Shiny modules

Shiny's module feature lets you break complicated UI and server logic into smaller, self-contained pieces. Compared to large monolithic Shiny apps, modules are easier to reuse and easier to reason about. See the article at https://shiny.posit.co/articles/modules.html to learn more.

moduleServer(id, module, session = getDefaultReactiveDomain())

Arguments

id

An ID string that corresponds with the ID used to call the module's UI function.

module

A Shiny module server function.

session

Session from which to make a child scope (the default should almost always be used).

Value

The return value, if any, from executing the module server function

Details

Starting in Shiny 1.5.0, we recommend using moduleServer instead of callModule(), because the syntax is a little easier to understand, and modules created with moduleServer can be tested with testServer().

Destroying module reactivity

When module UI is added and removed dynamically (e.g. via insertUI() and removeUI()), the server-side reactive objects created by moduleServer() continue to run after the UI is removed. The parent inserted the module's UI under an id, so it can tear down all reactive values, expressions, and observers in that scope by that same id:

# In parent server:
myModuleServer("myid")
removeUI(selector = "#myid")
session$destroy("myid")

If teardown must happen somewhere that doesn't have the parent session or id, a module can instead return its own session$destroy as a handle:

myModuleServer <- function(id) {
  moduleServer(id, function(input, output, session) {
    # ... module logic ...

    # Return a cleanup function for the caller to invoke
    list(result = ..., destroy = session$destroy)
  })
}

mod <- myModuleServer("myid")
removeUI(selector = "#myid")
mod$destroy()

See the session help topic for details on composability and data ownership patterns when using session$destroy().

See also

session, removeUI(), https://shiny.posit.co/articles/modules.html

Examples

# Define the UI for a module
counterUI <- function(id, label = "Counter") {
  ns <- NS(id)
  tagList(
    actionButton(ns("button"), label = label),
    verbatimTextOutput(ns("out"))
  )
}

# Define the server logic for a module
counterServer <- function(id) {
  moduleServer(
    id,
    function(input, output, session) {
      count <- reactiveVal(0)
      observeEvent(input$button, {
        count(count() + 1)
      })
      output$out <- renderText({
        count()
      })
      count
    }
  )
}

# Use the module in an app
ui <- fluidPage(
  counterUI("counter1", "Counter #1"),
  counterUI("counter2", "Counter #2")
)
server <- function(input, output, session) {
  counterServer("counter1")
  counterServer("counter2")
}
if (interactive()) {
  shinyApp(ui, server)
}



# If you want to pass extra parameters to the module's server logic, you can
# add them to your function. In this case `prefix` is text that will be
# printed before the count.
counterServer2 <- function(id, prefix = NULL) {
  moduleServer(
    id,
    function(input, output, session) {
      count <- reactiveVal(0)
      observeEvent(input$button, {
        count(count() + 1)
      })
      output$out <- renderText({
        paste0(prefix, count())
      })
      count
    }
  )
}

ui <- fluidPage(
  counterUI("counter", "Counter"),
)
server <- function(input, output, session) {
  counterServer2("counter", "The current count is: ")
}
if (interactive()) {
  shinyApp(ui, server)
}