The easiest way would be to handle all this in the consume stage with some external tool like ffprobe (from ffmpeg).

  def handle_event("save", _params, socket) do
    uploaded_files =
      consume_uploaded_entries(socket, :avatar, fn %{path: path}, _entry ->
        dest =
          Path.join([:code.priv_dir(:my_app), "static", "uploads", "#{Path.basename(path)}.mp4"])

        File.cp!(path, dest)
        # getting the duration with ffprobe
        {time_in_seconds, _error} = System.cmd("ffprobe", [
          "-i",
          dest,
          "-v",
          "quiet",
          "-show_entries",
          "format=duration",
          "-hide_banner",
          "-of",
          "default=noprint_wrappers=1:nokey=1"
        ])

        # printing the duration
        IO.inspect(time_in_seconds, label: "Duration")

        {:ok, Routes.static_path(socket, "/uploads/#{Path.basename(dest)}")}
      end)

    {:noreply, update(socket, :uploaded_files, &(&1 ++ uploaded_files))}
  end

To do it during the upload you probably will need to hook up to websocket and get the data directly from the binary data that is sent through the wire.

image1200×634 76 KB

To be honest I don’t even know if that’s possible, let’s hope that someone more knowledgeable than me can chime in, but that’s so over the top that that’s a solution that I would only look for if the requirements are so rigid that there’s no other way to do it.

In Javascript, you can process that metadata in the loadedmetadata event handler during a file upload. In theory, you could use Hooks and a Javascript uploader in app.js, but I don’t think you’d be able to use the LiveView Uploads feature.

I’ll use this approach if all else fails

The ffmpex module provides a wrapper for the FFmpeg library, giving access to FFprobe

Overview — ffmpex v0.10.0 (hexdocs.pm)

FFprobe.duration("/path/to/input.mp4")
# => result in seconds, e.g., 228.456939

Using your System.cmd/3 example, I was able to figure out how to retrieve metadata from audio, video and image files while consuming the files from the uploader.

I learned that the mediainfo library retrieves image metadata better than ffprobe, and was able to retrieve metadata for all 3 file types using mediainfo

I had a problem with mediainfo when integrating it with the System.cmd/3 because the cmd line format requires double quote characters around an attribute value. When used in the System.cmd/3 format, those value double quote characters need to be removed.

Cmd line width extraction from image file

mediainfo /filepath --Inform="Image;%Width%"

System.cmd/3 width extraction from image file

          {width, _error} = System.cmd("mediainfo", [
            dest,
            "--Inform=Image;%Width%"
          ])