Managing filter state

[Ngọc Tân Đỗ]

Hi All,
I am new to Gstreamer. I was confused when reading Plugin guide document
about Plugin's state at
http://gstreamer.freedesktop.org/data/doc/gstreamer/head/pwg/html/chapter-statemanage-states.html#section-statemanage-filters
At line :

 *ret = GST_ELEMENT_CLASS (parent_class)->change_state (element, transition);*


in this line why we call parent_class->change_state? I read and get
the explanation:"* upwards (NULL=>READY, READY=>PAUSED,
PAUSED=>PLAYING)
and downwards (PLAYING=>PAUSED, PAUSED=>READY, READY=>NULL) state
changes are handled in two separate blocks with the downwards state
change
handled only after we have chained up to the parent class's state
change function. This is necessary in order to safely handle
concurrent
access by multiple threads" *
but actually I can't understand the mechanism.Please help me resolve
this problem.Thanks
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.freedesktop.org/archives/gstreamer-devel/attachments/20121007/84a4f33d/attachment.html>