# Building Custom Modules

## Introduction

The modules in VizModules are designed to be composed and extended. You can build higher-level modules that add custom logic such as data filtering, transformations, or additional UI controls while reusing the full functionality of the base modules.

This vignette demonstrates how to create a custom module by building on top of the `scatterPlot` module.

## The Pattern

When building a custom module, you need to handle Shiny’s namespacing correctly. The key insight is:

1. Your module’s custom inputs need to be namespaced with `NS(id)`.
2. The base module’s UI and server functions should receive the bare `id`, not a namespaced version.
3. Data processing that requires access to your module’s inputs must happen inside a `moduleServer()` block.
4. The base module’s server should be called outside that `moduleServer()` block to avoid double-namespacing issues.

## A Minimal Example

Let’s build a custom module that adds a simple filtering checkbox to the `scatterPlot` module.

### The UI

    library(VizModules)

    minimalModuleUI <- function(id) {
        ns <- NS(id)
        tagList(
            h4("Minimal Module Controls"),
            # Custom input - uses the module's namespace
            checkboxInput(ns("filter_setosa"), "Start with Setosa Only", value = FALSE),
            hr(),
            # Base module UI - pass the bare 'id', not ns(id)
            dittoViz_scatterPlotInputsUI(id, iris)
        )
    }

    minimalModuleOutput <- function(id) {
        # Simply delegate to the base module's output UI
        dittoViz_scatterPlotOutputUI(id)
    }

Notice that `checkboxInput()` uses `ns("filter_setosa")` to namespace the custom input, while `dittoViz_scatterPlotInputsUI()` receives the bare `id`. This ensures the base module creates its inputs in the correct namespace.

### The Server

    minimalModuleServer <- function(id, data_reactive) {
        # Step 1: Process data inside a moduleServer block
        # This gives us access to inputs namespaced to 'id' (our module's inputs)
        filtered_data <- moduleServer(id, function(input, output, session) {
            reactive({
                req(data_reactive())
                df <- data_reactive()

                # Input specific to this custom module
                if (isTRUE(input$filter_setosa)) {
                    if ("Species" %in% names(df)) {
                        df <- df[df$Species == "setosa", ]
                    }
                }
                df
            })
        })

        # Step 2: Call the base module server OUTSIDE the moduleServer block
        # This is critical! If we called this inside the moduleServer above,
        # dittoViz_scatterPlotServer would look for inputs at id-id-inputName instead of id-inputName
        dittoViz_scatterPlotServer(id, filtered_data)
    }

Why this pattern?

- `moduleServer(id, ...)` gives us access to `input$filter_setosa`, which is namespaced to our wrapper’s `id`.
- By calling `dittoViz_scatterPlotServer(id, filtered_data)` outside the `moduleServer()` closure, the base module attaches to the same namespace as our UI, not a nested one.
- If we called `dittoViz_scatterPlotServer()` inside the `moduleServer()` block, it would create nested namespaces like `id-id-x_axis`, which wouldn’t match the actual input IDs in the UI.

### Putting It Together

    ui <- fluidPage(
        titlePanel("Minimal Module Example"),
        sidebarLayout(
            sidebarPanel(
                minimalModuleUI("demo")
            ),
            mainPanel(
                minimalModuleOutput("demo")
            )
        )
    )

    server <- function(input, output, session) {
        # Pass a reactive data source to the module
        minimalModuleServer("demo", reactive({
            iris
        }))
    }

    shinyApp(ui, server)

## Hiding Base Module Inputs

If your module pre-sets certain parameters, you can hide those inputs from the user to keep them from being changed:

    focusedModuleUI <- function(id) {
        ns <- NS(id)
        tagList(
            h4("Simplified Scatter Plot"),
            # Hide a few parameters
            dittoViz_scatterPlotInputsUI(id, iris,
                hide.inputs = c("shape.by", "color.by")
            )
        )
    }

## Best Practices

1. Keep wrapper logic focused: Each wrapper should add a cohesive set of related functionality.

2. Document the data requirements: If your wrapper expects certain columns or data types, document this clearly.

3. Use reactive expressions: Use reactive data inputs.

4. Test the namespace: If inputs aren’t working, check that you’re handling namespaces correctly. A common symptom of namespace issues is that inputs seem to have no effect.

5. Consider composability: Design your wrappers so they could potentially be wrapped by even higher-level modules.

## See Also

- The base modules (`dittoViz_scatterPlotInputsUI`, `dittoViz_scatterPlotOutputUI`, `dittoViz_scatterPlotServer`) are documented in the package reference.