Insert/Remove UI elements in R Shiny

example_app.R

library(shiny)

ui = fluidPage(
  titlePanel("My favorite movies"),
  hr(),
  
  # ------------------------------------- #
  # Input form
  tags$h3("Input form"),
  fluidRow(
    column(4, tags$label("Title")),
    column(4, tags$label("Director")),
    column(3, tags$label("Year released"))
  ),
  div(id = "placeholder"),
  actionButton(inputId = "new_row", label = "Add new movie", icon = icon("plus")),
  
  # ------------------------------------- #
  # Print user inputs
  hr(),
  tags$h3("User Inputs"),
  tableOutput("user_inputs")
)

server = function(input, output) {
  user_inputs = reactiveVal(data.frame(title = character(0), 
                                       director = character(0), 
                                       year = integer(0)))

  row_id = 1
  observeEvent(
    eventExpr = input$new_row, 
    handlerExpr = {
      new_row = add_formGroup(paste0("row", row_id), user_inputs)
      insertUI(new_row, selector = "#placeholder", where = "beforeBegin")
      row_id <<- row_id + 1
    }
  )
  
  output$user_inputs = renderTable(user_inputs())
}

add_formGroup = function(id, user_inputs){
  moduleServer(id, function(input, output, session){
    ns = NS(id)
    
    # Build UI
    ui = div(
      id = id,
      fluidRow(
        column(4, textInput(ns("title"), label = NULL, width = "100%")),
        column(4, textInput(ns("director"), label = NULL, width = "100%")),
        column(3, textInput(ns("year"), label = NULL, width = "100%")),
        column(1, actionButton(inputId = ns("remove_entry"), label = "X", style = "margin: 0px; padding: 0px; width: 34px; height: 34px"))
      )
    )
    
    # Observer for textInputs --> update 'user_inputs' reactiveVal
    observeEvent(
      eventExpr = {
        input$title 
        input$director
        input$year
      },
      
      handlerExpr = {
        new_row = c(title = input$title, director = input$director, year = input$year)
        inputs_df = isolate(user_inputs())
        inputs_df[id,] = new_row
        user_inputs(inputs_df)
      }
    )
    
    # Observer for 'remove_entry' --> remove UI
    observeEvent(
      eventExpr = input$remove_entry,
      handlerExpr = {
        inputs_df = isolate(user_inputs())
        user_inputs(inputs_df[rownames(inputs_df) != id,])
        removeUI(selector = paste0("#", id), session = session)
      }
    )
    
    return(ui)
  })
}

shinyApp(ui = ui, server = server)