The Projection is a read model connected to the PubSub.

Client code has to implement the Chronik.Projection.init function and the state transition Chronik.Projection.handle_event.

Example:

defmodule DomainEvents do
  defmodule CounterCreated do
    defstruct [:id]
  end

  defmodule CounterIncremented do
    defstruct [:id, :increment]
  end
end

defmodule CounterState do
  use Chronik.Projection

  alias DomainEvents.CounterCreated
  alias DomainEvents.CounterIncremented

  def init(_opts), do: {nil, []}

  def handle_event(%CounterCreated{}, nil) do
    0
  end
  def handle_event(%CounterIncremented{increment: increment}, value) do
    value + increment
  end
end