Expose the backend via UIManager

Summary:
### Summary

Exposes the animation backend via UIManager by introducing two new methods: `unstable_setAnimationBackend` and `unstable_getAnimationBackend`. The `NativeAnimatedNodesManagerProvider` is updated to initialize the animation backend using the `useSharedAnimatedBackend` feature flag.

Reviewed By: zeyap

Differential Revision: D81137954

packages/react-native/ReactCommon/react/renderer/animated/NativeAnimatedNodesManagerProvider.cpp

@@ -66,6 +66,8 @@ NativeAnimatedNodesManagerProvider::getOrCreate(
         };
 
     if (ReactNativeFeatureFlags::useSharedAnimatedBackend()) {
+      // TODO: this should be initialized outside of animated, but for now it
+      // was convenient to do it here
       animationBackend_ = std::make_shared<AnimationBackend>(
           std::move(startOnRenderCallback_),
           std::move(stopOnRenderCallback_),
@@ -73,6 +75,8 @@ NativeAnimatedNodesManagerProvider::getOrCreate(
 
       nativeAnimatedNodesManager_ =
           std::make_shared<NativeAnimatedNodesManager>(animationBackend_);
+
+      uiManager->unstable_setAnimationBackend(animationBackend_);
     } else {
       nativeAnimatedNodesManager_ =
           std::make_shared<NativeAnimatedNodesManager>(

packages/react-native/ReactCommon/react/renderer/uimanager/UIManager.cpp

@@ -678,6 +678,15 @@ void UIManager::setNativeAnimatedDelegate(
   nativeAnimatedDelegate_ = delegate;
 }
 
+void UIManager::unstable_setAnimationBackend(
+    std::weak_ptr<AnimationBackend> animationBackend) {
+  animationBackend_ = animationBackend;
+}
+
+std::weak_ptr<AnimationBackend> UIManager::unstable_getAnimationBackend() {
+  return animationBackend_;
+}
+
 void UIManager::animationTick() const {
   if (animationDelegate_ != nullptr &&
       animationDelegate_->shouldAnimateFrame()) {

packages/react-native/ReactCommon/react/renderer/uimanager/UIManager.h

@@ -36,6 +36,7 @@ namespace facebook::react {
 class UIManagerBinding;
 class UIManagerCommitHook;
 class UIManagerMountHook;
+class AnimationBackend;
 
 class UIManager final : public ShadowTreeDelegate {
  public:
@@ -62,6 +63,9 @@ class UIManager final : public ShadowTreeDelegate {
    * the pointer before being destroyed.
    */
   void setAnimationDelegate(UIManagerAnimationDelegate* delegate);
+  void unstable_setAnimationBackend(
+      std::weak_ptr<AnimationBackend> animationBackend);
+  std::weak_ptr<AnimationBackend> unstable_getAnimationBackend();
 
   /**
    * Execute stopSurface on any UIMAnagerAnimationDelegate.
@@ -258,6 +262,8 @@ class UIManager final : public ShadowTreeDelegate {
 
   std::unique_ptr<LazyShadowTreeRevisionConsistencyManager>
       lazyShadowTreeRevisionConsistencyManager_;
+
+  std::weak_ptr<AnimationBackend> animationBackend_;
 };
 
 } // namespace facebook::react