How to process Phoenix conn after render before it is sent as a response

Plug.Conn allows you to register a "before send" hook via register_before_send/2 function:

require Logger

Plug.Conn.register_before_send(conn, fn conn ->
  Logger.info("Sent a #{conn.status} response")
  conn
end)

It's very handy especially if you want to process conn with a plug. Here is an example from a simple minify_response library:

defmodule MinifyResponse.HTML do
  alias MinifyResponse.HTML

  def init(opts \\ []), do: opts

  def call %Plug.Conn{} = conn, _ \\ [] do
    Plug.Conn.register_before_send(conn, &HTML.minify_body/1)
  end

  def minify_body(%Plug.Conn{} = conn) do
    case List.keyfind(conn.resp_headers, "content-type", 0) do
      {_, "text/html" <> _} ->
        body = conn.resp_body
               |> Floki.parse
               |> Floki.raw_html

        %Plug.Conn{conn | resp_body: body}
      _ ->
        conn
    end
  end
end

As you can see in this example, you can for instance use the register_before_send/2 function to process already rendered HTML. However, this is just an example, and there are many other use cases worth exploring.

That's it! It's a very simple yet powerful trick. If you use it in your project to do some cool stuff - let me in the comments, I'd love to hear that!