Shiny: conditionally build UI - r

Question

I'm building a Shiny dashboard to show a large amount of data. People access the dashboard through a separate login page (non-Shiny) that sits in front, at which point a JWT is generated and put in a cookie. I've managed to read in the cookie in Shiny and parse the data, saving the token in a variable called userPermissions. Now, however, I'd like to show/hide tabs in the dashboard depending on the user permissions.
For example: I have data on both managers and assistants. I want to show the manager data to any user with userPermissions == 'manager', and the assistant data to anyone with userPermissions == assistant.
I thought the best way would be to use conditionalPanel() – here's a (simplified) reproducible example:
library(shiny)
# UI file
ui <- fluidPage(
# JS to read cookie -- simplified to just return value!
tags$head(tags$script(
HTML('
Shiny.addCustomMessageHandler("goReadTheCookie", function (message) {
Shiny.onInputChange("cookie", "manager");
})
')
)
# Title
,titlePanel('Test')
# Navbar
,navbarPage(
id="navbar"
,conditionalPanel(condition = "userPermissions() == 'manager'",
mainPanel(textOutput('Manager data')))
,conditionalPanel(condition = "userPermissions() == 'assistant'",
mainPanel(textOutput('Assistant data')))
)
))
# Server file
server <- shinyServer(function(input, output,session){
## Grab permissions from Cookie
# Prepare output
userPermissions <- reactiveVal("")
# Tell JS to return cookie
session$sendCustomMessage(type="goReadTheCookie", message=list(name="cookie_name"))
# Listen for cookie
observeEvent(input$cookie,{
## -- Bunch of code omitted for sake of brevity -- ##
userPermissions("manager")
})
})
# Run app
shinyApp(ui=ui, server=server)
The problem with this code is that, in a browser console, I get the error Can't find variable: userPermissions.
I guess what's going on here is that the entire ui is executed, before JS can grab and return the cookie. What would be the best way to solve this?
Or, maybe I'm going about this the wrong way. I obviously need to verify the cookie server-side (i.e., in R) not to divulge the secret; and preferably this check, and the hiding/showing is completed at the very start of the Shiny application (userPermissions won't change during the session). Maybe there's a different (& better) solution to get to that point?
Any help would be very much appreciated!

Answer 1

In the end I found that the function appendTab does exactly what I was looking for. This needs to be run in server.R though, within the function to look for the cookie (otherwise userPermissions indeed doesn't exist). I could then do:
appendTab("navbar" # the id of the navigation bar created in ui.R
,tabPanel("tab name"
,mainPanel(...))
)
where tabPanel(...) could be anything you'd normally put in ui.R.
The added benefit here is that hidden tabs are also not available in the HTML source, as they're never even passed from the server to the client!