iOS screen become black little by little - ios

Question

I'm implementing an iOS app. and found that sometimes the screen became black little by little.
for example, in a view controller, there are a collection view, a button, a page controller, and sometimes I found the collection view became black(or invisible), only black background is shown, after 1-2 seconds, the button is gone, then the whole screen is black. but if I put the app into background, then bring it back, everything is ok.
here is the code:
- (void)application:(UIApplication *)application performFetchWithCompletionHandler:(void (^)(UIBackgroundFetchResult))completionHandler
{
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^{
NSTimeInterval timeLeft = [UIApplication sharedApplication].backgroundTimeRemaining - 1.0;
if(timeLeft &lt 0)
{
completionHandler(UIBackgroundFetchResultNoData);
}
//create new thread to do something, and sleep at current thread
...create new threads
[NSThread sleep:timeLeft];
completionHandler(UIBackgroundFetchResultNewData);
});
}
the issue could be reproduced if I repeat the following actions several times:
Do some thing
Put app to background(press home button)
Bring app back to foreground
repeat 1 - 3
we found following error in organizer for our app:
: CoreAnimation: warning, deleted thread with uncommitted CATransaction; set CA_DEBUG_TRANSACTIONS=1 in environment to log backtraces.
after added , I got the following log:
Myapp[11496] : CoreAnimation: warning, deleted thread with uncommitted CATransaction; created by:
0 QuartzCore 0x31ca6a75 + 268
1 QuartzCore 0x31ca6929 + 224
2 QuartzCore 0x31cabddb + 86
3 QuartzCore 0x31cab9f7 + 150
4 UIKit 0x3203f919 + 344
5 UIKit 0x320bb11b + 138
6 UIKit 0x322b0ebf + 218
7 UIKit 0x322b1169 + 104
8 UIKit 0x322b1735 + 36
9 Myapp 0x002e538d __61-[AppDelegate application:performFetchWithCompletionHandler:]_block_invoke + 632
10 libdispatch.dylib 0x3a487d53 + 10
11 libdispatch.dylib 0x3a48d689 + 228
12 libdispatch.dylib 0x3a48d8dd + 56
13 libsystem_pthread.dylib 0x3a5b8c17 _pthread_wqthread + 298
14 libsystem_pthread.dylib 0x3a5b8adc start_wqthread + 8
my questions is:
how could application:performFetchWithCompletionHandler cause animation issue?
to answer questions:
1. I'm sure that phone is not going to sleep
2. source code. sorry the project is too big

Answer 1

Check that you are working with UI on main thread only, this should be the case.
Edit: - I've seen this one, although never used it(written by Peter Steinberger).
Also this answer can help avoiding several more problems.

Answer 2

The thread created to perform your fetch invokes a call to update your ui, when it finishes performing whatever task it is doing, and its no longer needed it is deallocated. UIUpdates MUST be performed on the main thread or they might not be performed right away.
Have you tried dispatching to the main thread whatever ui update you are performing there?

Answer 3

It's hard to know what you your completion handler is doing, but if it's updating UI try to dispatch them on the main thread
- (void)application:(UIApplication *)application performFetchWithCompletionHandler:(void (^)(UIBackgroundFetchResult))completionHandler
{
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^{
NSTimeInterval timeLeft = [UIApplication sharedApplication].backgroundTimeRemaining - 1.0;
if(timeLeft < 0)
{
dispatch_async(dispatch_get_main_queue(), ^{
completionHandler(UIBackgroundFetchResultNoData);
});
}
//create new thread to do something, and sleep at current thread
...create new threads
[NSThread sleep:timeLeft];
dispatch_async(dispatch_get_main_queue(), ^{
completionHandler(UIBackgroundFetchResultNewData);
});
});
}