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Building blocks for distributed and/or CQRS/ES systems

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x3m_system lib aggregate.ex
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lib/aggregate.ex 

defmodule X3m.System.Aggregate do

  defmodule State do

    @type t :: %__MODULE__{version: integer, client_state: any}

    defstruct version: -1, client_state: nil

  end



  @callback initial_state :: map()



  @spec initial_state(atom) :: State.t()

  def initial_state(aggregate_mod),

    do: %State{client_state: apply(aggregate_mod, :initial_state, [])}



  defmacro handle_msg(msg_name, fun) do

    quote do

      @spec unquote(msg_name)(X3m.System.Message.t(), X3m.System.Aggregate.State.t()) ::

              {:block | :noblock, X3m.System.Message.t(), X3m.System.Aggregate.State.t()}

      def unquote(msg_name)(%X3m.System.Message{} = message, state) do

        X3m.System.Instrumenter.execute(:handle_msg, %{}, %{

          aggregate: __MODULE__,

          message: unquote(msg_name)

        })



        client_state = state.client_state || raise("Aggregate state wasn't set")



        {label, %X3m.System.Message{} = message, client_state} =

          unquote(fun).(message, client_state)



        {label, message, %State{state | client_state: client_state}}

      end

    end

  end



  defmacro __using__(_opts) do

    quote do

      alias X3m.System.Aggregate

      alias X3m.System.Aggregate.State, as: AggregateState

      require Aggregate

      import Aggregate

      @behaviour Aggregate



      @doc false

      def apply_events([event | tail], last_version, %AggregateState{} = state) do

        new_client_state = apply_event(event, state.client_state)

        apply_events(tail, last_version, %AggregateState{state | client_state: new_client_state})

      end



      def apply_events([], last_version, %AggregateState{} = state),

        do: %AggregateState{state | version: last_version}



      @before_compile X3m.System.Aggregate

    end

  end



  defmacro __before_compile__(_env) do

    quote do

      defp apply_event(_event, state),

        do: state

    end

  end

end