Protocol to allow an event to be transformed before being passed to a consumer.

You can use an upcaster to change the shape of an event (e.g. add a new field with a default, rename a field) or rename an event.

Because the upcaster changes any historical event to the latest version, consumers (aggregates, event handlers, and process managers) only need to support the latest version.

Example

defimpl Commanded.Event.Upcaster, for: AnEvent do
  def upcast(%AnEvent{} = event, _metadata) do
    %AnEvent{name: name} = event

    %AnEvent{event | first_name: name}
  end
end