How to get screen resolution from JavaScript in R Shiny?

Question

I want to get screen resolution from JavaScript, stored in the GetScreenWidth variable on Shiny Server.

I tried the reference:

1. Receiving data from .js in server.R shiny
2. https://ryouready.wordpress.com/2013/11/20/sending-data-from-client-to-server-and-back-using-shiny/

So I have:

ui.R

shinyUI(
  bootstrapPage(

    verbatimTextOutput("results")
    ,tags$script('
        var jsWidth = screen.width;
        Shiny.onInputChange("GetScreenWidth",jsWidth);
    ')
   )
 )

server.R

 shinyServer(function(input,output){

     output$results=renderPrint({
     input$GetScreenWidth
   })

 })

It will return NULL by verbatimTextOutput.

How should I modify the code? Thanks!

Answer 1

The problem is that you're running the JavaScript code before Shiny is initialized. You can use the new feature that tells you when shiny is ready, here's example code

jscode <-
'$(document).on("shiny:connected", function(e) {
  var jsWidth = screen.width;
  Shiny.onInputChange("GetScreenWidth",jsWidth);
});
'

library(shiny)
runApp(shinyApp(
  ui = fluidPage(
    tags$script(jscode)
  ),
  server = function(input, output, session) {
    observe({
      cat(input$GetScreenWidth)
    })
  }
))