AVAssetTrack totalSampleDataLength is 0

I'm programming an iPhone app which uses an AVAssetReader to read samples from an AVAssetTrack, I need to allocate some memory which depe开发者_C百科nds on the number of samples in the AVAssetTrack, but for some reason totalSampleDataLength is returning 0.

Any ideas?

---

< warning - guess! >

Perhaps it's something to do with the AVAsynchronousKeyValueLoading protocol - do you have to ask it to load the value for that key before it's populated?