Listen to reactives in list

Question

I am trying to add reactives to a list and listen to them. I do not understand why in teh code below I do not see any output in dbg1 while it works in dgb2.

The only difference is that l2 will contain a reactive on startup, while l1 only after the first button press but are otherwise identical.

Any explanation for this?

library(shiny)
library(purrr)

ui <- fluidPage(
   actionButton("add1", "Add to List 1"), 
   actionButton("add2", "Add to List 2"), 
   actionButton("rnd", "Generate Random"),
   verbatimTextOutput("dbg1"),
   verbatimTextOutput("dbg2"))

server <- function(input, output, session) {
   l1 <- l2 <- list()
   
   observeEvent(input$add1, {
      l1 <<- c(l1, reactive({
         input$rnd
         sample(100, 1)
      }))
   })
   
   observeEvent(input$add2, {
      l2 <<- c(l2, reactive({
         input$rnd
         sample(100, 1)
      }))
   }, ignoreNULL = FALSE)
   
   output$dbg1 <- renderPrint(map(l1, ~ .x()))
   output$dbg2 <- renderPrint(map(l2, ~ .x())) 
}

shinyApp(ui, server)

---

Adter reading the answer from @stefan and the comment from @starja I want to render the issue more precisely.

Goal

I want a dynamic container of reactives. That is, a dynamically created amount of reactives, which do their thing depending on some inputs.

Problem

I think in my code the renderPrint for dbg1 is only called on startup. It realizes that there is no reactive context (which is indeed only added later) and therefor never recalls it. In the case of dbg1 it sees at least one reactive and thus comes back. So I guess that I have to make l1 reactive itself (as pointed out by @stefan)

Answer 1

I guess that the problem is that when dbg1 first checks l1, it does not see any reactive context (which is true). It does, however, not realize that l1 eventually contains some reactives and never "recalls" it.

Thus, I think we have to be more explicit about l1 by making it reactive itself (inspired by @Stefan):

server <- function(input, output, session) {
   l1 <- l2 <- list()

   r1 <- reactiveVal(list())
   r2 <- reactiveVal(list())
   
   observeEvent(input$add1, {
      r1(c(r1(), reactive({
         input$rnd
         sample(100, 1)
      })))
   })
   
   observeEvent(input$add2, {
      r2(c(r2(), reactive({
         input$rnd
         sample(100, 1)
      })))
      
   }, ignoreNULL = FALSE)
   
   output$dbg1 <- renderPrint(map(r1(), ~ .x()))
   output$dbg2 <- renderPrint(map(r2(), ~ .x())) 
}

Answer 2

Not sure what you are trying to achive in the end. But following this post you could update your lists and have them printed by making use of reactiveVal like so:

library(shiny)
library(purrr)

ui <- fluidPage(
  actionButton("add1", "Add to List 1"), 
  actionButton("add2", "Add to List 2"), 
  actionButton("rnd", "Generate Random"),
  verbatimTextOutput("dbg1"),
  verbatimTextOutput("dbg2"))

server <- function(input, output, session) {
  l1 <- reactiveVal(value = list())
  l2 <- reactiveVal(value = list())
  
  rnd <- eventReactive(input$rnd, {
    sample(100, 1)
  })
  
  observeEvent(input$add1, {
    old_value <- l1()
    l1(c(old_value, rnd()))
  })
  observeEvent(input$add2, {
    old_value <- l2()
    l2(c(old_value, rnd()))
  })
  
  output$dbg1 <- renderPrint(l1())
  output$dbg2 <- renderPrint(l2()) 
}

shinyApp(ui, server)