import pygst
pygst.require("0.10") 
import gst
import gobject

class Track:

    def __init__(self, pipeline):
        self.comp = gst.element_factory_make("gnlcomposition")
        self.comp.connect("pad-added", self.on_composition_pad_added)
        pipeline.add(self.comp)
        self.comp.set_state(gst.STATE_PLAYING)

    def on_composition_pad_added(self, comp, pad):
        caps = pad.get_caps()
        self.srcpad = pad
        if not comp.get_parent() or not convert.get_parent():
            return
        gst.Element.link(comp, adder)

def go():
    global num_tracks, tracks, action
    
    if action == ACTION_ADD_TRACK:
        print "Add Track"
        tracks.append(Track(pipeline))
        action = ACTION_ADD_EVENT
    elif action == ACTION_ADD_EVENT:
        print "Add Event"

        track = tracks[-1]
        silence = gst.element_factory_make("audiotestsrc")
        silence.set_property("wave", "silence")
        src = gst.element_factory_make("gnlsource")
        src.add(silence)
        src.set_property("start", 0 * gst.SECOND)
        src.set_property("duration", 1000 * gst.SECOND)
        track.comp.add(src)
        
        if len(tracks) < 3:
            action = ACTION_ADD_TRACK
        else:
            action = ACTION_REMOVE_TRACK
    else:
        print "Remove Track"
        track = tracks.pop()
        try:
            sink = track.srcpad.get_peer()
            gst.Element.unlink(track.comp, adder)
            adder.release_request_pad(sink)
        except:
            pass
        track.comp.set_state(gst.STATE_NULL)
        pipeline.remove(track.comp)
        if not tracks:
            action = ACTION_ADD_TRACK
    return True

adder = gst.element_factory_make("adder")
convert = gst.element_factory_make("audioconvert")
sink = gst.element_factory_make("alsasink")
pipeline = gst.element_factory_make("pipeline")
pipeline.add(adder, convert, sink)
gst.Element.link(adder, convert)
gst.Element.link(convert, sink)

ACTION_ADD_TRACK = 0
ACTION_ADD_EVENT = 1
ACTION_REMOVE_TRACK = 2
action = ACTION_ADD_TRACK

tracks = []

gobject.threads_init()
gobject.idle_add(go)
pipeline.set_state(gst.STATE_PLAYING)
gobject.MainLoop().run()
pipeline.set_state(gst.STATE_NULL)