UINavigationBar setBackgroundImage sometimes doesn't take immediate effect - objective-c

Question

I'm resetting the NavigationBar's background image by calling setBackgroundImage when showing an MFMailComposeViewController. But when I try to reset the image to its original by calling setBackgroundImage a second time in mailComposeController:didFinishWithResult:, it doesn't visibly change the background image.
The weird thing is that the log shows me that the NavigationBar's background image has changed, and after I pop a few view controllers (I'm not sure which are modal transitions and which aren't, because it's somebody else's code), the background image finally does change. But why doesn't it change immediately?
Here's my relevant code inside mailComposeController:didFinishWithResult::
[[UINavigationBar appearance] setBackgroundImage:[UIImage imageNamed:#"img"]
forBarMetrics:UIBarMetricsDefault];
NSLog(#"Navbar img = %#", [[UINavigationBar appearance]
backgroundImageForBarMetrics:UIBarMetricsDefault]);

Answer 1

Another suggestion is to make sure that you are setting the background image after you've dismissed the mailComposeController and after your own view controller has become active again.

Answer 2

Are you sure that your -mailComposeController:didFinishWithResult: is being called on the main thread, where you need to be doing all of your ui work?
Try executing your -setBackgroundImage:forBarMetrics: call on the main thread and see if this helps.
For example:
dispatch_async(dispatch_get_main_queue(), ^{
[[UINavigationBar appearance] setBackgroundImage:[UIImage imageNamed:#"img"]
forBarMetrics:UIBarMetricsDefault];
});