Shiny modules - including module CSS and JS without repeating

Question

I'm wondering what are the ways to include some CSS and possibly Javascript that's needed for a Shiny module to work, without duplicating that code when multiple instances of the module are called? For example, say I have a module with this UI:

my_module_UI <- function(id) {
  ns <- NS(id)
  tagList(
    tags$head(
      tags$link(rel = "stylesheet", type = "text/css", href = "special_button.css")
    ),
    actionButton(ns("button"), class = "special-button")
  )
}

my_module_server <- function(id) {
  moduleServer(
    id,
    function(input, output, session) {
    # Server-side functionality
   }
  )
}
      

This would work for styling the button appropriately. However, If I were to reuse this module at multiple times in the main app, the special_button.css stylesheet would be added multiple times. If I were to use inline CSS, the specific rules would be multiplied for every button instance.
Is there a way to provide these stylesheets as a singleton, while still keeping it a part of the module definition? I'd like to have everything related to the module kept inside the module, instead of providing JS and CSS in the main app.

Answer 1

You can wrap the link tag in htmltools::singleton:

my_module_UI <- function(id) {
    ns <- NS(id)
    tagList(
        tags$head(
            htmltools::singleton(
                tags$link(rel = "stylesheet", type = "text/css", href = "special_button.css")
            )
        ),
        actionButton(ns("button"), label = "button", class = "special-button")
    )
}