How to create dynamic number of observeEvent in another observeEvent?

Question

Here I asked an similar question and got a working answer. But the solution does not work if 'actionButton' of sub segment is replace by 'selectInput'. On each selection of selectInput creates two outputs. Please help.. Thanks....

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$addSeg,{
    new_id <- length(alld$ui) + 1
    sub_name <- paste0("addSub_", new_id)

    alld$ui[[new_id]] <- list(selectInput(sub_name,"Add a variable", choices = c("V1","V2"), selected  = NULL))

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

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

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

shinyApp(ui, server)

Answer 1

This behaviour occurs because the custom UI element is re-rendered every time a new element is added to the list. Once you click "V2" and the new text element is added, the selectInput itself re-renders and resets to V1, which is noticed by the observer you've created.

The following might be a solution for you:

  observeEvent(input$addSeg,{
    new_id <- length(alld$ui) + 1
    sub_name <- paste0("addSub_", new_id)

    alld$ui[[new_id]] <- list(
      selectInput(sub_name,
                  "Add a variable",
                  choices = c("", "V1","V2"),
                  selected  = "")
      )

    observeEvent(input[[sub_name]], {
      if (input[[sub_name]] == "") return()
      new_text_id <- length(alld$ui[[new_id]]) + 1
      alld$ui[[new_id]][[new_text_id]] <- HTML(paste0("Variable ",input[[sub_name]]," added<br>"))
    }, ignoreInit = TRUE)
  })

What I've done here is add an empty option to your selectInput s, and a condition to the corresponding observer that it shouldn't do anything if the input is empty. This way, I'm harnessing the "resetting" behaviour to be useful instead of annoying.