Thanks to Sebastian Dröge there is a new thing in GStreamer called streamid. It basically gives all streams inside a given file a unique id, making files with multiple streams a lot easier to deal with. This streamid is also supported by the GStreamer discoverer object. So once you identified the contents of a file with discoverer you can be sure to grab the exact stream you want coming out of (uri)decodebin by checking the pad for the streamid. The most common usecase for this is of course files with multiple audio streams in different languages.
From the output of Discoverer the stream id is really easy to get:
On the stream object you get out of Discoverer you just run a:
On the pad you get from decodebin or uridecodebin the patch is a bit more convoluted, but not
to hard once you know how (there might be some kind of convenience API added for this at some point).
Before you connect the pad you get from the bin you attach a pad to it like this:
src_pad.add_probe(Gst.PadProbeType.EVENT_DOWNSTREAM, self.padprobe, None)
Then you in the function you define you can extract the stream_id with the parse_stream_start call as seen below:
def padprobe(self, pad, probeinfo, userdata): event = probeinfo.get_event() eventtype=event.type if eventtype==Gst.EventType.STREAM_START: streamid = event.parse_stream_start() return Gst.PadProbeReturn.OK
I been using this code in my local copy of Transmageddon to start implementing support for files with multiple audio streams (also supporting multiple video streams would be easy, but I am not sure how useful it would be). Got a screenshot of my current development snapshot below, but I am still trying to figure out what would be a nice way to present it. The current setup will look quite crap if the incoming file got more than a few audio streams. Suggestions welcome :)