Hey there, let's dive into fixing these two pesky iOS problems you're dealing with. I've helped a lot of devs work through similar bugs, so let's break this down step by step.

1. Fixing the "Received CA callback for state, but active state queue is empty" Error #

This error usually pops up when Core Animation (CA) tries to trigger a state callback, but the animation queue it expects is already empty or invalid. Here are actionable fixes:

• Check animation lifecycle & avoid memory leaks
  Make sure you're not holding strong references to view controllers/layers in animation closures. Use weak self to prevent objects from sticking around after they should be deallocated:

  UIView.animate(withDuration: 0.3) { [weak self] in
      self?.someView.alpha = 0.0
  } completion: { [weak self] _ in
      self?.someView.removeFromSuperview()
  }
  

  Also, clean up animations when views are dismissed or deallocated—add this to your view controller's viewWillDisappear or deinit:

  override func viewWillDisappear(_ animated: Bool) {
      super.viewWillDisappear(animated)
      view.layer.removeAllAnimations()
  }
  

• Run all Core Animation operations on the main thread
  CA requires all animation-related work to happen on the main thread. If you're triggering animations from a background queue, wrap them in DispatchQueue.main.async:

  DispatchQueue.global().async {
      // Do background work
      DispatchQueue.main.async {
          // Trigger your animation here
      }
  }
  

• Debug third-party animation libraries
  If you're using a third-party library for animations, try updating to the latest version first. If the error persists, temporarily remove the library and see if the issue goes away—sometimes libraries have unhandled edge cases with CA queues.

2. Fixing UIPageViewController Crash Linked to the CA Error #

UIPageViewController relies heavily on Core Animation for page transitions, so the CA queue issue often triggers crashes here. Here's how to fix it:

• Ensure consistent data source methods
  Double-check your pageViewController(_:viewControllerBefore:) and pageViewController(_:viewControllerAfter:) methods. Make sure they return valid view controllers (no unexpected nil values) and don't reuse or return duplicate instances incorrectly. Inconsistent data can throw off the internal animation state of UIPageViewController.

• Prevent concurrent data source modifications
  If you're updating your page data source (like adding/removing view controllers) while the user is swiping, it can mess up the animation queue. Add a flag to block modifications during transitions:

  private var isPageTransitioning = false
  
  func pageViewController(_ pageViewController: UIPageViewController, willTransitionTo pendingViewControllers: [UIViewController]) {
      isPageTransitioning = true
  }
  
  func pageViewController(_ pageViewController: UIPageViewController, didFinishAnimating finished: Bool, previousViewControllers: [UIViewController], transitionCompleted completed: Bool) {
      isPageTransitioning = false
  }
  
  // When updating data source:
  func updatePageData() {
      guard !isPageTransitioning else { return }
      // Update your data array here
  }
  

• Validate custom transition animations
  If you're using a custom transition for UIPageViewController, make sure your UIViewControllerAnimatedTransitioning implementation properly cleans up after animations. Implement the animationEnded(_:) method to ensure all CA states are resolved:

  func animationEnded(_ transitionCompleted: Bool) {
      // Clean up any animation-related resources here
      // Ensure no leftover CA callbacks are pending
  }
  

• Fix memory leaks in page view controllers
  Use Xcode's Memory Graph Debugger to check for circular references in your page view controllers. Common culprits are strong references in closures or delegates. Make sure your UIPageViewController delegate is set to weak if it's a view controller:

  weak var pageDelegate: UIPageViewControllerDelegate?
  

Quick Debugging Tips #

• When the error occurs, check the Xcode call stack to see which animation or view is triggering the CA callback. This can narrow down the exact source of the problem.
• Use the Core Animation instrument in Xcode to track active animations and see if any are lingering after their parent views are deallocated.

内容的提问来源于stack exchange，提问作者sea