Shiny Sortable CheckboxGroupInput with Update on Change

app.R

sortableCheckboxGroupInput <- function(inputId, ...) {
  # build jquery-ui dependency for sortable
  jqueryUIDep <- htmlDependency("jqueryui", "1.10.4", c(href="shared/jqueryui/1.10.4"),
                                script = "jquery-ui.min.js",
                                stylesheet = "jquery-ui.min.css")
  
  # sortable enabler
  sortableScript <- "
function makeSortable(el) {
    el.sortable({
        //clone a helper which will append to the end so that it would visually
        //preserve the checked status while dragging
        helper: 'clone',
        placeholder: {
        element: function(item) {
            //clone the item and show it so the placeholder replicate the item 
            //and will be reflected in our values
            var clone = item.clone();
            clone.show();
  
            //uncheck the box so the original box won't mess with our values
            var checkbox = item.children(':checkbox');
            checkbox.prop('was-checked', checkbox.prop('checked'));
            checkbox.prop('checked', false);
          
            return clone;
          },
          update: function() {
              return;
          }
        },
        change: function(event, ui) {
            //propogate changes in position directly to the element being moved
            $(event.target).trigger('change');
        },
        stop: function(event, ui) {
          //now we want to restore the checked status if it was checked
          var checkbox = ui.item.children(':checkbox');
          checkbox.prop('checked', checkbox.prop('was-checked'));
        }
    });
}"
  
  # return label and select tag
  attachDependencies(
    tagList(
      singleton(tags$head(tags$script(sortableScript))),
      checkboxGroupInput(inputId, ...),
      tags$script(paste0("makeSortable($('#", inputId, "'));"))),
    jqueryUIDep)
}

shinyApp(
  ui = fluidPage(
    sortableCheckboxGroupInput("variables", "", 1:10, 1:10),
    textOutput("variables")
  ), 
  server = function(input, output) {
    output$variables <- renderText(unique(input$variables))
  }
)