How to create dynamic number of observeEvent in shiny?

Question

Here I have tried to replicate my original problem. The following shiny code will create 'Sub Segment' button if you click on 'Add a Segment'.

Now 'Add a Sub Segment' button should give a single line text on each click. But what I have noticed, It creates (1 + number of 'Add a Segment' button after the clicked one) many lines.

For single observeEvent it is working (the code is commented out).

library(shiny)

ui <- fluidPage(
  verbatimTextOutput("txt",placeholder = T), #"It is Created for Testing"
  actionButton("addSeg", "Add a Segment"),
  uiOutput("myUI")
)

server <- function(input, output, session) {
  alld <- reactiveValues()
  alld$ui <- list()

  # Action to add new Segment
  observeEvent(input$addSeg,{
    alld$ui[[length(alld$ui)+1]] <- list(actionButton(paste0("addSub_",(length(alld$ui)+1)), "Add a Sub Segment"))
  })

  # Action to add new Sub Segment
  # observeEvent(input[[paste0("addSub_",1)]],{
  #   alld$ui[[1]][[length(alld$ui[[1]])+1]] <- paste0("addSub_",1)
  # })

  observeEvent(input$addSeg,{
    lapply(1:length(alld$ui), function(i){
      observeEvent(input[[paste0("addSub_",i)]],{
        alld$ui[[i]][[length(alld$ui[[i]])+1]] <- HTML(paste0("<br>addSub_",i,"<br>"))
      })
    })
  })

  output$myUI <- renderUI({alld$ui})

  output$txt <- renderText(class(alld$ui))
}

shinyApp(ui, server)

Please Help...

Answer 1

Very interesting question, and a very commendable reprex. I managed to find a solution. You can replace your server object with this:

server <- function(input, output, session) {
  alld <- reactiveValues()
  alld$ui <- list()

  # Action to add new Segment
  observeEvent(input$addSeg,{
    new_id <- length(alld$ui) + 1
    sub_name <- paste0("addSub_", new_id)

    alld$ui[[new_id]] <- list(
      actionButton(sub_name, "Add a Sub Segment")
    )

    observeEvent(input[[sub_name]], {
      new_text_id <- length(alld$ui[[new_id]]) + 1
      alld$ui[[new_id]][[new_text_id]] <- HTML(paste0("<br>addSub<br>"))
    })
  })

  output$myUI <- renderUI({alld$ui})

  output$txt <- renderText({
    capture.output(str(alld$ui))
  })
}

Let's talk about your original code. Your first observer works just fine. The second one, however, is causing the unwanted behaviour. It returns a list of new observers, one for every addSub button currently in the app. This means that on the first click, it creates an observer for addSub_1 , and on the second click, it returns an observer for addSub_1 and addSub_2 . However, the first addSub_1 observer still exists. This means that when you click addSub_1 , there are two observers responding and the text is shown twice.

My solution is to combine your two observers into one. When you click addSeg , the button is created in the UI-list, and the observer that handles it is also created. This way, there are no duplicates and the app works as expected.