An R Shiny Modal in a Module

modalModule.R

# define the module UI
moduleUI <- function(id) {
  
  # create the namespace from the id
  ns <- NS(id)
  
  #  As this is a function, the last statement will be the return value.
  #  Make sure it contains all of the UI elements you want to display

  fluidPage(
    fluidRow(
      actionButton(ns("show"), "Show modal dialog"),
      verbatimTextOutput(ns("dataInfo"))
    )
  )

}

# define the module "business logic"
module <- function(input, output, session) {

    # reactiveValues object for storing current data set.
    vals <- reactiveValues(data = NULL)

    # Return the UI for a modal dialog with data selection input. If 'failed' is
    # TRUE, then display a message that the previous value was invalid.
    dataModal <- function(failed = FALSE) {
      
      # load the namespace
      ns <- session$ns

      # build the modal
      modalDialog(

        # make sure to wrap all id tokens in ns()
        textInput(ns("dataset"), "Choose data set",
          placeholder = 'Try "mtcars" or "abc"'),

        span('(Try the name of a valid data object like "mtcars", ',
             'then a name of a non-existent object like "abc")'),

        if (failed)
          div(tags$b("Invalid name of data object", style = "color: red;")),

        footer = tagList(
          modalButton("Cancel"),
          actionButton(ns("ok"), "OK") # wrapped in ns()
        )

      )
    }

    # Show modal when button is clicked.
    observeEvent(input$show, {
      showModal(dataModal())
    })

    # When OK button is pressed, attempt to load the data set. If successful,
    # remove the modal. If not show another modal, but this time with a failure
    # message.
    observeEvent(input$ok, {
      
      print(input$dataset)
      
      # Check that data object exists and is data frame.
      if (!is.null(input$dataset) && nzchar(input$dataset) &&
          exists(input$dataset) && is.data.frame(get(input$dataset))) {
        vals$data <- get(input$dataset)
        removeModal()
      } else {
        showModal(dataModal(failed = TRUE))
      }
    })

    # Display information about selected data
    output$dataInfo <- renderPrint({
      if (is.null(vals$data))
        "No data selected"
      else
        summary(vals$data)
    })
    
}

modalModuleApp.R

source("modalModule.R")

#  Modularised version of: https://shiny.rstudio.com/reference/shiny/0.14/modalDialog.html
#  Created using instructions at: https://stackoverflow.com/a/48127806/2925434

# get the module UI from the sourced file
ui = basicPage(
  moduleUI("main")
)

# call the module with the same id ("main") as above
server = function(input, output) {
  callModule(module, "main")
}

# run the app
shinyApp(ui, server)