how to change element when pipe is running!!!!!!!!!!!!!

[DBOYaoao]

hi everyone:
           I am a new gst developer,I have a trouble with change elemnt
while pipeline is running.
           my project is like this:A->B->C,B can change the
pixel-aspect-ratio,I use d to change b that can change the
pixel-aspect-ratio,but i don't know why can't change it. 
         my method is this: I use videoscale ! capsfilter ! xvimagsink to
change aspect-ratio
         capsfilterB aspect-ratio is 1:1      capsfilterD aspect-ratio is
16:9
        first I block the videoscale srcpad,and the movie stop,then I unlink
B with A and c,then connect D with A and C use gst_bin_link_many(),finally I
unblock the videoscale srcpad.but the movie not continue.
       However When I set capsfilterD's aspect-ratio as 1:1 , it work. 
       
block code like this:

             GstPad *videoscalesrcpad =
gst_element_get_static_pad(videoscale,"src");
					       
gst_pad_set_blocked_async(videoscalesrcpad,TRUE,padBlockcb,videoscalesrcpad);
					while(!gst_pad_is_blocking(videoscalesrcpad))
					{
						g_print("wait for the srcpad blocking\n");
					}
					gst_element_set_state(capsfilter_1,GST_STATE_NULL);
					g_print(".....................src pad blocked..........\n");
					GstPad *caps1sinkpad = gst_element_get_static_pad(capsfilter_1,"sink");

					if(!gst_pad_is_linked(caps1sinkpad))
					{
						g_print("caps sink not link\n");
					}
					if(!gst_pad_unlink(videoscalesrcpad,caps1sinkpad))
					{
						g_print("unlinlk failed \n");
						return ;
					}
					GstPad *outsinkpad = gst_element_get_static_pad(outsink,"sink");
					GstPad *caps1srcpad = gst_element_get_static_pad(capsfilter_1,"src");

					gst_pad_unlink(caps1srcpad,outsinkpad);
unblock code like this:
                   GstCaps *filtercaps = NULL;
					filtercaps =
gst_caps_from_string("video/x-raw-yuv,pixel-aspect-ratio=\(fraction\)1/1");
					g_object_set(G_OBJECT(capsfilter_2),"caps",filtercaps,NULL);
					gst_caps_unref(filtercaps);

					gst_bin_add(videobin,capsfilter_2);
					gst_element_link_many(videoscale,capsfilter_2,outsink,NULL);
					gst_element_set_state(capsfilter_2,GST_STATE_PLAYING);

#endif
					GstPad *videoscalesrcpad =
gst_element_get_static_pad(videoscale,"src");
					gst_pad_set_blocked_async(videoscalesrcpad,FALSE,padBlockcb,NULL);

Thanks for anybody help!!!!!!



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