Three Shiny apps that export UI components to a separate Shiny module.

app1.R

# Simple example of exporting UI to another function (this works)

library(shiny)

tabUI <- function(id) {
  ns <- NS(id)
  fluidRow(
    textOutput(ns("filter"))
  )
}
filtersUI <- function(id) {
  ns <- NS(id)
  
  fluidRow(
    uiOutput(ns("ui"))
  )
}

filters <- function(input, output, session, ui) {
  observe({
    output$ui <- ui()
  })
  
}

tab <- function(input, output, session) {
  
  ns <- session$ns
  
  ui <- renderUI({
    selectizeInput(ns("select"), "Model", choices = unique(mpg$model))
  })
  
  output$filter <- renderText(input$select)
  
  return(reactive(ui))
}



ui <- fixedPage(
  wellPanel(
  tabUI("tab")
  ),
  wellPanel(
  filtersUI("filters")
  )
  
)

server <- function(input, output, session) {
  tab <- callModule(tab, "tab")
  
  callModule(filters, "filters", tab)
  
  
}


shinyApp(ui, server)

app2.R

# Example of exporting UI where a selectInput is dependent on another input from the same exported UI (this doesn't work)

library(shiny)

tabUI <- function(id) {
  ns <- NS(id)
  fluidRow(
    textOutput(ns("filter"))
  )
}

filtersUI <- function(id) {
  ns <- NS(id)
  
  fluidRow(
    uiOutput(ns("ui"))
  )
}

filters <- function(input, output, session, ui) {
  observe({
    output$ui <- ui()
  })
  
}

tab <- function(input, output, session) {

  ns <- session$ns
  
  model <- reactive({
    req(!is.null(input$select2))
    unique(mpg$model[mpg$class == input$select2])
  })
  
  ui <- renderUI({
    withTags(
      div(
        div(
          selectizeInput(ns("select2"), "Class", choices = unique(mpg$class))
        ),
        div(
          selectizeInput(ns("select"), "Model", choices = model())
        )
      )
    )
  })
  
  
  output$filter <- renderText(input$select)
  
  return(reactive(ui))
}

ui <- fixedPage(
  wellPanel(
    tabUI("tab")
  ),
  wellPanel(
    filtersUI("filters")
  )
)

server <- function(input, output, session) {
  tab <- callModule(tab, "tab")
  callModule(filters, "filters", tab)
}


shinyApp(ui, server)

app3.R

# Example of exporting UI where a selectInput is dependent on another input from a different exported UI (this works)

library(shiny)

tabUI <- function(id) {
  ns <- NS(id)
  fluidRow(
    textOutput(ns("filter"))
  )
}

filtersUI <- function(id) {
  ns <- NS(id)
  
  fluidRow(
    uiOutput(ns("ui")),
    uiOutput(ns("ui2"))
  )
}

filters <- function(input, output, session, ui) {
  observe({
    output$ui <- ui$ui()
    output$ui2 <- ui$ui2()
  })
  
}

tab <- function(input, output, session) {

  ns <- session$ns
  
  model <- reactive({
    req(!is.null(input$select2))
    unique(mpg$model[mpg$class == input$select2])
  })
  
  ui <- renderUI({
    withTags(
      div(
        div(
          selectizeInput(ns("select2"), "Class", choices = unique(mpg$class))
        )
      )
    )
  })
  
  ui2 <- renderUI({
    withTags(
      div(
        div(
          selectizeInput(ns("select"), "Model", choices = model())
        )
      )
    )
  })
  
  
  output$filter <- renderText(input$select)
  
  return(list(ui = reactive(ui), ui2 = reactive(ui2)))
}

ui <- fixedPage(
  wellPanel(
    tabUI("tab")
  ),
  wellPanel(
    filtersUI("filters")
  )
)

server <- function(input, output, session) {
  tab <- callModule(tab, "tab")
  callModule(filters, "filters", tab)
}


shinyApp(ui, server)

app4.R

library(shiny)

tabUI <- function(id) {
  ns <- NS(id)
  fluidRow(
    textOutput(ns("filter"))
  )
}

tabUI2 <- function(id) {
  ns <- NS(id)
  tagList(
    uiOutput(ns("ui")),
    uiOutput(ns("ui2"))
  )
}

filtersUI <- function(id) {
  ns <- NS(id)
  
  fluidRow(
    uiOutput(ns("filters"))
  )
}

filters <- function(input, output, session) {
  output$filters <- renderUI({
    tabUI2("tab")
  })
}

tab <- function(input, output, session) {
  
  ns <- session$ns
  
  model <- reactive({
    req(!is.null(input$select2))
    unique(mpg$model[mpg$class == input$select2])
  })
  
  output$ui <- renderUI({
    withTags(
      div(
        div(
          selectizeInput(ns("select2"), "Class", choices = unique(mpg$class))
        )
      )
    )
  })
  
  output$ui2 <- renderUI({
    withTags(
      div(
        div(
          selectizeInput(ns("select"), "Model", choices = model())
        )
      )
    )
  })
  
  
  output$filter <- renderText(input$select)
  
  invisible()
}

ui <- fixedPage(
  wellPanel(
    tabUI("tab")
  ),
  wellPanel(
    filtersUI("filters")
  )
)

server <- function(input, output, session) {
  callModule(tab, "tab")
  callModule(filters, "filters")
}


shinyApp(ui, server)

the mpg dataset comes from ggplot package, in case anyone else is wondering