@callback handle_demand(
  pad :: Membrane.Pad.ref(),
  size :: non_neg_integer(),
  unit :: Membrane.Buffer.Metric.unit(),
  context :: Membrane.Element.CallbackContext.t(),
  state :: Membrane.Element.state()
) ::
  {[
     Membrane.Element.Action.common_actions()
     | Membrane.Element.Action.stream_actions()
   ], Membrane.Element.state()}

Callback called when buffers should be emitted by a source, filter or endpoint.

It is called only for output pads in the :manual flow control mode, as in their case demand is triggered by the input pad of the subsequent element.

In sources and endpoint, appropriate amount of data should be sent here.

In filters, this callback should usually return :demand action with size sufficient for supplying incoming demand. This will result in calling c:Membrane.WithInputPads.handle_buffer/4, which is to supply the demand.

If a source or an endpoint is unable to produce enough buffers, or a filter underestimated returned demand, the :redemand action should be used (see Membrane.Element.Action.redemand/0).

Context passed to this callback contains additional field :incoming_demand.