modalDialog(
  ...,
  title = NULL,
  footer = modalButton("Dismiss"),
  size = c("m", "s", "l"),
  easyClose = FALSE,
  fade = TRUE
)

modalButton(label, icon = NULL)

if (interactive()) {
# Display an important message that can be dismissed only by clicking the
# dismiss button.
shinyApp(
  ui = basicPage(
    actionButton("show", "Show modal dialog")
  ),
  server = function(input, output) {
    observeEvent(input$show, {
      showModal(modalDialog(
        title = "Important message",
        "This is an important message!"
      ))
    })
  }
)


# Display a message that can be dismissed by clicking outside the modal dialog,
# or by pressing Esc.
shinyApp(
  ui = basicPage(
    actionButton("show", "Show modal dialog")
  ),
  server = function(input, output) {
    observeEvent(input$show, {
      showModal(modalDialog(
        title = "Somewhat important message",
        "This is a somewhat important message.",
        easyClose = TRUE,
        footer = NULL
      ))
    })
  }
)


# Display a modal that requires valid input before continuing.
shinyApp(
  ui = basicPage(
    actionButton("show", "Show modal dialog"),
    verbatimTextOutput("dataInfo")
  ),

  server = function(input, output) {
    # 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) {
      modalDialog(
        textInput("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("ok", "OK")
        )
      )
    }

    # 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$ok, {
      # Check that data object exists and is data frame.
      if (!is.null(input$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)
    })
  }
)
}