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03-add-and-remove-source.md

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Adding and Removing Sources

This example is based upon 01-add-source.py. A Pipeline with a live audiotestsrc, an audiomixer and an autoaudiosink is created. After 2 seconds, a second audiotestsrc is created, added to the pipeline and linked to the Mixer. Another 2 seconds later it is stopped, unlinked and removed again. The process repeats as long as the experiment runs.

You should read Adding a Source before this, because important Aspects that have been explained there are not repeated here.

The most important new Lines have been marked as such:

  1. Because the Sequencing is done in a background thread and the Sequence will run over and over again, we need a way to stop the Sequence when the Main-Program terminates. In Python it is best to use the threading.Event Class and its wait-Method to achieve this.

  2. We want to remove all Elements that are added. One way to do this is to keep references to the actual Elements, like we do in these global variables. A even better way would be to collect the added Elements in a Bin, a Cluster of Elements, which can be added and removed as a whole. See Adding and Removing RTP-Sources for an Example of using a Bin.

  3. In Adding a Source we used Element.link_filtered to create a similar link between the audiotestsrc and the audiomixer. link_filtered creates and adds capsfilter-Element for us behind the scenes and places it between the Test-Source and the Audiomixer. When we later try to remove the Test-Source, the capsfilter-Element would not automatically removed for us and we would need to figure out a way to get hold of a reference to this Element.

    To avoid that, this Example explicitly created a capsfilter and links it, so we already have a Reference to it. Another Option, explored in the next Experiment Adding and Removing RTP-Sources is to use a Bin. The automatically created capsfilter-Element would then be a Child of that Bin and removed when the Bin is removed.

    For the same reason a Sink-Pad on the Mixer is requested explicitly. Usually a call to Element.link targeting an Element with request-Pads will automatically request a Pad for you, but then you need to figure out a way to explicitly select this pad later, when you intend to release the requested Mixer-Pad.

  4. Debugging these Scenarios can be quite complex. Gst.debug_bin_to_dot_file_with_ts is a Utility-Method which generates a GraphViz-File in the directory named in the Environment-Variable GST_DEBUG_DUMP_DOT_DIR. To generate such files, you can run the Experiment like this:

    GST_DEBUG_DUMP_DOT_DIR=. ./04-add-and-remove-network-source.py

    To view theses Files, which can become rather large, I suggest xdot which can be used on all major OSes and is really handy to view and examine these large dot-File graphs.

  5. Actually removing a Source is rather straight forward. First the State of all involved Elements is set to NULL, starting with the Source and moving down the Buffer-Flow. This will release any Resources held by the Elements and ensures that the Sources does not try to push new Buffers into an Element that has already been stopped.

    Next the Elements are removed from the Pipeline in the same order. This will also remove the Links between them.

    Lastly the Sink-Pad will be released to the Mixer.