Animation Backend docs (#55488)

Summary:
Pull Request resolved: #55488

This diff introduces docs for the Animation Backend

# Changelog
[General][Added] - AnimationBackend docs

Reviewed By: NickGerleman

Differential Revision: D92826716

fbshipit-source-id: f3f48f66d85c6a586a520f8015a0f988decc5885

Author: Bartlomiej Bloniarz

__docs__/README.md

@@ -49,6 +49,7 @@ TODO: Explain the different components of React Native at a high level.
         - [passChildrenWhenCloningPersistedNodes](../packages/react-native/ReactCommon/react/renderer/core/__docs__/passChildrenWhenCloning.md)
       - Layout
       - Mounting
+      - [Animation Backend](../packages/react-native/ReactCommon/react/renderer/animationbackend/__docs__/AnimationBackend.md)
     - Native Modules / TurboModules
     - JS Runtime
       - [Event Loop](../packages/react-native/ReactCommon/react/renderer/runtimescheduler/__docs__/README.md)

packages/react-native/ReactCommon/react/renderer/animationbackend/__docs__/AnimationBackend.md

@@ -0,0 +1,88 @@
+# Shared Animation Backend
+
+[🏠 Home](../../../../../../../__docs__/README.md)
+
+Shared Animation Backend is a part of the React Native renderer that enables
+animation frameworks to update props of React components without going through
+React's JavaScript rendering pipeline.
+
+Animation Backend allows for updates of both layout and non-layout props. If
+there are no layout updates, the animations will go through the
+`synchronouslyUpdateProps` path, otherwise a Fabric commit will be performed. To
+synchronize the changes with React, we use the `AnimationBackendCommitHook`.
+
+## 🚀 Usage
+
+The backend is not meant to be used directly. Its main purpose is to serve as a
+layer over the Fabric renderer that animation frameworks (like Animated or
+Reanimated) can use. A framework that wants to make use of the backend has to
+register a callback with the `start` method. The callback will be executed on
+each animation frame (supported by native mechanisms like
+DisplayLink/Choreographer) and should return a list of prop updates that should
+be applied for the given timestamp. To stop receiving callbacks the framework
+should call `stop` with the appropriate callbackID. For updates that need to be
+applied synchronously (e.g. gesture events), one can use the `trigger` method.
+
+## 📐 Design
+
+![Animation Backend](./AnimationBackend.svg)
+
+The main purpose of the Animation Backend is to bring long-term stability to
+animation solutions. The New Architecture was designed to maintain React props
+in check with the `ShadowTree`. This poses a problem for any scenario when we
+want to apply updates to the `ShadowTree` without going through the React
+rendering pipeline, which is exactly what animations want to do. Any commit that
+happens outside of React on a different thread will be overwritten by following
+React updates, as the new `ShadowTree` revision created by React is based on the
+revision that is currently held by React. To mitigate that issue, we use the
+`AnimationBackendCommitHook` that runs on every React commit and fixes
+overridden props. In this section, we give a more detailed description of the
+components that make up the Shared Animation Backend.
+
+### AnimationBackend
+
+This is the heart of the backend. This component is responsible for managing
+user-provided callbacks, segregating props, and applying them either through the
+`synchronouslyUpdateProps` path (when there are no layout updates), or through a
+Fabric commit performed on the main thread. This component is exposed through
+`UIManager` with the `UIManagerAnimationBackend` interface.
+
+### AnimatedProps
+
+This is the structure that should be returned by the callback registered with
+the `start` method. It represents updates that are to be applied for a single
+component identified by `ShadowNodeFamily` in `AnimationMutation`. You shouldn't
+interact with this class directly, it's better to use it through the
+`PropsBuilder`. `AnimatedProps` support the `RawProps` format, but they also can
+hold a vector of `AnimatedProp` (a new C++ structure) for a select subset of the
+usual `ViewProps`. These can be applied to the cloned props in a more efficient
+way.
+
+### AnimationBackendCommitHook
+
+This component is responsible for keeping animation updates synchronized with
+React updates. In Fabric, when React performs a commit, it disregards the
+currently mounted revision of the `ShadowTree`. Instead, it bases the new
+revision on the `ShadowNodes` it holds. These nodes do not represent any prop
+updates that happened when the Animation Backend made a commit on the main
+thread (or applied non-layout props through the fast-path). This would create an
+issue where a rerender that happens during the animation would display a wrong
+state for a split second. To fix this, we also write animation updates to
+`AnimatedPropsRegistry`. When React performs a commit, the
+`AnimationBackendCommitHook` is able to overwrite any stale props by using the
+value from the registry. Since the commit hook runs on React commits, it is safe
+for it to use RSNRU to propagate these updates to React. After the commit hook
+runs, we clear the registry. If the prop is not explicitly changed by the user
+in a render, it will remain unchanged by React on subsequent rerenders.
+
+Additionally, in the callback registered in `start`, one can return a list of
+surfaces that should perform a commit on the JS thread, to have the RSNRU
+propagate the updates to React. This was used by Animated before the backend was
+created.
+
+### AnimationChoreographer
+
+This is an abstraction layer that wraps the native frame callback mechanisms. On
+iOS, it has a corresponding `RCTDisplayLink`, and on Android, it uses the
+`Choreographer`. For any new platform that wants to adopt the Animation Backend,
+this is the part that needs to be implemented.