'Notify GStreamer that we are about to change state of playback.
This function *MUST* be called before changing URIs or doing
changes like updating data that is being pushed. The reason for this
is that GStreamer will reset all its state when it changes to
:attr:`Gst.State.READY`.'

def prepare_change(self):

    return self._set_state(Gst.State.READY)
