UIAlertView takes a long time to appear when called in a completion block

问题

Part of my app requires calendar access, which requires a call to the EKEventStore method -(void)requestAccessToEntityType:(EKEntityType)entityType completion:(EKEventStoreRequestAccessCompletionHandler)completion as of iOS 7.

I added the request, and everything runs smoothly if the user selects to allow access, but a problem arises if the user denies or has previously denied access. I added a UIAlertView to notify the user if access is denied, but the UIAlertView consistently takes 20-30 seconds to appear, and completely disables the UI during that time. Debugging has shown that [alertView show] runs before the delay, even though it doesn't actually show until after the delay.

Why is this delay happening and how can I remove it?

[eventStore requestAccessToEntityType:EKEntityTypeEvent
                           completion:^(BOOL granted, NSError *error) {
                               if (granted) {
                                   [self createCalendarEvent];
                               } else {
                                   UIAlertView *alertView = [[UIAlertView alloc] initWithTitle:@"Calendar Access Denied"
                                                                                       message:@"Please enable access in Privacy Settings to use this feature."
                                                                                      delegate:nil
                                                                             cancelButtonTitle:@"OK"
                                                                             otherButtonTitles:nil];
                                   [alertView show];
                               }
                           }];

回答1:

[alertView show] is not thread safe, so it is adding its UI change to the queue from which the completion block was dispatched rather than the main queue. I resolved this issue by adding dispatch_async(dispatch_get_main_queue(), ^{}); around the code inside the completion block:

[eventStore requestAccessToEntityType:EKEntityTypeEvent
                           completion:^(BOOL granted, NSError *error) {
                               dispatch_async(dispatch_get_main_queue(), ^{
                                   if (granted) {
                                       [self createCalendarEvent];
                                   } else {
                                       UIAlertView *alertView = [[UIAlertView alloc] initWithTitle:@"Calendar Access Denied"
                                                                                           message:@"Please enable access in Privacy Settings to use this feature."
                                                                                          delegate:nil
                                                                                 cancelButtonTitle:@"OK"
                                                                                 otherButtonTitles:nil];
                                       [alertView show];
                                   }
                               });
                           }];

来源：https://stackoverflow.com/questions/22333584/uialertview-takes-a-long-time-to-appear-when-called-in-a-completion-block