Dynamically starting children to Demux Mp4 tracks

    structure = [
      child(:video_source, %Membrane.File.Source{
        location: @input_file
      })
      |> child(:demuxer, Membrane.MP4.Demuxer.ISOM)
      # Video
      |> via_out(Pad.ref(:output, 1))
      |> child(:vido_tmp, %Membrane.H264.Parser{
        output_stream_structure: :annexb
      })
      |> do_video()
      |> child(:video_out, %Membrane.H264.Parser{
        generate_best_effort_timestamps: %{framerate: {25, 1}},
        output_stream_structure: :avc1
      }),
      # Audio
      get_child(:demuxer)
      |> via_out(Pad.ref(:output, 2))
      |> do_audio(),
      # Mux
      child(:muxer, Membrane.MP4.Muxer.ISOM)
      |> child(:sink, %Membrane.File.Sink{location: @out_file})
      get_child(:video_out) |> get_child(:muxer),
      get_child(:audio_out) |> get_child(:muxer)
    ]

  @impl true
  def handle_init(_ctx, _path) do
    structure = [
      child(:video_source, %Membrane.File.Source{
        location: @input_file
      })
      |> child(:demuxer, Membrane.MP4.Demuxer.ISOM),
      # Mux
      child(:muxer, Membrane.MP4.Muxer.ISOM)
      |> child(:sink, %Membrane.File.Sink{location: @out_file})
    ]

    {[spec: structure], %{}}
  end

  @impl true
  def handle_child_notification({:new_tracks, tracks}, :demuxer, _ctx, state) do
    spec =
      tracks
      |> Enum.map(fn
        {track_id, %Membrane.H264{}} ->
          get_child(:demuxer)
          |> via_out(Pad.ref(:output, track_id))
          |> do_video()

        {track_id, %Membrane.AAC{}} ->
          get_child(:demuxer)
          |> via_out(Pad.ref(:output, track_id))
          |> do_audio()
      end)

    {[spec: spec], state}
  end