How to return a variable from a module to the server in an R Shiny app?

Question

I'm having a surprising amount of difficulty returning a variable from a module to the server in an R Shiny app. In a module, I'd like to return a value when a button press is observed, so I've wrapped the return() statement in a block inside observeEvent(). However, the desired value is not returned, the entire observeEvent() block appears to be.

I've attempted to create a minimal working example outlining the problem below:

ui.R

# ui.R

fluidPage(
  input_module_ui("input"),
  actionButton("print_input_button",
               label = "Print Input")
)

server.R

# server.R

function(input, output, session) {

  # Calling input module.
  input_module_return <- callModule(input_module, "input")

  observeEvent(input$print_input_button, {
    print(input_module_return)
  })

}

global.R

# global.R

source("modules/input.R")

input.R

# input.R

input_module_ui <- function(id) {

  ns <- NS(id)

  tagList(
    textInput(ns("text_input"),
              label = h2("Input Text:")),
    actionButton(ns("submit_input"),
                 label = "Submit Input")
  )

}

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

  print("I should only print once")

  observeEvent(input$submit_input, {
    print("Return input")
    return(input$text_input)
  })

}

When testing this app, I entered "test" in the text input box and submitted my input. However, when I attempted to print my input, instead of printing "test" as I'd expect, the following was printed:

<Observer>
  Public:
    .autoDestroy: TRUE
    .autoDestroyHandle: function () 
    clone: function (deep = FALSE) 
    .createContext: function () 
    .ctx: environment
    destroy: function () 
    .destroyed: FALSE
    .domain: session_proxy
    .execCount: 3
    .func: function () 
    initialize: function (observerFunc, label, suspended = FALSE, priority = 0, 
    .invalidateCallbacks: list
    .label: observeEvent(input$submit_input)
    .onDomainEnded: function () 
    onInvalidate: function (callback) 
    .onResume: function () 
    .prevId: 1896
    .priority: 0
    resume: function () 
    run: function () 
    self: Observer, R6
    setAutoDestroy: function (autoDestroy) 
    setPriority: function (priority = 0) 
    suspend: function () 
    .suspended: FALSE

I believe that this corresponds to the last block in input.R:

observeEvent(input$submit_input, {
    print("Return input")
    return(input$text_input)
  })

How can I get this app to work as intended and return input$text_input when input$submit_input is observed?

Answer 1

You were quite close to getting this to work. The trick with shiny modules is that passing variables into and out of them requires passing the variables as reactive values. I made two small changes to your code to get what I think you're hoping to see.

First was to return a reactive version of input$text_input from the server module (rather than from the observer itself, which should just tell the app what you want to happen):

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

  print("I should only print once")

  observeEvent(input$submit_input, {
    print("Return input")
  })

  return(reactive({input$text_input}))

}

The second change is that now the output from input_module is reactive. If you want the values rather than the function contents, you need to resolve the object using (). So, in your server function:

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

  # Calling input module.
  input_module_return <- callModule(input_module, "input")

  observeEvent(input$print_input_button, {
    print(input_module_return())
  })

}

Output:

Listening on http://127.0.0.1:6796
[1] "I should only print once"
[1] "Return input"
[1] "test"

Answer 2

I found the code above to error out in my situation. Thus I thought I could provide a more generalised answer.

library(shiny)
library(tidyverse)

lower_UI <- function(id) {
  tagList(
    uiOutput(NS(id, "text3")),
    textOutput(NS(id,"verbose3")),
    actionButton(NS(id,"goButton"), label = "beep me", class = "btn-success")
  )
}

lower <- function(id) {
  moduleServer(id, function(input, output, session) {

    retV = reactiveValues(output = NULL)
    output$text3 <- renderUI(textInput(session$ns("text3"), "test", "test"))
    output$verbose3 <- renderText(input$text3)
    observe({

      retV$output = input$text3
    }) %>% bindEvent(input$goButton)

    return(reactive(retV$output))
})
}

upper_UI <- function(id) {
  lower_UI(NS(id, "test"))
}

upper <- function(id) {
  moduleServer(id, function(input, output, session) {

  uRV <- reactiveValues(one = NULL)
  output$one <- lower("test")

  observe({
    print(uRV$one())
    })
  })
}

ui <- fluidPage(
  upper_UI("upper")
)

server <- function(input, output, session) {
  upper("upper")
}

shinyApp(ui, server)