sessionInputs

runApp(list(
  ui = bootstrapPage(
    uiOutput("nUI"),
    plotOutput('plot'),
    submitButton("PlotIt")
  ),
  server = function(input, output, session) {
    output$plot <- renderPlot({ hist(runif(input$obs)) })
    
    output$nUI <- renderUI({ 
      query <- parseQueryString(session$clientData$url_search)
      
      if(is.null(input$obs) & !is.null(query$obs)) {
        obs <- query$obs
      } else {
        if(!is.null(input$obs)) {
          obs <- input$obs
        } else {
          obs <- 100
        }
      }
      numericInput('obs', 'Number of obs', obs) 
      })
  }
))