Gstreamer Change State Function - gstreamer

Question

I am new to Gstreamer. I have a question about change_state function for plugin.
As I read in this guide: http://gstreamer.freedesktop.org/data/doc/gstreamer/head/pwg/html/chapter-statemanage-states.html#section-statemanage-filters
static GstStateChangeReturn
gst_my_filter_change_state (GstElement *element, GstStateChange transition)
{
GstStateChangeReturn ret = GST_STATE_CHANGE_SUCCESS;
GstMyFilter *filter = GST_MY_FILTER (element);
switch (transition) {
**//Downwards state change;**
ret = GST_ELEMENT_CLASS (parent_class)->change_state (element, transition);
if (ret == GST_STATE_CHANGE_FAILURE)
return ret;
**//upward state change**
}
return ret;
}
I really don't know how we can use parent_class and call parent_class->change_state
Because in init function of this element:
gst_my_filter_class_init (GstMyFilterClass *klass)
{
GstElementClass *element_class = GST_ELEMENT_CLASS (klass);
element_class->change_state = gst_my_filter_change_state;**strong text**
}
element_class->change_state was assigned to gst_my_filter_change_state. Why can we still call element_class->change_state in gst_my_filter_change_state when element_class->change_state is assign to another function.
Thank you!

Answer 1

Change gst_my_filter_class_init (GstMyFilterClass *klass) to something like this:
gst_my_filter_class_init (GstMyFilterClass *klass)
{
GstElementClass *element_class = GST_ELEMENT_CLASS (klass);
parent_class = (GstXyzClass *) g_type_class_peek_parent (klass);
element_class->change_state = gst_my_filter_change_state;**strong text**
}
And add a static global variable GstXyzClass *parent_class; somewhere near the top of your plugin. Just GstXyzClass will be the type of the Element you are inheriting from, e.g. GstElementClass. Just look at other plugin sources for examples.

Answer 2

Thanks for your answer. Actually,this code I quoted from that guide can run normally.But the thing I can not understand is in header file we declare: GstMyFilterClass {GstElementClass parent_class;} It means GstElementClass is the father class of GstMyFilterClass
but why can we use parent_class in source file of this plugin(.c file)? Sorry for my lacking of knowledge in Gobject , but as I know GstMyFilterClass is a struct and(not class like C++) and the attribute parent_class can not be used in function of plugin(in C++ we can easily use attribute in method of class). And in the gst_my_filter_class_init :
gst_my_filter_class_init (GstMyFilterClass *klass)
{
GstElementClass *element_class = GST_ELEMENT_CLASS (klass);
element_class->change_state = gst_my_filter_change_state;//assign to function pointer state change
}
Does The statement:GstElementClass *element_class = GST_ELEMENT_CLASS (klass); mean we cast GST_ELEMENT_CLASS (klass) to get its parent class(GstElementClass parent_class)?
If it is true,so the change_state function pointer of parent_class is not Null. So In
gst_my_filter_change_state (GstElement *element, GstStateChange transition)
{
GstStateChangeReturn ret = GST_STATE_CHANGE_SUCCESS;
GstMyFilter *filter = GST_MY_FILTER (element);
switch (transition) {
**//Downwards state change;**
ret = GST_ELEMENT_CLASS (parent_class)->change_state (element, transition);
if (ret == GST_STATE_CHANGE_FAILURE)
return ret;
**//upward state change**
}
return ret;
}
what is GST_ELEMENT_CLASS (parent_class)->change_state (element, transition)?
As I know every GstElementClass has a default function change_state but in this situation function change_state has assigned to another pointer function(gst_my_filter_class_init do this assignment) Is it right? Hope to receive your answer soon. Thank you very much