Construct the shared animation backend after the UIManager has a delegate (#57817)

[zeyap]

Summary:

Changelog:

[Internal] - Construct the shared animation backend after the UIManager has a delegate

AnimationBackend's constructor calls UIManager::addOnSurfaceStartCallback so that AnimatedPropsRegistry::initializeSurface runs for each surface as it starts. That call is a no-op unless the UIManager already has a delegate:

void UIManager::addOnSurfaceStartCallback(
    UIManagerDelegate::OnSurfaceStartCallback&& callback) {
  if (delegate_ != nullptr) {
    delegate_->uiManagerShouldAddOnSurfaceStartCallback(std::move(callback));
  }
}

Scheduler's constructor was building the backend immediately after constructing the UIManager and roughly seventy lines before uiManager->setDelegate(this), so the callback was dropped every time and Scheduler::onSurfaceStartCallbacks_ never received it. AnimatedPropsRegistry::update skips surfaces missing from surfaceContexts_, so correctness was left resting on getMap() default-constructing the entry via operator[] — that is, on a commit hook happening to run before the first animated update on a freshly started surface. When that ordering does not hold, early animated updates on a new surface are dropped.

Move the backend construction to just after setDelegate, and add a comment recording the ordering constraint. This is still well before any surface can start, so the registry is populated for every surface from the first frame.

The neighbouring getShadowTreeRegistry().enumerate(...) in the same constructor is also dead today — the UIManager is two lines old and its registry is necessarily empty — but it is left in place deliberately. It mirrors the enumerate-then-register pattern in NativeAnimatedNodesManagerProvider and ViewTransitionModule, both of which are constructed lazily and do need it, and it keeps AnimationBackend's constructor correct if it is ever moved to a lazy call site.

Reviewed By: christophpurrer

Differential Revision: D114759538

[meta-codesync]

@zeyap has exported this pull request. If you are a Meta employee, you can view the originating Diff in D114759538.

[meta-codesync]

This pull request has been merged in 471c612.