How do I determine the duration of a sample buffer uses in an AVAssetWriter

Question

I have a sample buffer that I'm using to capture video for an AVAssetWriter. I'm trying to figure out a way to determine how much video (time) I have captured. currently I am able to access the current buffer timestamp by using

CMTime pts = CMSampleBufferGetPresentationTimeStamp(sampleBuffer);

I'm looking for a way to determine the duration of the buffer, i've tried

CMTime dur = CMSampleBufferGetOutputDuration(sampleBuffer); but it seems to fluctuate 

between .002333 and nan for some reason

Answer 1

为什么不只为第一个样本保留一个时间戳，然后做一个非常简单的计算（最新的时间戳减去第一个）？