I see GStreamer Plugin Writer’s Guide PDF file, I don't understand mechanism that how do the element pad ask for (requests) sometimes and request pad on peer pad?

[zhang007z]

according to GStreamer Plugin Writer’s Guide PDF file,I read Chapter 12.
Request and Sometimes pads
(website:http://gstreamer.freedesktop.org/data/doc/gstreamer/head/pwg/html/).
I quote this document sentences : " This mostly depends on stream content:
demuxers will generally parse the stream header, decide what
elementary(video, audio, subtitle, etc.) streams are embedded inside the
system stream, and will then create asometimes pad for each of those
elementary streams."
I understand that the creating process of sometimes and request pad throught
given example in this document ,but I don't understand that how to
communicate to make this pad produce new sometimes and request pad between
this element pad and peer element pad ?
I don't know that when is gst_my_filter_loopfunc function invoked by what my
code ? 
 you can specifically illustrate this communication (include query event and
which functions does this process invoke?)between downstream pad and
upstream pad if you know this issue . you also could enumerate some example
to be convenient to my understand,such as demuxer or mixer 

if you take time for answering my stupid issue, think you in advance very
much.



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