download.ex

pid = Progress.start_link "test.jpg"
HTTPoison.get! "url", %{}, stream_to: pid

Progress.ex

defmodule Progress do 
  def start_link name do 
    {:ok, file} = File.open name, [:write]
    spawn_link(fn -> loop(file, 0, 0) end)
  end

  def loop file, size, total do
    receive do 
      %HTTPoison.AsyncChunk{chunk: chunk} ->
        IO.inspect {size, total}
        IO.binwrite file, chunk
        chunk_size = byte_size chunk
        loop file, (size + chunk_size), total
      %HTTPoison.AsyncEnd{} -> 
        File.close file
      %HTTPoison.Error{} ->    
        File.close file
      %HTTPoison.AsyncHeaders{headers: headers} -> 
        loop file, size, content_length(headers)
      %HTTPoison.AsyncRedirect{to: to} ->
        reget to
        loop file, size, total
      _ ->
        loop file, size, total
    end
  end

  defp content_length headers do  
    Enum.find_value(headers,
      fn {"Content-Length", total} -> String.to_integer total      
         _                         -> false end)
  end

  defp reget! to do
    HTTPoison.get! to, [], [stream_to: self, hackney: [follow_redirect: true]]
  end
end