Wait for inputs to be updated from the server, then execute a callback

on_inputs_initialized.R

#' Run callback when all inputs are initialized
#'
#' @param ids A character vector of input ids to check.
#' @param callback A function to run after all inputs are initialized.
#' @param session A Shiny session
#' @importFrom shiny reactiveVal observe getDefaultReactiveDomain
#' @importFrom purrr map reduce
#' @importFrom checkmate test_null
on_inputs_initialized <- function(ids, callback, session = getDefaultReactiveDomain()) {
  initialized_ <- reactiveVal(FALSE)
  initialized_observer <- observe({
    flag <- ids |>
      map(\(x) session$input[[x]]) |>
      map(\(x) !test_null(x)) |>
      reduce(`&`)
    initialized_(flag)
  })

  observe({
    if (initialized_()) {
      callback()
      attr(initialized_, ".impl")$freeze()
      initialized_observer$destroy()
    }
  })
}