Rails' before_filter equivalent in Phoenix

This is better as a comment but I lack the rep; with the current version of Phoenix (1.3.4, August of 2018), if you use the top answer's code, you would only want to do plug :store_something: do not use plug :action as it is redundant. The actions will run after the plugs which you listed.

If you include plug :action you will get (Plug.Conn.AlreadySentError) the response was already sent as the action will run twice and Phoenix will be mad at you.

You can indeed achieve this with a Plug and Plug.Conn.assign.

defmodule TestApp.PageController do
  use TestApp.Web, :controller

  plug :store_something
  # This line is only needed in old phoenix, if your controller doesn't
  # have it already, don't add it.
  plug :action

  def index(conn, _params) do
    IO.inspect(conn.assigns[:something]) # => :some_data
    render conn, "index.html"
  end

  defp store_something(conn, _params) do
    assign(conn, :something, :some_data)
  end
end

Remember to add the plug declaration before your action plug, as they are executed in-order.