How can I have a Shiny app calling a module, which in turn calls another module?

I have a simple Shiny app which calls 2 modules - this works.

However, I'm trying to transform it so that Shiny app calls a module, which in turn calls a sub-module. Please see code below, I can't work out why it won't work:

I'm trying to solve this problem so that I understand how to translate another application into Golem.

################## Sub-module ###########
mod_b_ui <- function(id) {
    ns <- NS(id)
    tagList(
        actionButton(ns("validate"), "Print")
    )
}

mod_b_server <- function(id, react) {
    moduleServer(id, function(input, output, session) {
        observeEvent( input$validate , {
            print(react())
        })
    })
}

################## Module ############
mod_ui <- function(id) {
    ns <- NS(id)
    tagList(
        sliderInput(ns("choice"), "Choice", 1, 10, 5),
        mod_b_ui("mod_ui_2")
    )
}

mod_server <- function(id) {
    moduleServer(id, function(input, output, session) {
        res <- reactive({input$choice})
        mod_b_server("mod_ui_2", react = res)
    })
}

################### Application ############
library(shiny)
app_ui <- function() {
    fluidPage(
        mod_ui("mod_ui_1")
    )
}

app_server <- function(input, output, session) {
    res <- mod_server("mod_ui_1")
}

shinyApp(app_ui, app_server)

Solution

Whenever you are calling a submodule, you need to namespace the id of the submodule.

In other words, you should do mod_b_ui(ns("mod_ui_2")) inside mod_ui:

mod_ui <- function(id) {
  ns <- NS(id)
  tagList(
    sliderInput(ns("choice"), "Choice", 1, 10, 5),
    mod_b_ui(ns("mod_ui_2"))
  )
}

Here is the full working app.

################## Sub-module ###########
mod_b_ui <- function(id) {
  ns <- NS(id)
  tagList(
    actionButton(ns("validate"), "Print")
  )
}

mod_b_server <- function(id, react) {
  moduleServer(id, function(input, output, session) {
    observeEvent( input$validate , {
      print(react())
    })
  })
}

################## Module ############
mod_ui <- function(id) {
  ns <- NS(id)
  tagList(
    sliderInput(ns("choice"), "Choice", 1, 10, 5),
    mod_b_ui(ns("mod_ui_2"))
  )
}

mod_server <- function(id) {
  moduleServer(id, function(input, output, session) {
    res <- reactive({input$choice})
    mod_b_server("mod_ui_2", react = res)
  })
}

################### Application ############
library(shiny)
app_ui <- function() {
  fluidPage(
    mod_ui("mod_ui_1")
  )
}

app_server <- function(input, output, session) {
  res <- mod_server("mod_ui_1")
}

shinyApp(app_ui, app_server)