title: How do I use AVAssetWriter?
body: <p>I’d like to take some video frames and encode them into a video. It looks like that’s exactly what <code>AVAssetWriter</code> was meant for, but no matter how I eyeball the docs and Google I can’t find any way to actually use it. From the docs it looks like I need an input (<code>AVAssetWriterInput</code>) to feed the writer from. Fine. But the <code>AVAssetWriterInput</code> class is abstract and the only subclass that I know of in 4.1 is <code>AVAssetWriterInputPixelBufferAdaptor</code> that requires a <code>AVAssetWriterInput</code> in its initializer…? Am I missing something obvious here?</p>