shiny checkbox unchecks - cross-referring to inputs in same renderUI call - r

Question

I intended to have some optional extra buttons appearing in my shiny app using renderUI. I would prefer to have new buttons inserted in the one and same renderUI call. I wrote a renderUI expession were the second button only is rendered if input of fist button is not null and(&&) is TRUE. It does not work as first button rapidly unchecks itself again whenever checked. I made a solution by splitting into two renderUI calls.
My question are:
Why exactly does the first code piece fail?
Would it be possible to achieve what the second code piece does in only one renderUI call?
.
library(shiny)
ui = fluidPage(
uiOutput("checkUI")
)
server = function(input,output) {
output$checkUI = renderUI({
list(
checkboxInput('check1',"check me first"),
if(!is.null(input$check1)&&input$check1==T) {
checkboxInput('check2',"check me to win")
} else {
NULL
}
)
})
}
shinyApp(ui,server)
but this works...
ui = fluidPage(
uiOutput("checkUI"),
uiOutput("textUI")
)
server = function(input,output) {
#UI first button
output$checkUI = renderUI({
list(
checkboxInput('check1',"check me first")
)
})
#UI second button
output$textUI = renderUI({
list(
if(!is.null(input$check1) && input$check1)
checkboxInput('check2',"check me to win") else NULL
)
})
}
shinyApp(ui,server)

Answer 1

Your first piece of code fails because the renderUI runs every time there is a change in input, so when you check the first check box. Since you have:
checkboxInput('check1',"check me first")
The renderUI immediately resets the input$check1 and then the renderUI is run again, unsetting the checkbox. This is why the second checkbox briefly flashes.
Luckily this is a common problem, so there is a Shiny solution to this with conditionalPanels:
library(shiny)
ui = fluidPage(
checkboxInput('check1',"check me first"),
conditionalPanel(
condition = "input.check1 == true",
checkboxInput("check2", "check me to win")
)
)
server = function(input,output){
}
shinyApp(ui,server)