In my usage so far, an "Event" doesn't get its own type; it's an object that transports attributes. The listeners also use a class method to route events to methods on the object. In this way, you can test event handlers as you would any other ruby method.

event_sourcing_example.rb

# app/commands/receive_bonus_command.rb
# Commands look like ActiveModel+virtus "ish" objects
class ReceiveBonusCommand < Command
  attribute :income_source_id, String
  attribute :bonus_amount,     Integer
  attribute :effective_date,   Date
  
  validates :account_id, presence: true
  
  def validate
    unless effective_date > Date.current
      errors[:effective_date] << "must be in the future"
    end
  end
  
  def apply
    # Email the user congratulations; we wouldn't want to resend this
    # email if we replay the :received_bonus event.
    CongratulationsMailer.deliver(self)
    # Because Command's initialize method asks for an event_store, IoC
    # container (ruse) injected it in for us.
    event_store.fire(
      :received_bonus,
      payload: {
        income_source_id: income_source_id,
        bonus_amount:     bonus_amount,
        effective_date:   effective_date,
      }
    )
  end
end

# app/models/income_source.rb
class IncomeSource < Entity
  attribute :compensation_changes, Array[CompensationChange]
  
  # In practice, since the event's payload has a property called
  # income_source_id, and the name of this aggregate root is
  # IncomeSource, listen can automatically fetch an instance from
  # the repo. Here, the .listen class macro is actually wired up
  # to glue code between the Entity class and the EventStore.
  listen received_bonus: 'handle_received_bonus',
    instantiate: ->(event) { repository.find(event.payload.income_source_id) }
  
  def handle_received_bonus(event)
    comp_change = CompensationChange.new(
      date: event.effective_date,
      amount: event.bonus_amount,
    )
    self.compensation_changes << comp_change
    repository.store self
  end
end

The use of Entity.listen in IncomeSource is explicitly telling the framework how to fetch the particular IncomeSource instance that wants to listen on the :received_bonus event. In practice, I can omit passing the proc via instantiate since I followed the convention of naming the id after the model. I wanted to be explicit in this example, so you can follow how everything is intended to work.