Create a block

Write custom blocks in pure R to extend blockr with your own logic. A block is a specialized Shiny module that returns an expression (the R code it generates) and a state (its current input values).

Block anatomy

Every block has three parts:

Block
├── UI function        → defines the user interface (Shiny UI)
├── Server function    → handles reactive logic, returns expr + state
└── Constructor        → wraps UI + server, initializes defaults

UI function

A standard Shiny module UI. Takes a single id argument and returns shiny.tag objects:

my_block_ui <- function(id) {
  ns <- shiny::NS(id)
  shiny::tagList(
    shiny::selectInput(ns("column"), "Column", choices = NULL),
    shiny::numericInput(ns("threshold"), "Threshold", value = 0)
  )
}

Server function

Returns a list with two reactive elements: expr (a quoted R expression) and state (a named list of current input values):

my_block_server <- function(id, data) {
  shiny::moduleServer(id, function(input, output, session) {
    # Update choices based on incoming data
    shiny::observe({
      shiny::updateSelectInput(session, "column",
        choices = names(data()))
    })

    list(
      expr = shiny::reactive({
        rlang::expr(
          dplyr::filter(data, !!rlang::sym(input$column) > !!input$threshold)
        )
      }),
      state = shiny::reactive(list(
        column = input$column,
        threshold = input$threshold
      ))
    )
  })
}

Constructor

Wraps UI and server together with default state values:

new_my_block <- function(column = character(), threshold = 0) {
  new_block(
    ui = my_block_ui,
    expr_server = my_block_server,
    state = list(column = column, threshold = threshold),
    class = c("my_block", "transform_block")
  )
}

Registering your block

Blocks must be registered so they appear in the block menu. Use .onLoad() in your package:

.onLoad <- function(libname, pkgname) {
  blockr.core::register_block(
    constructor = new_my_block,
    name = "My custom filter",
    description = "Filter rows by a numeric threshold",
    category = "transform",
    package = pkgname
  )
}

You can also register blocks dynamically at runtime:

register_block(
  constructor = new_my_block,
  name = "My custom filter",
  description = "Filter rows by a numeric threshold",
  category = "transform"
)

Query what's available with available_blocks().

Further reading

• Full create-block vignette: detailed walkthrough with advanced examples
• Block registry vignette: registry system details
• Extend blockr vignette: plugins and custom UI