Modify and Read Global variable after session in R Shiny

[ImSanjay]

I am trying to read a global variable which is declared before UI. In a scenario I am modifying the global variable during the session and in observer function. But when I want to print that modified variable after session kill, it prints old values itself. I want to keep this modified variable(in my requirement every time it changes on each session) for each session.

library(shiny)    

timeoutSeconds <- 5
varA <- 3 #Declaring Global variable    

#Trigger the function after the 5 seconds 
inactivity <- sprintf("
function idleTimer() {
var t = setTimeout(logout, %s);
window.onmousemove = resetTimer; // catches mouse movements
window.onmousedown = resetTimer; // catches mouse movements
window.onclick = resetTimer;     // catches mouse clicks
window.onscroll = resetTimer;    // catches scrolling
window.onkeypress = resetTimer;  //catches keyboard actions

function logout() {
   Shiny.setInputValue('timeOut', '%ss')
  }

function resetTimer() {
   clearTimeout(t);
   t = setTimeout(logout, %s);  // time is in milliseconds (1000 is 1 second)
   }
}
idleTimer();", 
timeoutSeconds*1000, 
timeoutSeconds, 
timeoutSeconds*1000)

#UI 
ui <- fluidPage(
  tags$script(inactivity)  

)

#Server
server <- shinyServer(function(input,output,session){


  observeEvent(input$timeOut, {
    varA <-varA + 1 #Modifing this global variable for each session

    print(paste0("Session (", session$token, ") timed out at: ", Sys.time()))
    showModal(modalDialog(
      title = "Timeout",
      paste("Session timeout due to", input$timeOut, "inactivity -", Sys.time()),
      footer = NULL
    ))
    session$close()
  })

  session$onSessionEnded(function() {

    #Should print 4
    print(varA) #Printing the modified  variable  during sessions

  })

})

runApp(ui,server)

I want to print 4 in the above code. I have tried multiple times, but may be it doesn't work like that. can you please help me anyone on this.

[pieterjanvc]

Hi,

You have to make the variable a reactive Shiny variable:
Within your server function first declare it, then modify.

Run this dummy app to see in action which variables update

library(shiny)

ui <- fluidPage(
  actionButton("myButton", "Click me"),
  uiOutput("myText")
)

server <- function(input, output, session) {
  
  var1 = 0
  var2 = reactiveVal(0)
  
  observeEvent(input$myButton, {
    
    #Try and update values
    var1 = var1 + 1
    var2(var2() + 1) #reactive variables are called/updated like a function
    
    #Print values on UI 
    output$myText = renderUI({
      HTML(paste("<h2>Static R variable var1 =", var1, 
            "<br>and Reactive Shiny Variable var2 =", var2(), "</h2>"))
    })
    
    
  })
  
}

shinyApp(ui, server)

[ImSanjay]

is it no possible to update within observeEvent function?

[pieterjanvc]

Hi,

I updated my previous post, as it was a bit confusing.

It shows you how static R variables can only be declared once in Shiny and will always reset to the original value when called again in a function. Shiny (reactive) variables on the other hand can get updated and will retain their value. It becomes important to learn when to use which but it can cause a lot of confusion in the beginning, but there are some good reasons (long story) why this is the case.

Hope this helps,
PJ