You're seeing just the function handle_redemand, go back to Membrane.Core.Element.DemandHandler module for more information.
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handle_redemand(pad_ref, state)

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handle_redemand(Membrane.Pad.ref_t(), Membrane.Core.Element.State.t()) ::
  {:ok, Membrane.Core.Element.State.t()}

Called when redemand action was returned.

  • If element is currently supplying demand it means that after finishing supply_demand it will call handle_delayed_demands.
  • If element isn't supplying demand at the moment handle_demand is invoked right away, and it will invoke handle_demand callback, which will probably return :redemand and :buffers actions and in that way source will synchronously supply demand.