AVMutableVideoComposition - CoreAnimation: warning, deleted thread with uncommitted CATransaction - xcode

Question

I am working on some basic video compositions using AVMutableComposition - currently a video layer (AVMutableVideoComposition) with a text layer (CATextLayer).
It all looks ok but when I let it export via "AVMutableComposition exportAsynchronouslyWithCompletionHandler " it goes off and completes but returns this error:
CoreAnimation: warning, deleted thread with uncommitted CATransaction; created by:
0 QuartzCore 0x00007fff8a106959 _ZN2CA11Transaction4pushEv + 219
1 QuartzCore 0x00007fff8a106531 _ZN2CA11Transaction15ensure_implicitEv + 273
2 QuartzCore 0x00007fff8a10d66f _ZN2CA5Layer13thread_flags_EPNS_11TransactionE + 37
3 QuartzCore 0x00007fff8a10d5a7 _ZN2CA5Layer4markEPNS_11TransactionEjj + 79
4 QuartzCore 0x00007fff8a112cac _ZN2CA5Layer27contents_visibility_changedEPNS_11TransactionEb + 216
5 QuartzCore 0x00007fff8a112b65 _ZN2CA5Layer12mark_visibleEPNS_11TransactionEb + 261
6 QuartzCore 0x00007fff8a112b26 _ZN2CA5Layer12mark_visibleEPNS_11TransactionEb + 198
7 QuartzCore 0x00007fff8a112b26 _ZN2CA5Layer12mark_visibleEPNS_11TransactionEb + 198
8 QuartzCore 0x00007fff8a1128d1 _ZN2CA5Layer11set_visibleEj + 335
9 QuartzCore 0x00007fff8a1126b9 _ZN2CA7Context9set_layerEPKv + 75
10 MediaToolbox 0x00007fff857f155b FigCoreAnimationRendererInvalidate + 108
11 CoreFoundation 0x00007fff8ec763df CFRelease + 511
12 MediaToolbox 0x00007fff857d3a6b FigVideoCompositionProcessorInvalidate + 675
13 MediaToolbox 0x00007fff85791341 FigAssetWriterCreateWithURL + 18573
14 MediaToolbox 0x00007fff85791f7b FigAssetWriterCreateWithURL + 21703
15 CoreMediaAuthoringCrunchers 0x00000001046e2b99 AssetAudioSourcer_CreateInstance + 3865
I find that this goes away if I comment-out the following line - however the CATextLayer is not rendered:
videoComposition.animationTool = [AVVideoCompositionCoreAnimationTool videoCompositionCoreAnimationToolWithPostProcessingAsVideoLayer:videoLayer inLayer:parentLayer];
Any thoughts anyone?
Adam

Answer 1

You must ensure any UI drawing occurs on the main thread. The line you commented out probably does some drawing internally, so you want to make sure this code is executed on the main thread. A couple ways to do this is by using the function dispatch_async() or the methods performSelectorOnMainThread:withObject:waitUntilDone: or performSelectorOnMainThread:withObject:waitUntilDone:modes:
- (void) someMethod
{
// You may need to load a container object to pass as myCustomData whose
// contents you then access in myCustomDrawing: if the data isn't accessible
// as instance data.
[...]
// 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.
videoComposition.animationTool = [AVVideoCompositionCoreAnimationTool videoCompositionCoreAnimationToolWithPostProcessingAsVideoLayer:videoLayer
inLayer:parentLayer];
}
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?