# `Chronicle.EventType`
[🔗](https://github.com/Cratis/Chronicle.Elixir/blob/main/lib/chronicle/event_type.ex#L4)

Macro for defining Chronicle event types.

Use `Chronicle.EventType` in an event struct module to annotate it with a
stable event type identifier and generation number. Chronicle uses these to
register the event schema and route events to the correct observers.

## Usage

    defmodule MyApp.Events.AccountOpened do
      use Chronicle.EventType, id: "account-opened-v1"
      defstruct [:account_id, :owner_name, :initial_balance]
    end

With an explicit generation:

    defmodule MyApp.Events.FundsDeposited do
      use Chronicle.EventType, id: "funds-deposited", generation: 2
      defstruct [:account_id, :amount, :currency]
    end

## Introspection

Modules that `use Chronicle.EventType` expose metadata via
`__chronicle_event_type__/1`:

    MyApp.Events.AccountOpened.__chronicle_event_type__(:id)
    #=> "account-opened-v1"

    MyApp.Events.AccountOpened.__chronicle_event_type__(:generation)
    #=> 1

Chronicle also generates a `Jason.Encoder` implementation automatically so
events can be serialized to JSON for storage. Event fields are encoded using
their atom keys as-is (snake_case).

# `__chronicle_event_type__`

```elixir
@callback __chronicle_event_type__(key :: :id | :generation) :: term()
```

Returns metadata for this event type module.

Accepts `:id` or `:generation` as the key.

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