A dispatcher that accumulates demand from all consumers before broadcasting events to all of them.

If a producer uses GenStage.BroadcastDispatcher, its subscribers can specify an optional :selector function that receives the event and returns a boolean in the subscription options.

Assume producer and consumer are stages exchanging events of type %{:key => String.t, any => any}, then by calling

GenStage.sync_subscribe(consumer,
  to: producer,
  selector: fn %{key: key} -> String.starts_with?(key, "foo-") end)

consumer will receive only the events broadcasted from producer for which the selector function returns a truthy value.

The :selector option can be specified in sync and async subscriptions, as well as in the :subscribe_to list in the return tuple of GenStage.init/1. For example:

def init(:ok) do
  {:consumer, :ok, subscribe_to:
    [{producer, selector: fn %{key: key} -> String.starts_with?(key, "foo-") end}]}`
end