Removing Shiny Objects

Inputs

The front-end part of shiny inputs can easily be removed by using the removeUI function, however this does not impact the server side, where the input is still accessible.

library(shiny)

ui <- fluidPage(
  numericInput("number", "Select number:", 5L, 1L, 10L),
  actionButton("delete", "Remove input"),
  p("Selected number:", textOutput("number_out", inline = TRUE))
)

server <- function(input, output, session) {
  output$number_out <- renderText(input$number)

  observeEvent(input$delete, removeUI("#number"))
}

shinyApp(ui, server)

Shiny application removing the numeric input with removeUI

With removeInput, both the front and back end remove the input, and any observer or output that references the removed input will be updated. Whilst the trick of using .subset2(input, "impl")$remove(id) is known to remove the input, it does not trigger anything. Within the input object, the names are all stored within the .namesOrder and needs to be removed from here too.

After the input has been removed, the session needs to be aware that the input has been removed, otherwise nothing will be updated. This is where the invalidation of the various values is required, referencing the names and value dependency environments within the input object.

With all this resolved, the input is now fully removed from the shiny instance.

library(shiny)
library(shiny.destroy)

ui <- fluidPage(
  numericInput("number", "Select number:", 5L, 1L, 10L),
  actionButton("delete", "Remove input"),
  p("Selected number:", textOutput("number_out", inline = TRUE))
)

server <- function(input, output, session) {
  output$number_out <- renderText(input$number %||% "input unavailable")

  observeEvent(
    input$delete,
    removeInput("number", selector = ":has(> #number)")
  )
}

shinyApp(ui, server)

Shiny application removing the numeric input with removeInput

Output

Outputs can be removed in shiny applications by using a combination of removeUI and assigning NULL to the relevant output ID server-side; removeOutput is a wrapper for both operations. When assigning the output NULL, rather than removing the output entirely, it instead creates a reactive using req(FALSE) so that the output will never be updated.

library(shiny)
library(shiny.destroy)

ui <- fluidPage(
  numericInput("bins", "Number of bins:", min = 1L, max = 50L, value = 30L),
  actionButton("delete", "Remove output"),
  plotOutput("distPlot", height = "200px", width = "400px")
)

server <- function(input, output, session) {
  output$distPlot <- renderPlot({
    x <- faithful[, 2L]
    bins <- seq(min(x), max(x), length.out = input$bins + 1L)
    hist(
      x,
      breaks = bins,
      col = "darkgray",
      border = "white",
      xlab = "Waiting time to next eruption (in mins)",
      main = "Histogram of waiting times"
    )
  })

  observeEvent(input$delete, removeOutput("distPlot"))
}

shinyApp(ui, server)

Shiny application removing the plot output with removeOutput

Module

In larger shiny applications, modules are beneficial to reduce the application down into smaller, reusable chunks. There are situations where these modules can be dynamically added to the application, but also want to be removed. By using removeUI, we can remove the HTML of the module, however there are still a load of server-side processes that will be running even with the module “removed”. {shiny.destroy} makes sure that any trace of the module is no longer available within the application.

shiny::runExample(
  "01_boxes",
  package = "shiny.destroy"
)

The inputs and outputs are removed in the same way as an individual input or output, however additional objects in a module need to be addressed to properly destroy the module. observe and observeEvent are eager, and execute whenever any of the triggers are updated. If you assign an observer to a variable, you can see that it too is an R6 object with several methods. To prevent an observer from evaluating, you can use the $destroy() method.

library(shiny)

ui <- fluidPage(
  actionButton("update", "Increase"),
  actionButton("stop", "Stop increasing"),
  p("Updates:", textOutput("number_out", inline = TRUE))
)

server <- function(input, output, session) {
  clicks <- reactiveVal(0L)

  update_obs <- observeEvent(input$update, clicks(clicks() + 1L))

  observeEvent(input$stop, update_obs$destroy())

  output$number_out <- renderText(clicks())
}

shinyApp(ui, server)

Shiny application stopping the counter observer increase

When it comes to modules, these triggers can potentially be inputs or reactive values that are passed into the module. Removing a module from the shiny session won’t disable these observers, so in order for destroyModule to remove these observers, the module function needs to be passed into makeModuleServerDestroyable. This assigns all observers to the user session data so that when the module is destroyed, the observers can be too.