Shiny renderUI selectInput returned NULL

Question

I am trying to use a reactivity model with one input affecting several outputs as describe in the shiny cheat sheet. I need to use renderUI because the choices list is rende

Answer 1

This might work if you use is.null() and return your "default" for input$A:

A.r <- reactive({
   if(is.null(input$A)) return (1)

   input$A 

})

Answer 2

Shiny has a function called observeEvent which I almost always use instead of observer. It basically runs some code only when a reactive value changes, and by default it ignored NULL values. So here is the code to make your example work (all I had to do is change your observe({ line to observeEvent(A.r(), {

library(shiny)

runApp(list(

  ui = bootstrapPage(
    fluidPage( uiOutput('ui.A')   )
  ),


  server = function(input, output){

    output$ui.A = renderUI({
      selectInput("A", label = h4("input A"), 
                  choices = list(A_1=1, A_2=2), 
                  selected = 1)
    })

    A.r <- reactive({input$A })

    observeEvent(A.r(), { 

      A <- A.r()
      str(A)

    })

  }))