Protocol for applying custom events to a workflow.

Built-in events (e.g. FactProduced, ActivationConsumed, JoinCompleted) are handled by pattern-matched clauses in Workflow.apply_event/2 and do not need to implement this protocol — they are dispatched before the protocol fallback.

External libraries that define custom Invokable node types can introduce their own event structs and implement EventApplicator so that apply_event/2 knows how to fold them into the workflow.

Example

defmodule MyApp.Events.CustomStepCompleted do
  defstruct [:node_hash, :result_value]
end

defimpl Runic.Workflow.EventApplicator, for: MyApp.Events.CustomStepCompleted do
  def apply(event, workflow) do
    # Use Workflow public API to fold the event into the workflow
    fact = Runic.Workflow.Fact.new(value: event.result_value, ancestry: {event.node_hash, nil})
    Runic.Workflow.log_fact(workflow, fact)
  end
end

Serialization

Custom events serialize naturally via ETF (:erlang.term_to_binary/1). For cross-language interop, implement serialization at the Store adapter level using the event struct fields directly.