title: R shiny conditionalPanel output value
body: <p>There are many questions about <code>conditionalPanel</code> in R shiny, but I still don't understand how I can use values created by server.R for <code>conditionalPanel</code>. Here is what I would like to do: I have a URL like <a href="http://some-url.com/php/session_check.php?sid=session_id" rel="noreferrer">http://some-url.com/php/session_check.php?sid=session_id</a>. When the <code>session_id</code> starts with a 0, like <a href="http://some-url.com/php/session_check.php?sid=00221245" rel="noreferrer">http://some-url.com/php/session_check.php?sid=00221245</a> a string with a username is returned (e.g. 'testuser'). When the session_id starts with any other number but 0, like <a href="http://some-url.com/php/session_check.php?sid=10221245" rel="noreferrer">http://some-url.com/php/session_check.php?sid=10221245</a> a 0 is returned. Now I would like to hide a panel, depending on whether the a 0 or a username is returned. Therefore I try to do something like this:</p>
  
  <pre><code>conditionalPanel(
   condition="output.disable_ui!=0"
  </code></pre>
  
  <p>I know that this is the wrong, but I don't really understand how the <code>condition</code> argument works for <code>outputs</code>, as the same would work if I would do this for any <code>input</code> from <code>ui.R</code>.</p>
  
  <p>Here is my sample code:</p>
  
  <p><strong>server.R</strong></p>
  
  <pre><code>library(shiny)
  library(raster)
  library(rgdal)
  
  shinyServer(function(input, output, clientData) {
  
    output$disable_ui&lt;-reactive({
      query&lt;-parseQueryString(clientData$url_search)
      url_path&lt;-paste(sep="","http://some-url.com/php/session_check.php?sid=",query, collapse="")
      read.table(url_path)
    })
  
    data &lt;- reactive({  
      inFile &lt;- input$example_layer 
  
      if (is.null(inFile)) 
        return(NULL)
      raster.file&lt;- raster(inFile$datapath) 
    })
  
    output$raster.plot &lt;- renderPrint({
      "Nothing to see here"
    })
  })
  </code></pre>
  
  <p><strong>ui.R</strong></p>
  
  <pre><code>library(shiny)
  
  shinyUI(pageWithSidebar(
  
    headerPanel("test"),
  
    sidebarPanel(
      conditionalPanel(
        condition="output.disable_ui!=0",
  
      #File Upload
      fileInput('example_layer', 'Choose Raster Layer (ASCII)', multiple=FALSE, accept='asc')
  
    )),
  
    mainPanel(
      verbatimTextOutput("raster.plot")
    )
  ))
  </code></pre>