R shiny conditionalPanel output value - r

Question

There are many questions about conditionalPanel in R shiny, but I still don't understand how I can use values created by server.R for conditionalPanel. Here is what I would like to do: I have a URL like http://some-url.com/php/session_check.php?sid=session_id. When the session_id starts with a 0, like http://some-url.com/php/session_check.php?sid=00221245 a string with a username is returned (e.g. 'testuser'). When the session_id starts with any other number but 0, like http://some-url.com/php/session_check.php?sid=10221245 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:
conditionalPanel(
condition="output.disable_ui!=0"
I know that this is the wrong, but I don't really understand how the condition argument works for outputs, as the same would work if I would do this for any input from ui.R.
Here is my sample code:
server.R
library(shiny)
library(raster)
library(rgdal)
shinyServer(function(input, output, clientData) {
output$disable_ui<-reactive({
query<-parseQueryString(clientData$url_search)
url_path<-paste(sep="","http://some-url.com/php/session_check.php?sid=",query, collapse="")
read.table(url_path)
})
data <- reactive({
inFile <- input$example_layer
if (is.null(inFile))
return(NULL)
raster.file<- raster(inFile$datapath)
})
output$raster.plot <- renderPrint({
"Nothing to see here"
})
})
ui.R
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")
)
))

Answer 1

#Julien Navarre is right: the output must be rendered. Except if you set its option suspendWhenHidden to FALSE:
output$disable_ui<-reactive({
query<-parseQueryString(clientData$url_search)
url_path<-paste(sep="","http://some-url.com/php/session_check.php?sid=",query, collapse="")
read.table(url_path)
})
outputOptions(output, 'disable_ui', suspendWhenHidden=FALSE)

Answer 2

I think that the output must be rendered in the UI if you want to use it after in the condition of a conditionalPanel.
With you example, the HTML for the conditional panel will look like something like this :
<div data-display-if="output.disable_ui!=0">
If no elements in your page (created as outputs in the server side) have the id "disable_ui" then the condition "output.disable_ui!=0" is always TRUE, and the conditional panel always displayed.
A simple example :
shiny::runApp(list(
ui = pageWithSidebar(
headerPanel("test"),
sidebarPanel(
selectInput(
"var", "Var",
0:9)),
mainPanel(
verbatimTextOutput("id"),
conditionalPanel(
condition="output.id!=0",
h4('Visible')
)
)
),
server = function(input, output) {
output$id<-reactive({input$var})
}
))
If you select a number different of 0 the conditional panel will be displayed.
Now, comment the line verbatimTextOutput("id"),, there is no more element with the id "id" in the page and then the condition of the conditional panel <div data-display-if="output.id!=0"> can't be FALSE.

Answer 3

This is the real answer to this question:
Use this inside of your server function:
outputOptions(output, "outputId", suspendWhenHidden = FALSE)
You will then be able to use the output.item in your conditionalPanel.
Answer from here:
https://github.com/daattali/advanced-shiny/blob/master/server-to-ui-variable/app.R
And here:
https://github.com/rstudio/shiny/issues/1318