Shiny App: Adding unlimited number of input bars - r

Question

I want to build an app in which the user can add as many as input slots as he wants. I could only build an app that let the user to add only one more input slot. Here is my code:
ui <- fluidPage(
sidebarLayout(
sidebarPanel(
selectizeInput("a", "Something", choices = "blah blah"),
uiOutput("b"),
actionButton(inputId = "rm", label = "-"),
actionButton(inputId = "add", label = "+"),
),
mainPanel(
textOutput("test")
)
)
)
server <- function(input, output) {
observeEvent(input$add ,{
output$b <- renderUI({
selectizeInput("b", "Another thing", choices = "blah blah")
})
})
observeEvent(input$rm ,{
output$b <- renderUI({
NULL
})
})
}
shinyApp(ui = ui, server = server)
I have no idea how I can extend this to let the user add as many as input slots as he wants. Is this even possible?

Answer 1

We can try this approach:
We can access new added inputs with input$a1, input$a2 ... input$ax
Edit: added an observer to see the new inputs generated in the console. The first input created after pressing + button will be called input$a1.
observe({
print(names(input))
print(input$a1)
})
library(shiny)
ui <- fluidPage(
sidebarLayout(
sidebarPanel(
selectizeInput("a", "Something", choices = "blah blah"),
actionButton(inputId = "rm", label = "-"),
actionButton(inputId = "add", label = "+"),
),
mainPanel(
textOutput("test")
)
)
)
server <- function(input, output) {
input_counter <- reactiveVal(0)
observeEvent(input$add, {
input_counter(input_counter() + 1)
insertUI(
selector = "#rm", where = "beforeBegin",
ui = div(id = paste0("selectize_div", input_counter()), selectizeInput(paste0("a", input_counter()), label = "Another thing", choices = c("bla", "blabla")))
)
})
observeEvent(input$rm, {
removeUI(
selector = paste0("#selectize_div", input_counter())
)
input_counter(input_counter() - 1)
})
observe({
print(names(input))
print(input$a1)
})
}
shinyApp(ui, server)