# `Runic.Workflow.EventApplicator`
[🔗](https://github.com/zblanco/runic/blob/main/lib/workflow/event_applicator.ex#L1)

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.

# `t`

```elixir
@type t() :: term()
```

All the types that implement this protocol.

# `apply`

```elixir
@spec apply(event :: struct(), workflow :: Runic.Workflow.t()) :: Runic.Workflow.t()
```

Applies this event to the workflow, returning the updated workflow.

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*Consult [api-reference.md](api-reference.md) for complete listing*