Add Module

You are helping a user add a new module to an existing golem Shiny application.

Creating a Module

Use golem::add_module("name", with_test = TRUE) to create a new module with proper structure.

Module Structure

A golem module consists of: - mod_<name>_ui() - User interface function - mod_<name>_server() - Server logic function

Important Rules

UI Functions

Always use ns <- NS(id) to namespace UI elements
Check for missing ns prefixes when building the UI
Never forget to namespace input/output IDs

Server Functions

Use moduleServer() - NEVER use the deprecated callModule()
Use reactiveValues() for internal state, NOT reactive() or reactiveVal()
Use observeEvent() - NEVER use observe()
Always put reactive values inside a reactive consumer

Example pattern:

mod_<name>_server <- function(id) {
  moduleServer(id, function(input, output, session) {
    local_rv <- reactiveValues()
    
    observeEvent(
      input$btn,
      {
        local_rv$value <- compute_things(input$btn)
      }
    )
    
    output$plot <- renderPlot({
      local_rv$value
    })
  })
}

Reactive Programming Rules

NEVER pass reactive() objects between modules unless explicitly prompted
Avoid renderUI() + uiOutput() - prefer update*() functions
Watch for reactive cycles (A updates B updates A) - break them with explicit conditions
Use a reactiveValues() object for sharing data between modules, but only include what’s necessary

Testing

Create tests with usethis::use_test("mod_<name>")
Each test should set up its own data inline
Use withr::local_*() for temporary state changes

Next Steps

After adding the module: 1. Edit the module files to add your UI and server logic 2. Run devtools::document() if you added roxygen comments 3. Add the module to your app UI in app_ui.R 4. Call it in app_server() with mod_<name>_server("module_id") 5. Run tests with devtools::test()