+ * # Usecases
+ * React Native platform-specific mounting layer has limitations when it comes to notifying view components about
+ * (coming or just happened) changes in the view tree. Implementing that generically for all components would make
+ * everything way to slow. For instance, the mounting layer does not have dedicated APIs to notify some component that:
+ * - Some ancestor of the component was reparented;
+ * - Some descendant of the component was added, removed or reparented;
+ * - Some ancestor of the component got new layout metrics (which might affect the absolute position of the component);
+ * - The transaction which affected the component's children just finished.
+ *
+ * If some very specific component (e.g. a performance logger) needs to handle some of the similar use-cases, it might
+ * rely on this protocol.
+ *
+ * # How to use
+ * - Declare conformance to this protocol for the ComponentView class.
+ * - Implement methods *only* suitable for a particular use case. Do not implement all methods if it is not strictly
+ * required.
+ * - Alternatively, an observer can be registered explicitly via `RCTSurface`.
+ *
+ * # Implementation details
+ * The framework checks all registered view classes for conformance to the protocol and for a set of implemented
+ * methods, then it stores this information for future use. When a view got created, the framework checks the info
+ * associated with the class and adds the view object to the list of listeners of the particular events (if needed).
+ * When a view got destroyed, the framework removes the view from suitable collections.
+ */
+@protocol RCTMountingTransactionObserving <NSObject>
+
+@optional
+
+/*
+ * Called right before the fist mutation instruction is executed.
+ * Is not being called for a component view which is being mounted as part of the transaction (because the view is not
+ * registered as an observer yet).
+ */
+- (void)mountingTransactionWillMountWithMetadata:(facebook::react::MountingTransactionMetadata const &)metadata;
+
+/*
+ * Called right after the last mutation instruction is executed.
+ * Is not being called for a component view which was being unmounted as part of the transaction (because the view is
+ * not registered as an observer already).
+ */
+- (void)mountingTransactionDidMountWithMetadata:(facebook::react::MountingTransactionMetadata const &)metadata;
+
+@end
+
+NS_ASSUME_NONNULL_END

ReactCommon/fabric/mounting/MountingTransactionMetadata.cpp

@@ -0,0 +1,12 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+#include "MountingTransactionMetadata.h"
+
+namespace facebook {
+namespace react {} // namespace react
+} // namespace facebook

ReactCommon/fabric/mounting/MountingTransactionMetadata.h

@@ -0,0 +1,31 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+#pragma once
+
+#include <react/mounting/MountingTelemetry.h>
+#include <react/mounting/MountingTransaction.h>
+
+namespace facebook {
+namespace react {
+
+/*
+ * Contains all (meta)information related to a MountingTransaction except a list
+ * of mutation instructions.
+ * The class is meant to be used when a cosumer should not have access to all
+ * information about the transaction (incapsulation) but still needs to observe
+ * it to produce some side-effects.
+ */
+class MountingTransactionMetadata final {
+ public:
+  SurfaceId surfaceId;
+  MountingTransaction::Number number;
+  MountingTelemetry telemetry;
+};
+
+} // namespace react
+} // namespace facebook