I have a Shiny app that allows the user to load a csv and then plot the data.
I then have some custom functions to create different UI elements, that are passed into the server, for example:

   GroupByUI <- function(id,
                      data,
                      label = 'Group By',
                      selected = 'All',
                      None = FALSE,
                      All = TRUE) {
  renderUI({
    xsecs <- data %>%
      select_if(is.character) %>%
      names()

    if (None) {
      xsecs <- c('None', xsecs)
    }

    if (All == FALSE) {
      xsecs <- xsecs[-which(xsecs == 'All')]

      selectInput(inputId = paste0('groupby_', id),
                  label = label,
                  choices = as.list(xsecs),
                  selected = as.list(xsecs)[1])

    } else {

      selectInput(inputId = paste0('groupby_', id),
                  label = label,
                  choices = as.list(xsecs),
                  selected = selected)
    }
  })

} 

and in the server I have:

output$GroupBy_Area <- GroupByUI(
  id = "area",
  data = dataFlt()
)

and this works when I load a first csv (e.g. the Group By selectInput is updated the correct variable names).

For context, dataFlt() is a reactive and it’s set to NULL and updates every time I load a new/different csv.

dataFlt <- reactive({
  req(uploadedData$data)
  datafl <- uploadedData$data
})

The problem that I have is, that if I have loaded a csv, plotted the data and then I change the csv that I load (e.g. I change the data I want to plot, variables’ names change), the options in the Group By selectInput do not update accordingly UNLESS I move the renderUI directly to the server:

output$GroupBy_Area <- renderUI({
  req(dataFlt())
  GroupByUI(id = "area", 
            data = dataFlt())
})

and I remove the renderUI from the function:

GroupByUI <- function(id, data, label = 'Group By', selected = 'All', None = FALSE, All = TRUE) {
  xsecs <- data %>%
    select_if(is.character) %>%
    names()

  if (None) {
    xsecs <- c('None', xsecs)
  }

  if (All == FALSE) {
    xsecs <- xsecs[-which(xsecs == 'All')]

    selectInput(inputId = paste0('groupby_', id),
                label = label,
                choices = as.list(xsecs),
                selected = as.list(xsecs)[1])

  } else {
    selectInput(inputId = paste0('groupby_', id),
                label = label,
                choices = as.list(xsecs),
                selected = selected)
  }
}

With this latest code (moving the renderUI directly to the server), it works every time I change the data I load.

What I don’t understand is why. What’s the difference? And why do the original code works the first time?