Custom gst.Element in GTK2

import gst
import gtk
import gobject
gobject.threads_init()

class TestFilter(gst.Element):

    __gstdetails__ = (
        "TestFilter plugin",
        "test_filter.py",
        "Description",
        "Contact"
    )

    # src pad template: used to forward buffers
    _srctemplate = gst.PadTemplate (
        'video_src',
        gst.PAD_SRC,
        gst.PAD_ALWAYS,
        gst.Caps('video/x-raw-yuv')
    )

    # sink pad template: used to receive buffers
    _sinktemplate = gst.PadTemplate (
        'video_sink',
        gst.PAD_SINK,
        gst.PAD_ALWAYS,
        gst.Caps('video/x-raw-yuv')
    )

    # register pad templates
    __gsttemplates__ = (_srctemplate, _sinktemplate)

    def __init__(self):
        gst.Element.__init__(self)

        # src pad
        self.srcpad = gst.Pad(self._srctemplate, 'video_src')

        # sink pad
        self.sinkpad = gst.Pad(self._sinktemplate, 'video_sink')
        self.sinkpad.set_setcaps_function(self._sink_setcaps)
        self.sinkpad.set_chain_function(self._sink_chain)

        # add pads
        self.add_pad(self.srcpad)
        self.add_pad(self.sinkpad)

    def _sink_setcaps(self, pad, caps):
        return True

    def _sink_chain(self, pad, buf):
        # ... do analyzing stuff ...
        print('...analyzing buffer...')
        # forward buffer
        return self.srcpad.push(buf)

gobject.type_register(TestFilter)


if __name__=='__main__':
    def on_message(bus, message):
        if message.type == gst.MESSAGE_EOS:
            player.set_state(gst.STATE_NULL)
        elif message.type == gst.MESSAGE_ERROR:
          player.set_state(gst.STATE_NULL)
          (err, debug) = message.parse_error()
          print "Error: %s" % err, debug

    def on_sync_message(bus, message):
        if message.structure.get_name() == "prepare-xwindow-id":
            win_id = videowidget.window.xid
            imagesink = message.src
            imagesink.set_property("force-aspect-ratio", True)
            imagesink.set_xwindow_id(win_id)

    def on_delete(*args):
        player.set_state(gst.STATE_NULL)
        gtk.main_quit()

    win = gtk.Window()
    win.connect("delete_event", on_delete)
    videowidget = gtk.DrawingArea()
    videowidget.set_size_request(400, 250)
    vbox = gtk.VBox(False, 0)
    vbox.pack_start(videowidget, True, True, 0)
    win.add(vbox)
    win.show_all()

    # Setup GStreamer
    player = gst.element_factory_make("playbin", "MultimediaPlayer")
    bus = player.get_bus()
    bus.add_signal_watch()
    bus.enable_sync_message_emission()
    bus.connect("message", on_message)
    bus.connect("sync-message::element", on_sync_message)

    # create a Bin containing my own TestFilter, textoverlay and autovideosink
    testfilter = TestFilter()
    autovideosink = gst.element_factory_make('autovideosink')
    textoverlay = gst.element_factory_make('textoverlay', 'textoverlay')
    video_bin = gst.Bin()
    video_bin.add(testfilter)
    video_bin.add(textoverlay)
    video_bin.add(autovideosink)
    sink_pad = gst.GhostPad('video_sink', testfilter.get_pad('video_sink'))
    video_bin.add_pad(sink_pad)
    gst.element_link_many(testfilter, textoverlay, autovideosink)
    player.set_property('video-sink', video_bin)

    textoverlay.set_property('text', 'ABC')


    player.set_property('uri', "file:///.../somefile.avi")
    player.set_state(gst.STATE_PLAYING)

    gtk.main()