According to About the App Launch Sequence, application:didFinishLaunchingWithOptions: won't be called during app prewarming on iOS 15. The Objective-C app works as documented. However, the Swift app behaves differently. application(_:didFinishLaunchingWithOptions:) is called during app prewarming. I believe this mismatch is an unintentional bug, and it brings a few issues. I can still observe this issue on iOS 15.2. Is there a plan to fix this bug?

Is there any threading assumption/requirement for ASIdentifierManager.advertisingIdentifier? Please see the complete issue and stack trace here. The main thread was waiting for the worker thread, which was blocked in ASIdentifierManager.advertisingIdentifier. Thread 68: 0 libsystem_kernel.dylib 0x188abf0f4 mach_msg_trap (in libsystem_kernel.dylib) + 8 1 libsystem_kernel.dylib 0x188abe5a0 mach_msg (in libsystem_kernel.dylib) + 72 2 libdispatch.dylib 0x188924880 _dispatch_mach_send_and_wait_for_reply (in libdispatch.dylib) + 500 3 libdispatch.dylib 0x188924d10 dispatch_mach_send_with_result_and_wait_for_reply$VARIANT$mp (in libdispatch.dylib) + 52 4 libxpc.dylib 0x188b8391c xpc_connection_send_message_with_reply_sync (in libxpc.dylib) + 204 5 Foundation 0x189aafa28 NSXPCCONNECTION_IS_WAITING_FOR_A_SYNCHRONOUS_REPLY (in Foundation) + 12 6 Foundation 0x189892f60 -[NSXPCConnection sendInvocation:orArguments:count:methodSignature:selector:withProxy:] (in Foundation) + 3608 7 CoreFoundation 0x188f3276c forwarding (in CoreFoundation) + 552 8 CoreFoundation 0x188f3475c forwarding_prep_0 (in CoreFoundation) + 92 9 CoreServices 0x1b1896ce4 -[LSApplicationWorkspace deviceIdentifierForAdvertising] (in CoreServices) + 160 10 AdSupport 0x198f70a60 -[ASIdentifierManager advertisingIdentifier] (in AdSupport) + 56 I don't see any thread related information in the document. I tried to recreate the scenario, but I could not reproduce the issue with the simplified test below. #import "ViewController.h" #import "AdSupport/AdSupport.h" #import "AppTrackingTransparency/AppTrackingTransparency.h" @interface ViewController () {  dispatch_queue_t _queue; } @end @implementation ViewController - (void)viewDidLoad {  [super viewDidLoad];  _queue = dispatch_queue_create("IdentityWorkerQueue", DISPATCH_QUEUE_SERIAL);  dispatch_set_target_queue(_queue,               dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0));  if (@available(iOS 14, *)) {   [ATTrackingManager requestTrackingAuthorizationWithCompletionHandler:    ^(ATTrackingManagerAuthorizationStatus status) {    NSLog(@"Status: %lu", (unsigned long)status);   }];  } } - (IBAction)action:(id)sender {  __block NSString *adId1 = nil;  dispatch_async(self->_queue, ^{   sleep(1);   adId1 = ASIdentifierManager.sharedManager.advertisingIdentifier.UUIDString;  });  __block NSString *adId2 = nil;  dispatch_sync(self->_queue, ^{   adId2 = adId1;  }); } @end Is there any threading assumption/requirement for ASIdentifierManager.advertisingIdentifier? For example, would it wait for the main thread to finish a certain task when an error happens?

I am testing AppTrackingTransparency - https://developer.apple.com/documentation/apptrackingtransparency framework. I was able to get a valid IDFA on an iOS 14 simulator by following steps in the doc. When I turned off "Settings > Privacy > Apple Advertising > Personalized Ads", the IDFA became 0s, which is expected. However, after I turned "Personalized Ads" back on, I still got zeroed-out IDFA. I tried several ways to reset the simulator, e.g. Device > Erase All Content and Settings…, but I cannot get it back to a working state. I cannot get a valid IDFA anymore. Environment: Xcode 12.0 beta (12A6159) macOS Catalina 10.15.5