Find IDR in AVAsset

I'm assuming you want the IDR info in decode order rather than presentation order. This makes a difference if your movie contains bidirectionally predicted frames. If your H.264 video file is a QuickTime movie, you can do something like this:

func getSyncInfo(url: URL) -> [Bool] {
    var idrInfo = [Bool]()

    let asset = AVURLAsset(url: url)
    guard let reader = try? AVAssetReader(asset: asset) else {
        return idrInfo
    }
    let videoTrack = asset.tracks(withMediaType: AVMediaType.video).first!
    guard let cursor = videoTrack.makeSampleCursorAtFirstSampleInDecodeOrder() else {
        return idrInfo
    }
    repeat {
        let syncInfo = cursor.currentSampleSyncInfo
        idrInfo.append(syncInfo.sampleIsFullSync.boolValue ? true : false)
    } while cursor.stepInDecodeOrder(byCount: 1) == 1
    return idrInfo
}

This is unlikely, but if the CMSampleBuffer contains an access unit of a raw Annex B bitstream, then you have to:

1. Get the raw data bytes from the CMBlockBuffers
2. Remove encapsulation prevention bytes
3. Search for start codes the look like 0x00 0x00 0x01
4. Parse the nal_unit_type byte following the start code
5. Mask out the higher 3 bits to get nal_unit_type
6. Check that the nal_unit_type is 5.