defprotocol Membrane.EventProtocol do
  @moduledoc """
  Protocol that allows to configure behaviour of `Membrane.Event`s.

  Each event has to implement or derive this protocol.
  """

  @type t :: struct

  @doc """
  Specifies whether event is sent right away (not sticky), or it is 'pushed' by
  the next sent buffer (sticky). Defaults to false (not sticky).

  Returning a sticky event from a callback stores it in a queue. When the next
  buffer is to be sent, all events from the queue are sent before it. An example
  can be the `Membrane.Event.StartOfStream` event.
  """
  @spec sticky?(t) :: boolean
  def sticky?(_event)

  @doc """
  Determines whether event is synchronized with buffers (sync) or not (async).
  Defaults to false (sync).

  Buffers and sync events are always received in the same order they are
  sent. Async events are handled before any enqueued buffers that are waiting to
  be processed (e.g. in `Membrane.PullBuffer`).
  """
  @spec async?(t) :: boolean
  def async?(_event)
end

defmodule Membrane.EventProtocol.DefaultImpl do
  @moduledoc """
  Default implementation of `Membrane.EventProtocol`.

  If `use`d in `defimpl`, not implemented callbacks fallback to default ones.
  """
  defmacro __using__(_args) do
    quote do
      @impl true
      def sticky?(_event), do: false

      @impl true
      def async?(_event), do: false

      defoverridable async?: 1, sticky?: 1
    end
  end
end

defimpl Membrane.EventProtocol, for: Any do
  use Membrane.EventProtocol.DefaultImpl
end