Shiny: Error when referring to renderUI's id inside the module

Question

I'm building a Shiny module with renderUI where I refer to IDs produced inside the module server function:

library(shiny)
library(DT)
library(dplyr)

module_ui <- function(id){
  ns <- shiny::NS(id)
  shiny::tagList(
    fluidPage(uiOutput('test_ui'))

  )
}


module <- function(input, output, session) {

  ns <- session$ns

  output$test_ui <- renderUI({

    shiny::fluidPage(
      shiny::selectizeInput(
        inputId = ns('plot_vars'),
        label = 'Choose variables to plot',
        choices = colnames(mtcars),
        selected = colnames(select(mtcars, mpg, wt)),
        multiple = TRUE
      ),
      verbatimTextOutput(ns('text')),
      DT::dataTableOutput(ns('d_plot'))
    )

    })

  output$text <- renderText({
    input$plot_vars
  })

  output$d_plot <- DT::renderDataTable({

    input_data <- mtcars[, input$plot_vars]

    DT::datatable(input_data)

  })

}


ui <- module_ui('XXX')
server <- callModule(module, 'XXX')

shinyApp(ui, server)

Despite applying ns() religiously throughout, I'm getting the following error when running server module function:

Error in session$makeScope(id) : attempt to apply non-function

Can't see what's causing the problem here, any hints would be very much appreciated!

Answer 1

First, you have to use ns in module_ui :

module_ui <- function(id){
  ns <- NS(id)
  tagList(
    fluidPage(uiOutput(ns('test_ui')))
  )
}

You have to define ui and server like this:

ui <- fluidPage(
  module_ui('XXX')
)
server <- function(input, output, session){
  callModule(module, 'XXX')
}

Something else: set drop = FALSE when you select some columns of a dataframe:

input_data <- mtcars[, input$plot_vars, drop = FALSE]

otherwise you get a vector if you select only one column, with drop=FALSE you get a dataframe with a single column.