Detect screen size and pixel density on the server-side?

Question

I've been doing some research and I think I know the answer already, but I'm wondering if there's any means by which you can get a device's screen size and pixel density without the use of javascript or relying on CSS3 media queries.

Essentially, I'm looking into what it would take to get the screen resolution and pixel density so that the server could decide which image to server in a URI request.

So far I've not found anything that says this is even possible but I thought hey, why not ask?

Answer 1

I had the same problem and solved it using a getter and a setter route for the window height. If the $height variabale is 0 the get_heigt.erb is served, otherwise the index.erb This is a one user app, so I use a global variable, with different users you would have to keep that info in cookies. Here the code that matters.

Controller:

get "/" do
  if $height == 0
    erb :get_height
  else
    erb :index
  end
end

get "/get_height" do
  erb :get_height
end

get "/set_height" do
  $height = params[:height]
  redirect "/"
end

get_height.erb

<script type="text/javascript">
  function send_message(message) {
    var xhttp = new XMLHttpRequest();
    xhttp.onreadystatechange = function() {
      document.location.reload(true);
    }
    xhttp.open("GET", "http://localhost:4567/" + message + "?height=" + window.innerHeight, true);
    xhttp.send();
  }

  send_message('set_height');
</script>