How to install AVAssetWriter with maxduration and change url

I want to record a movie from the camera. Just use the Apple Rosywriter demo delivery. But what I want to do is set the maximum recording value, and when the duration reaches maxduration, the AVAssetWriter can change the url to write the movie to another file. But the problem is the url is read-only and I cannot change it. I know there is another way to do it - add movieFileOutput to AVCaptureSession. But I need to process the PixelBuffer before writing it. So I need to add AVCaptureVideoDataOutput. How can I achieve this? I have to apologize for my bad english.