Ios swift – iOS GameCenter crash in private API `GKNotificationBannerViewController::hideBannerQuickly`

I found this crash log in Firebase:

Crashed: com.apple.main-thread
0  libdispatch.dylib              0x4b20 dispatch_semaphore_signal + 8
1  GameCenterUI                   0x9ebd8 __56-[GKNotificationBannerViewController hideBannerQuickly:]_block_invoke_2 + 40
2  libdispatch.dylib              0x3f88 _dispatch_client_callout + 20
3  libdispatch.dylib              0x7418 _dispatch_continuation_pop + 504
4  libdispatch.dylib              0x1aa58 _dispatch_source_invoke + 1588
5  libdispatch.dylib              0x12748 _dispatch_main_queue_drain + 756
6  libdispatch.dylib              0x12444 _dispatch_main_queue_callback_4CF + 44
7  CoreFoundation                 0x9a6c8 __CFRUNLOOP_IS_SERVICING_THE_MAIN_DISPATCH_QUEUE__ + 16
8  CoreFoundation                 0x7c02c __CFRunLoopRun + 2036
9  CoreFoundation                 0x80eb0 CFRunLoopRunSpecific + 612
10 GraphicsServices               0x1368 GSEventRunModal + 164
11 UIKitCore                      0x3a1668 -[UIApplication _run] + 888
12 UIKitCore                      0x3a12cc UIApplicationMain + 340
13 libswiftUIKit.dylib            0x35308 UIApplicationMain(_:_:_:_:) + 104
14 BinMinesweeper                 0x7050 main + 4345163856 (AppSceneDelegate.swift:4345163856)
15 ???                            0x1e6d6c960 (Missing)

The above example came from iOS 16.3.1, iPhone XR. I can’t reproduce this crash. It happens pretty rarely (less than 1% of users).

I assume GKNotificationBannerViewController is the top banner "Welcome user_abc" presented when you launch the app.

The only time I interact with Game Center is when user click a leaderboard button, I show the game center VC. Here are some code:

import GameKit

public enum GameCenterUtil {
  
  private static var g_isEnabled: Bool = false
  
  public static func setupIfNeeded() {
    GKLocalPlayer.local.authenticateHandler = { loginVC, error in
      if loginVC != nil {
        // Nothing. Do not present login. 
        // User generally don't use game center. It's annoying.
        return
      }
      if error != nil {
        g_isEnabled = false
        return
      }
      g_isEnabled = true
    }
  }
  
  public static func reportScore(_ score: Int, category: String) {
    guard g_isEnabled else { return }
    GKLeaderboard.submitScore(score, context: 0, player: GKLocalPlayer.local, leaderboardIDs: [category]) { error in
      // nothing
    }
  }
  
  public static func presentLeaderboard(in vc: UIViewController) {
    // when disabled, this will be an alert.
    let gameCenterVC = GKGameCenterViewController()
    gameCenterVC.gameCenterDelegate = LeaderboardDismisser.shared
    vc.present(gameCenterVC, animated: true, completion: nil)
  }
}

final class LeaderboardDismisser: NSObject, GKGameCenterControllerDelegate {
  static let shared = LeaderboardDismisser()
  func gameCenterViewControllerDidFinish(_ gameCenterViewController: GKGameCenterViewController) {
    // This is required. When user is not logged in, the alert prompts up. If we don't dismiss it, the game vc will be not responding. 
    gameCenterViewController.dismiss(animated: true, completion: nil)
  }
}

Then in didFinishLaunching, i call GameCenterUtil.setupIfNeeded(), and in leaderboard button callback, I call GameCenterUtil.presentLeaderboard(in: vc).

Edit:
There is also a very similar crash, also unable to repro and happens rarely:

Crashed: com.apple.GameKit.banner
0  libdispatch.dylib              0x4a60 dispatch_semaphore_wait + 8
1  GameCenterUI                   0x9abe4 __42+[GKNotificationBannerWindow enqueBanner:]_block_invoke_2 + 60
2  libdispatch.dylib              0x2320 _dispatch_call_block_and_release + 32
3  libdispatch.dylib              0x3eac _dispatch_client_callout + 20
4  libdispatch.dylib              0xb534 _dispatch_lane_serial_drain + 668
5  libdispatch.dylib              0xc0d8 _dispatch_lane_invoke + 436
6  libdispatch.dylib              0x16cdc _dispatch_workloop_worker_thread + 648
7  libsystem_pthread.dylib        0xddc _pthread_wqthread + 288
8  libsystem_pthread.dylib        0xb7c start_wqthread + 8

EDIT 2:

This is how I present/dismiss the game VC:

In button click, I simply call the present function:

    let button = MySpriteButton(texture: GAME_TEXTURE(.leaderboardBig))
    {
      PLAY_APP_SOUND(.click)
      GameCenterUtil.presentLeaderboard(in: vc)
    }

Then I auto dismiss it when user enter background. This is a workaround for a bug earlier (refer to the comment in the code):

// MySceneDelegate.swift
open func sceneDidEnterBackground(_ scene: UIScene) {

   // When we background the app with game center VC presented, 
   // Then a while later when we foreground the app, sometimes the game center VC becomes transparent for some reason, 
   // and covers the whole screen, making whole app not interactive
   // This could be a bug in game center
   // A naive solution is simply dismiss the game center VC when entering background.
   if
     let topVC = NAV_VC?.topViewController,
     let gameCenterVC = topVC.presentedViewController as? GKGameCenterViewController
   {
     gameCenterVC.dismiss(animated: false, completion: nil)
   }
}

Now I have a feeling that this workaround may be the reason. Because the doc (https://developer.apple.com/documentation/gamekit/gknotificationbanner) says:

If the game is in the foreground, the banner appears immediately. If the game is in the background, the banner appears when the game becomes active.

This sounds like a queue structure. And one of my crash log has GKNotificationBannerWindow enqueBanner:. The 2 sound related.

I think I still want to keep this workaround, but not sure how it caused the crash. I have tried to delay the dismiss for 1 seconds in my latest release, but it doesn’t fix the issue:

  // A helper function
  func fetchTopGameCenterVC() -> GKGameCenterViewController? {
    if
      let topVC = NAV_VC?.topViewController,
      let gameCenterVC = topVC.presentedViewController as? GKGameCenterViewController
    {
      return gameCenterVC
    }
    return nil
  }

  
// MySceneDelegate.swift

  open func sceneDidEnterBackground(_ scene: UIScene) {
    if let vcBeforeDelay = fetchTopGameCenterVC() {
      DispatchQueue.main.asyncAfter(deadline: .now() + 1) {
        if let vcAfterDelay = fetchTopGameCenterVC(),
           vcAfterDelay === vcBeforeDelay
        {
          vcAfterDelay.dismiss(animated: false, completion: nil)
        }
      }
    }
  }