@callback ask(demand :: pos_integer(), from :: {pid(), reference()}, state :: term()) ::
  {:ok, actual_demand :: non_neg_integer(), new_state}
when new_state: term()

Called every time a consumer sends demand.

The demand will always be a positive integer (more than 0). This callback must return the actual_demand as part of its return tuple. The returned demand is then sent to producers.

It is guaranteed the reference given in from points to a reference previously given in subscribe.

@callback cancel(from :: {pid(), reference()}, state :: term()) ::
  {:ok, demand :: non_neg_integer(), new_state}
when new_state: term()

Called every time a subscription is cancelled or the consumer goes down.

It is guaranteed the reference given in from points to a reference previously given in subscribe.

@callback dispatch(events :: [term(), ...], length :: pos_integer(), state :: term()) ::
  {:ok, leftover_events :: [term()], new_state}
when new_state: term()

Called every time a producer wants to dispatch an event.

The events will always be a non empty list. This callback may receive more events than previously asked and therefore must return events it cannot not effectively deliver as part of its return tuple. Any leftover_events will be stored by producers in their buffer.

It is important to emphasize that leftover_events can happen in any dispatcher implementation. After all, a consumer can subscribe, ask for events and crash. Eventually the events the consumer asked will be delivered while the consumer no longer exists, meaning they must be returned as left_over events until another consumer subscribes.

It is guaranteed the reference given in from points to a reference previously given in subscribe. It is also recommended for events to be sent with Process.send/3 and the [:noconnect] option as the consumers are all monitored by the producer. For example:

Process.send(consumer, {:"$gen_consumer", {self(), consumer_ref}, events}, [:noconnect])

@callback info(msg :: term(), state :: term()) :: {:ok, new_state} when new_state: term()

Used to send an info message to the current process.

In case the dispatcher is doing buffering, the message must only be sent after all currently buffered consumer messages are delivered.

@callback init(opts :: options()) :: {:ok, state} when state: any()

Called on initialization with the options given on GenStage.init/1.

@callback subscribe(opts :: keyword(), from :: {pid(), reference()}, state :: term()) ::
  {:ok, demand :: non_neg_integer(), new_state} | {:error, term()}
when new_state: term()

Called every time the producer gets a new subscriber.