CoreAnimation warning deleted thread with uncommitted CATransaction

Question

I am having issues with the following warning:

CoreAnimation: warning, deleted thread with uncommitted CATransaction; set CA_DEBUG_TRANSACTIONS=1 in environment to l

Answer 1

In keeping with standard Cocoa paradigms, the recommended solution here is to perform your Core Animation work on the main thread, easily done with GCD:

dispatch_async(dispatch_get_main_queue(), ^{
    [self.delegate redrawSomething];
});

In general it's poor form to call objects in contexts they don't expect, so a good rule of thumb is to always dispatch onto the main thread when delivering messages to external modules.

Some frameworks—like Core Location—with emit a log message if they are called from any context other than the main thread. Others will emit cryptic messages, such as your example here with Core Animation.

Answer 2

Your suspicions are right. If NSOperation completes before CoreAnimation is done performing, then you get a nice warning:

*CoreAnimation: warning, deleted thread with uncommitted CATransaction; set CA_DEBUG_TRANSACTIONS=1 in environment to log backtraces.*

This can also happen under some circumstances when a block that is dispatched on a queue triggers some work from CoreAnimation and returns before the CoreAnimation finishes.

The solution I use is simple: On a block or NSOperation that requests work from CoreAnimation, I check that the work has indeed been completed before exiting.

To give you a proof-of-concept example, this is a block to be dispatched on a dispatch queue. In order to avoid the warning, we check that the CoreAnimation is done before exiting.

^{

   // 1. Creating a completion indicator

   BOOL __block animationHasCompleted = NO;

   // 2. Requesting core animation do do some work. Using animator for instance.

   [NSAnimationContext runAnimationGroup:^(NSAnimationContext *context){
      [[object animator] perform-a-nice-animation];
   } completionHandler:^{
      animationHasCompleted = YES;
   }];

   // 3. Doing other stuff…

   …

   // 4. Waiting for core animation to complete before exiting

   while (animationHasCompleted == NO)
   {
       usleep(10000);
   }

}

Answer 3

Another way of ensuring any UI drawing occurs on the main thread, as described by Numist, is using the method performSelectorOnMainThread:withObject:waitUntilDone: or alternatively performSelectorOnMainThread:withObject:waitUntilDone:modes:

- (void) someMethod
{
    [...]

    // Perform all drawing/UI updates on the main thread.
    [self performSelectorOnMainThread:@selector(myCustomDrawing:)
                           withObject:myCustomData
                        waitUntilDone:YES];

    [...]
}

- (void) myCustomDrawing:(id)myCustomData
{
    // Perform any drawing/UI updates here.
}

For a related post on the difference between dispatch_async() and performSelectorOnMainThread:withObjects:waitUntilDone: see Whats the difference between performSelectorOnMainThread and dispatch_async on main queue?