Can't get Leaflet.Spin plugin working in R Shiny

[woodward]

I have a shiny app which involves drawing a large number of lines on a map. I would like to use a spinner to show the user that rendering is underway. Most shiny approaches don't work because they only show the spinner while the data is being sent to leaflet, not when leaflet is rendering in javascript. The Leaflet.Spin plugin looks promising but I have been struggling to get it to work. The examples I have been following are

See here.
And here.

How do I get the js events to fire properly and show Leaflet.Spin when the lines (circles in this example) are rendering? The spinner now works, but events fire for each individual circle added, so if number of circles declines, spinner doesn't turn off correctly. Thanks!

PS: I offered a bounty for this question over on Stackoverflow.

library(shiny)
library(leaflet)
library(htmltools) # for htmlDependency
library(htmlwidgets) # for onRender

# https://gist.github.com/jcheng5/c084a59717f18e947a17955007dc5f92
# https://stackoverflow.com/questions/52846472/leaflet-plugin-and-leafletproxy-with-polylinedecorator-as-example
spinPlugin <- htmlDependency(
  "spin.js", 
  "2.3.2",
  src = c(href = "https://cdnjs.cloudflare.com/ajax/libs/spin.js/2.3.2"),
  script = "spin.min.js") # there's no spin.css

leafletspinPlugin <- htmlDependency(
  "Leaflet.Spin", 
  "1.1.2",
  src = c(href = "https://cdnjs.cloudflare.com/ajax/libs/Leaflet.Spin/1.1.2"),
  script = "leaflet.spin.min.js")

registerPlugin <- function(map, plugin) {
  map$dependencies <- c(map$dependencies, list(plugin))
  map
}

# Note: Ctrl-Shift-J opens the javascript console in the browser
spin_event <- "function(el, x) {
  console.log('spin event added'); 
  var mymap = this;
  mymap.on('layerremove', function(e) {
    console.log('layerremove fired');
    mymap.spin(true);
  });
  mymap.on('layeradd', function(e) {
    console.log('layeradd fired');
    mymap.spin(false); 
  });
}"

dlat <- 1 / 111000 * 100 # degrees per metre

ui <- fluidRow(
  tags$h2("Using Leaflet.Spin in Shiny"),
  actionButton("plotbutton", label = "Show Spinner While Adding Markers"),
  leafletOutput("map")
)

server <- function(input, output, session) {
  output$map <- renderLeaflet({
    cat("renderLeaflet\n")
    leaflet() %>%
      addTiles() %>%
      setView(175.322, -37.789, zoom = 17) %>% 
      registerPlugin(spinPlugin) %>% 
      registerPlugin(leafletspinPlugin) %>% 
      onRender(spin_event) %>% 
      clearShapes() %>% # initialise spinner
      addCircles(
        lng = 175.322,
        lat = -37.789,
        radius = 0,
        opacity = 0
      )
  })
  
  observeEvent(input$plotbutton, {
    cat("input$plotbutton\n")
    n <- ceiling(runif(1) * 10000)
    leafletProxy("map") %>%
      clearShapes() %>% 
      addCircles(
        lng = 175.322 + (runif(n) * 2 - 1) * dlat * 6,
        lat = -37.789 + (runif(n) * 2 - 1) * dlat * 1.5,
        radius = dlat * runif(n) * dlat
      )
  })
}

shinyApp(ui = ui, server = server)