Example of progressive downloading with HTTPotion, showing percentage and downloaded bytes as you go.

1-progressive_downloader.ex

defmodule ProgressiveDownloader do
  def run do
    url = "https://ia600308.us.archive.org/2/items/HealthYo1953/HealthYo1953_512kb.mp4"

    HTTPotion.get url, stream_to: self, timeout: :infinity

    receive_data(total_bytes: :unknown, data: "")
  end

  defp receive_data(total_bytes: total_bytes, data: data) do
    receive do

      %HTTPotion.AsyncHeaders{headers: h} ->

        {total_bytes, _} = h[:"Content-Length"] |> Integer.parse
        IO.puts "Let's download #{mb total_bytes}…"
        receive_data(total_bytes: total_bytes, data: data)

      %HTTPotion.AsyncChunk{chunk: new_data} ->

        accumulated_data = data <> new_data
        accumulated_bytes = byte_size(accumulated_data)
        percent = accumulated_bytes / total_bytes * 100 |> Float.round(2)
        IO.puts "#{percent}% (#{mb accumulated_bytes})"
        receive_data(total_bytes: total_bytes, data: accumulated_data)

      %HTTPotion.AsyncEnd{} ->

        path = "/tmp/posture.mp4"
        File.write!(path, data)
        IO.puts "All downloaded! See: #{path}"

    end
  end

  defp mb(bytes) do
    number = bytes / 1_048_576 |> Float.round(2)
    "#{number} MB"
  end
end

2-output.md

iex -S mix  # In a project with HTTPotion in mix.exs.
iex(1)> ProgressiveDownloader.run
Let's download 43.2 MB…
2.31% (1.0 MB)
4.63% (2.0 MB)
6.94% (3.0 MB)
9.26% (4.0 MB)
11.57% (5.0 MB)
13.89% (6.0 MB)
16.2% (7.0 MB)
18.52% (8.0 MB)
20.83% (9.0 MB)
23.15% (10.0 MB)
25.46% (11.0 MB)
27.78% (12.0 MB)
30.09% (13.0 MB)
32.4% (14.0 MB)
34.72% (15.0 MB)
37.03% (16.0 MB)
39.35% (17.0 MB)
41.66% (18.0 MB)
43.98% (19.0 MB)
46.29% (20.0 MB)
48.61% (21.0 MB)
50.92% (22.0 MB)
53.24% (23.0 MB)
55.55% (24.0 MB)
57.87% (25.0 MB)
60.18% (26.0 MB)
62.49% (27.0 MB)
64.81% (28.0 MB)
67.12% (29.0 MB)
69.44% (30.0 MB)
71.75% (31.0 MB)
74.07% (32.0 MB)
76.38% (33.0 MB)
78.7% (34.0 MB)
81.01% (35.0 MB)
83.33% (36.0 MB)
85.64% (37.0 MB)
87.96% (38.0 MB)
90.27% (39.0 MB)
92.58% (40.0 MB)
94.9% (41.0 MB)
97.21% (42.0 MB)
99.53% (43.0 MB)
100.0% (43.2 MB)
All downloaded! See: /tmp/posture.mp4

I tried to use this snippet, but apparently there's something wrong with HTTPotion at some point:

iex(1)⋅❯ ProgressiveDownloader.run
** (Protocol.UndefinedError) protocol List.Chars not implemented for #PID<0.139.0>
    (elixir) lib/list/chars.ex:1: List.Chars.impl_for!/1
    (elixir) lib/list/chars.ex:12: List.Chars.to_char_list/1
             lib/httpotion.ex:157: anonymous fn/1 in HTTPotion.request/5
    (elixir) lib/enum.ex:1043: anonymous fn/3 in Enum.map/2
    (elixir) lib/enum.ex:1385: Enum."-reduce/3-lists^foldl/2-0-"/3
    (elixir) lib/enum.ex:1043: Enum.map/2
             lib/httpotion.ex:157: HTTPotion.request/5
    (sploit) lib/1-progressive_downloader.ex:5: ProgressiveDownloader.run/0