R shiny passing reactive to selectInput choices

server.R

### This will create the dynamic dropdown list ###

output$carControls <- renderUI({
    selectInput("cars", "Choose cars", rownames(mtcars))
})


## End dynamic drop down list ###

## Display selected results ##

txt <- reactive({ input$cars })
output$selectedText <- renderText({  paste("you selected: ", txt() ,sep="") })


## End Display selected results ##

ui.R

uiOutput("carControls"),
  br(),
  textOutput("selectedText")

You need to use renderUI on the server side for dynamic UIs. Here is a minimal example. Note that the second drop-down menu is reactive and adjusts to the dataset you choose in the first one. The code should be self-explanatory if you have dealt with shiny before.

runApp(list(
  ui = bootstrapPage(
    selectInput('dataset', 'Choose Dataset', c('mtcars', 'iris')),
    uiOutput('columns')
  ),
  server = function(input, output){
    output$columns = renderUI({
      mydata = get(input$dataset)
      selectInput('columns2', 'Columns', names(mydata))
    })
  }
))

EDIT. Another Solution using updateSelectInput

runApp(list(
  ui = bootstrapPage(
    selectInput('dataset', 'Choose Dataset', c('mtcars', 'iris')),
    selectInput('columns', 'Columns', "")
  ),
  server = function(input, output, session){
    outVar = reactive({
      mydata = get(input$dataset)
      names(mydata)
    })
    observe({
      updateSelectInput(session, "columns",
      choices = outVar()
    )})
  }
))

EDIT2: Modified Example using parse. In this app, the text formula entered is used to dynamically populate the dropdown menu below with the list of variables.

library(shiny)
runApp(list(
  ui = bootstrapPage(
    textInput("text", "Enter Formula", "a=b+c"),
    uiOutput('variables')
  ),
  server = function(input, output){
    outVar <- reactive({
      vars <- all.vars(parse(text = input$text))
      vars <- as.list(vars)
      return(vars)
    })

    output$variables = renderUI({
      selectInput('variables2', 'Variables', outVar())
    })
  }
))

As far as I can tell, the problem is that input$inBody does not retrieve a character even though the selectInput function is given a character as value, namely value = "a+b+c". The solution is therefore to wrap input$inBody in a as.character

The following works:

The observe approach with updateSelectInput:

observe({
     input$inBody
     vars <- all.vars(parse(text=as.character(input$inBody)))
     vars <- as.list(vars)
     updateSelectInput(session = session, inputId = "inVar", choices = vars)
})

The reactive approach with selectInput:

outVar <- reactive({
    vars <- all.vars(parse(text=as.character(input$inBody)))
    vars <- as.list(vars)
    return(vars)
})

output$inVar2 <- renderUI({
    selectInput(inputId = "inVar2", label = h4("Select:"), choices =  outVar())
})

Edit: I have edited my question with an explanation based on Ramnath's feedback. Ramnath has explained the problem and provided a better solution, which I give as an edit of my question. I'll keep this answer for the record.