Long delay before view controller appears when using GCD and NSNotificationCenter - ios

Question

Having odd behavior that I just can't figure out - when I'm pushing my second view controller onto a UINavigationController, there is a 5-10 second delay before the new view appears. The second view controller is being pushed based on an NSNotificationCenter notification.
The oddness: the delay only occurs when the event is posted from a background thread and not on the main thread (see comments in sample code below).
Why is there such a delay? This occurs on both simulator and device.
Here's the console output. As you can see, all logs appear in a timely manner, just not the 2nd view controller which suddenly appears after about 5-10 seconds.
2015-04-27 06:58:47.973 DelayedViews[49845:5005806] Background thread started
2015-04-27 06:58:49.978 DelayedViews[49845:5005806] Pushing second view controller
2015-04-27 06:58:49.979 DelayedViews[49845:5005806] Done pushing second view controller
2015-04-27 06:58:49.980 DelayedViews[49845:5005806] In completion block
2015-04-27 06:58:49.980 DelayedViews[49845:5005783] Back on main thread
And here's my simple example AppDelegate.m which demonstrates the issue.
#import "AppDelegate.h"
#interface AppDelegate ()
#property (nonatomic, strong) UINavigationController *navController;
#end
#implementation AppDelegate
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
self.window = [[UIWindow alloc] initWithFrame: [[UIScreen mainScreen] bounds]];
self.navController = [[UINavigationController alloc] init];
self.window.rootViewController = self.navController;
UIViewController *vc = self.viewControllerOne;
[self.navController pushViewController:vc animated:YES];
// register for notification center
[[NSNotificationCenter defaultCenter] addObserver:self selector:#selector(pushSecondViewController) name:#"PushSecondVC" object:nil];
// start background thread that posts notification and calls completion block
void (^completion)() = ^void() {
NSLog(#"In completion block");
dispatch_async(dispatch_get_main_queue(), ^{
NSLog(#"Back on main thread");
// If I do the notification here, the 2nd view controller is pushed immediately
// [[NSNotificationCenter defaultCenter] postNotificationName:#"PushSecondVC" object:self];
});
};
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
NSLog(#"Background thread started");
[NSThread sleepForTimeInterval:2]; // simulate some work
// if the notification is posted here, there is a 5-10 second delay on simulator before second VC appears
// even though the 2nd VC was pushed onto navigationcontroller.
[[NSNotificationCenter defaultCenter] postNotificationName:#"PushSecondVC" object:self];
// call completion block now
completion();
});
self.window.backgroundColor = [UIColor whiteColor];
[self.window makeKeyAndVisible];
return YES;
}
- (UIViewController*)viewControllerOne {
UIViewController *vc = [[UIViewController alloc] init];
UILabel *label = [[UILabel alloc] initWithFrame:CGRectMake(8, 100, 100, 100)];
label.text = #"View #1";
[vc.view addSubview:label];
return vc;
}
- (UIViewController*)viewControllerTwo {
UIViewController *vc = [[UIViewController alloc] init];
UILabel *label = [[UILabel alloc] initWithFrame:CGRectMake(50, 200, 100, 100)];
label.text = #"View #2";
[vc.view addSubview:label];
return vc;
}
- (void)pushSecondViewController {
NSLog(#"Pushing second view controller");
UIViewController *vc = self.viewControllerTwo;
[self.navController pushViewController:vc animated:YES];
NSLog(#"Done pushing second view controller");
}
#end

Answer 1

Thanks, A-Live, that appears to be it.
I changed the pushSecondViewController to the following and it seems to work perfectly now. I wonder what is going on under-the-covers and why it takes so long for the view to appear?
- (void)pushSecondViewController {
if (![[NSThread currentThread] isMainThread]) {
dispatch_sync(dispatch_get_main_queue(), ^{
[self pushSecondViewController];
});
return;
}
NSLog(#"Pushing second view controller");
UIViewController *vc = self.viewControllerTwo;
[self.navController pushViewController:vc animated:NO];
NSLog(#"Done pushing second view controller");
}