build-shiny-module

Build Shiny Module

Create reusable Shiny UI/server module pairs with proper namespace isolation, reactive communication, and composability.

When to Use

• Extracting a reusable component from a growing Shiny app
• Building a UI widget that will be used in multiple places
• Encapsulating complex reactive logic behind a clean interface
• Composing larger applications from smaller, testable units

Inputs

• Required: Module purpose and functionality description
• Required: Input/output contract (what the module receives and returns)
• Optional: Whether the module nests other modules (default: no)
• Optional: Framework context (golem, rhino, or vanilla)

Procedure

Step 1: Define the Module Interface

Before writing code, define what the module accepts and returns:

Module: data_filter
Inputs: reactive dataset, column names to filter on
Outputs: reactive filtered dataset
UI: filter controls (selectInput, sliderInput, dateRangeInput)

Got: Clear contract specifying reactive inputs, reactive outputs, and UI elements.

If fail: If the interface is unclear, the module is probably too broad. Split it into smaller modules with single responsibilities.

Step 2: Create the Module UI Function

#' Data Filter Module UI
#'
#' @param id Module namespace ID
#' @return A tagList of filter controls
#' @export
dataFilterUI <- function(id) {
  ns <- NS(id)
  tagList(
    selectInput(
      ns("column"),
      "Filter column",
      choices = NULL
    ),
    uiOutput(ns("filter_control")),
    actionButton(ns("apply"), "Apply Filter", class = "btn-primary")
  )
}

Key rules:

• Function name follows <name>UI convention
• First argument is always id
• Create ns <- NS(id) at the top
• Wrap every inputId and outputId with ns()
• Return a tagList() to allow flexible placement

Got: UI function that creates namespaced input/output elements.

If fail: If IDs collide when using the module twice, check that every ID is wrapped with ns(). Common miss: IDs inside renderUI() or uiOutput() — these need ns() too.